Strength power and rapid force assessment after total knee arthroplasty

Clinical interpretation

Quadriceps performance should be measured explicitly after total knee arthroplasty (TKA), but maximal strength, voluntary activation, movement power and rapid force production are different constructs. A normal-looking manual muscle grade or an improving walking time cannot establish their recovery. Early postoperative weakness may reflect activation failure as well as loss of muscle tissue; later performance can remain limited despite recovery to a poor preoperative baseline. Assessment should therefore preserve both limbs’ absolute results, the exact postoperative time, pain and testing conditions, and the mechanical quantity actually measured. [1–8]

For routine quantitative monitoring, a reproducible isometric knee-extension protocol is a reasonable core. A stabilized handheld or fixed dynamometer can provide useful data, but results from different devices, angles and lever arms must not be treated as interchangeable. Add hip strength when it addresses an identifiable functional question. Measure dynamic power or rate of force/torque development when the equipment, signal processing and patient’s ability permit a defensible protocol; do not infer these qualities from an ordinary strength value. The evidence supports broader assessment, but does not validate a universal strength target, power cutoff or RFD-based risk calculator. [3–14]

The prognosis literature requires unusually careful reading. Some cohorts genuinely measure an early variable before a later outcome. Other papers use “predictor” for a concurrent association, or include information from the outcome time in the model. A 2024 strength-recovery model uses a three-month-derived change score to explain three-month strength. A larger 2026 nomogram has temporal validation but requires contralateral strength measured at the same one-year visit as the target. Neither is an early postoperative forecasting tool as published. [15, 16]

Scope and evidence approach

The primary population is adults after elective primary TKA for osteoarthritis (OA). Preoperative end-stage OA assessments are distinguished from postoperative measurements. Unilateral, simultaneous bilateral and staged replacements are separated; the nonoperated limb may itself have OA or a previous arthroplasty. Revision, unicompartmental, fracture and inflammatory indications are not pooled into primary OA-TKA inference. The report concerns assessment, interpretation and prognosis, not comparative effectiveness of exercise or surgery.

Dated PubMed and Scopus searches through 2 October 2026 were supplemented with rapid-force, power, activation and hip-abductor terms and citation chasing. The principal PubMed query returned 638 official records; the targeted supplement returned 63, with overlap. Official NCBI ESearch and EFetch reconciled connector page duplication. These counts describe retrieval, not eligibility. This is a critical narrative review, without a claim of formal systematic-review completeness, duplicate independent screening or meta-analysis.

Constructs and units

Maximum force and joint torque

A dynamometer applied to the distal leg measures force at its contact point, often in newtons or kilogram-force. Knee-extension torque is force multiplied by the perpendicular moment arm, normally expressed in newton-metres. The distance and direction matter. Two patients generating the same contact force with different lever arms do not necessarily generate the same joint torque. A number in N/kg is not interchangeable with Nm/kg, and kgf/kg is not a unitless percentage of body weight unless the conversion and definition are explicit.

Isometric strength describes the highest voluntary force or torque at a specified joint configuration. Isokinetic strength describes torque under controlled angular velocity, usually across a range of motion. Isoinertial one-repetition maximum describes the greatest external load moved under a particular machine and movement protocol. A one-repetition load is neither isometric torque nor peak power. Values are also affected by gravity correction, alignment, stabilization, contraction instruction, encouragement and whether the maximum or average of trials is used. [3, 7, 13]

Manual muscle testing can describe severe weakness or inability to move against gravity, but its ordinal categories have limited resolution for quantitative recovery in larger muscle groups. A score of five is not evidence of symmetry, age-relevant strength, normal activation or sufficient rapid force. Conversely, inability to complete a demanding maximal test is a clinically meaningful observation and should not be hidden by selecting only successful trials.

Voluntary activation and muscle size

Voluntary activation concerns how fully a person can recruit available muscle capacity during an attempted maximal contraction. Burst-superimposition and interpolated-twitch/doublet methods add electrical stimulation during the voluntary effort and compare the extra torque with an appropriate reference. The central activation ratio and doublet-interpolation percentage are not identical equations. Results depend on stimulation adequacy, electrode placement, potentiation, the timing of the stimulus and the participant’s effort. [2, 17]

Anatomical cross-sectional area or thickness measures muscle size; it does not directly quantify activation, specific force, or whole-muscle functional capacity. Surface EMG amplitude is an electrical recording, not a direct percentage of voluntary activation or force. Pain and swelling can influence performance, but an observational association among these measures does not isolate a causal pathway. The clinical assessment should distinguish “low voluntary output” from a proven mechanism of that weakness.

Power and rapid force production

Mechanical power is the product of force and linear velocity, or joint torque and angular velocity, expressed in watts. An isometric contraction has no external joint motion; its RFD/RTD is not mechanical power, even when papers group both under “explosive strength” or “power.” RFD describes the rate of increase in force, in N/s; RTD describes torque increase, in Nm/s. Peak derivative, average slope over a fixed interval, slope to a percentage of maximum, and maximum force divided by time to peak are different calculations. Their values must not be merged. [4–8]

Rapid-force estimates are especially sensitive to contraction onset, baseline noise, pretension, countermovement, sampling frequency and filtering. A 50-ms interval contains very few samples at 100 Hz. That observation does not retrospectively invalidate a published study, but it constrains precision and transfer to another acquisition system. A camera-derived knee-extension speed, force-plate loading rate during gait and isolated quadriceps RTD are also distinct variables. Calling all three “RFD” would create false equivalence.

Table 1 Distinguishing muscle performance constructs

[2–14]. N, Nm, N/kg, Nm/kg and kgf/kg are not interchangeable quantities.

Early weakness and the recovery trajectory

Mizner and colleagues measured 20 patients around ten days before and 27 days after primary unilateral TKA for OA. At one month, quadriceps strength was approximately 62% lower, voluntary activation 17% lower and maximal cross-sectional area 10% lower than preoperatively. Change in activation and size together explained 85% of the variation in strength loss; activation contributed more strongly. Pain during testing did not explain a significant proportion of activation change in this small sample. [2]

This is compelling mechanistic context, but not proof that pain never matters or that atrophy is unimportant. The study excluded substantial other-joint impairment and several comorbidities. Activation was measured using a burst-superimposition central activation ratio at 75° knee flexion. Its regression concerns simultaneous changes over the early postoperative interval; it is not a validated individual forecast. Other work relating local nociceptive sensitivity to postoperative activation and strength supports a more complex relationship than a simple pain score alone captures. The latter article body has now been recovered and its six-week change analysis, measurement methods and observational mediation checked. These observational results do not establish that changing pain causes a specified change in activation or strength. [2, 19]

Measured recovery depends on the reference. Returning to the preoperative level means returning to end-stage OA performance, not reaching healthy peers. A side-to-side ratio can improve if the nonoperated side deteriorates. The 2023 review describes persistent heterogeneity in recovery and interlimb differences; its linked correction changes the country assigned to one study, not the pooled strength results. Its estimates should not be turned into a universal timetable or used to fill missing individual data. [20, 21]

Organize measurements around clinically relevant visits while recording the actual days after surgery. Preoperative, inpatient, early outpatient, three-to-six-month and one-year categories are useful summaries, not biological deadlines. Compare both limbs with their own prior values, then consider an appropriately matched reference sample. Rehabilitation exposure, contralateral symptoms, surgical complications and changes in body mass affect interpretation. A postoperative trajectory is observed under the care actually received; it should not be described as untreated natural recovery.

Measurement properties of knee dynamometry

Handheld dynamometry during early recovery

The study published in Disability and Rehabilitation in 2021 examined 56 primary unilateral OA-TKA patients before surgery and at two and six weeks. Handheld testing used a therapist-resisted make test at 60° knee flexion and 90° hip flexion, with the dynamometer near the distal tibia. Electromechanical testing used the same nominal angles and contact location. Warm-up trials preceded maximal efforts; the first two maximal trials, with repositioning, supplied within-session reliability. The same assessor performed all testing, and handheld testing preceded electromechanical testing by approximately 30 minutes. [3]

ICC(2,1) values for the handheld device were 0.94 before surgery, 0.93 at two weeks and 0.91 at six weeks. SEMs were 26.1, 9.4 and 16.3 N, respectively. Complete paired-trial denominators decreased from 51 to 45 and 43. These are within-session repeatability estimates, not between-day reliability or interrater reproducibility. The smaller early-postoperative SEM partly reflects weaker forces and a narrower range, not a claim that early clinical decisions are intrinsically more certain. [3]

Device agreement is the decisive counterweight. Handheld testing systematically underestimated force relative to the electromechanical device. Mean device differences were 129.5 N preoperatively, 45.9 N at two weeks and 94.8 N at six weeks; corresponding 95% limits of agreement were wide. Moderate-to-high correlations showed related rankings, not interchangeable measurements. A patient switched from one device to the other could appear to change because of method bias. The reported recovery standardized response mean of 1.57 for handheld testing describes cohort change from two to six weeks; it is not a patient-anchored MIC. [3]

An acute-inpatient study of 45 TKA patients reports high intrarater and interrater reliability for knee extension and flexion, with intrarater extension SEM 6.17 Nm and MDC95 17.01 Nm. The accessible abstract describes three trials, short rests, another assessor after five minutes and same-rater repetition after one hour. Full protocol and ICC-model details were not recovered; exact postoperative day and trial aggregation should therefore be confirmed before importing its numerical threshold. It does establish that inpatient measurement research exists. [9]

Preoperative reliability and why stage cannot be ignored

Koblbauer and colleagues studied patients awaiting TKA using modified handheld dynamometry. Interrater analyses included 32 people; the between-day subgroup included 13, with a reported mean interval of 11 days and range of 2–27 days. For affected knee extension, interrater ICC was 0.96, SEM 0.21 N/kg and smallest detectable difference 0.58 N/kg, or 21.7%. Between-day affected-extension estimates differed by examiner, with detectable differences of 19.0% and 31.4%. Flexion error was larger. [10]

The article calls the model ICC(2,1) while describing an average of three trials. That ambiguity should be retained rather than silently rewritten as a different ICC model. The method gives direct evidence for preoperative end-stage OA, not early postoperative reliability. Its high ICC alongside substantial individual error illustrates why between-person discrimination and precision of a change score must both be assessed. The paper’s broad conclusion against individual clinical use should also be interpreted in the context of its particular modification, interval, sample size and error; it does not invalidate every stabilized dynamometer protocol.

Fixed equipment and alternative portable devices

Lienhard and colleagues compared isometric, isokinetic and isoinertial strength in 29 TKA patients. The abstract reports ICCs of 0.947–0.966 and SEMs of 5.1%–9.3%, with associations with walking and perceived function. Full methods were not recovered, so the model, timing, stability interval and modality-specific error cannot be fully verified. The correct inference is that several strength modalities can yield useful measurements under studied protocols, not that one-repetition loads, isokinetic torque and isometric values share a common numerical scale. [13]

A 2025 Locomo Scan study provides a contrasting portable approach in 64 patients around one month after TKA, near discharge from a comparatively long inpatient stay. Participants pressed the knee region into a load-measuring device in long sitting with slight flexion, using three ten-second efforts. Results were normalized in kgf/kg. Same-session “Case 1” ICCs were 0.903 and 0.889 for the two testers; between-tester “Case 2” ICC was 0.818 for the maximum of three trials. The latter MDC95 was 0.132 kgf/kg, or 35.1%, despite no statistically significant systematic bias. [14]

That substantial random error matters more for small individual gains than the label “high reliability.” Different testers assessed the patient on consecutive days, so between-tester differences also contain day-to-day variation. Same-tester between-day error was not measured. The device’s pressing task is not equivalent to a distal-tibial dynamometer measuring knee-extension torque. It may improve feasibility, but its values cannot be converted into a 90° Nm/kg target merely by multiplying by a generic lever arm.

Table 2 Protocol specific strength reliability and error

[3, 9–12, 14]. All values remain study-specific. A high ICC does not remove individual measurement error; an MDC is not a MIC.

Hip strength and the nonoperated limb

Hip-abductor assessment can clarify frontal-plane capacity and functional limitations, but it is not a substitute for quadriceps testing. Alnahdi and colleagues studied 210 people six to 48 months after primary unilateral OA-TKA and a small retest subgroup. The hip test used side lying, neutral hip alignment, an extended knee and a nonelastic belt securing the dynamometer just above the lateral femoral epicondyle. Three trials were obtained; retest analysis ultimately included 15 people after an outlier was excluded. ICC(2,3) was 0.95 (95% CI 0.86–0.98), with MDC95 47.6 N and a wide confidence interval. [11]

The result concerns an average-trial, belt-stabilized later-stage protocol. It does not establish error for a single manually resisted effort in the first week. Hip strength added information to concurrent performance after covariates and quadriceps strength were considered, but this was cross-sectional evidence, not proof that increasing hip force by a specific amount produces a corresponding functional gain. Force values were handled with anthropometric adjustment rather than treating raw force and torque as equivalent. [11]

A separate early-postoperative hip study assessed 30 people approximately nine days after TKA. Its abstract reports ICC(3,1) of 0.82 with belt resistance and 0.80 with therapist resistance. Individual detectable changes were much larger than group-average changes: 33 N versus 8 N for belt resistance and 57 N versus 14 N for manual resistance. These estimates are explicitly different decision levels. A group-average error threshold must never become an individual change flag. Full-text protocol and confidence-interval derivation remain access gaps. [12]

Evidence for functional association also varies with stage. A post hoc trial analysis of 162 patients measured hip-abductor and knee-extensor performance preoperatively and at one and three months. Hip strength made an independent contribution to performance before surgery, but not consistently at postoperative visits after quadriceps strength and other factors were considered. This is compatible with the later-stage Alnahdi findings because the populations, stage and models differ. It argues against a universal claim that hip strength is always the dominant determinant. [18]

The other limb deserves direct measurement. In Zeni and Snyder-Mackler’s longitudinal cohort, weaker early-postoperative nonoperated quadriceps strength added information about one- and two-year TUG and stair performance after baseline function and other covariates were included. Participants with substantial contralateral symptoms had been excluded, so the finding cannot simply be generalized to severe bilateral disease. More importantly, a limb labeled “nonoperated” should never be relabeled “healthy” without evidence. [22]

Rapid force and torque development

Protocol differences are substantive

Maffiuletti and colleagues examined 31 people approximately six months after unilateral OA-TKA. Testing used a fixed isometric apparatus at 60° knee flexion and 90° hip flexion, with instructions to contract quickly and forcefully. The signal was sampled at 100 Hz; onset was defined at approximately 15 N above baseline. They calculated peak RFD and fixed-window RFD at 0–50, 0–100 and 0–200 ms. Interlimb asymmetry for fixed-window RFD was greater than for maximal force, and several RFD asymmetry measures were associated with self-reported knee function. [5]

This supports the idea that maximal strength can miss a rapid-force deficit. It does not establish a diagnostic cutoff or a future-outcome prediction. The paper’s asymmetry formula was (nonoperated minus operated)/nonoperated multiplied by 100, not operated/nonoperated multiplied by 100. Its use of a fixed absolute onset threshold and limited sampling resolution must travel with the result. A percentage asymmetry is meaningful only alongside the two absolute values and the formula.

Winters and colleagues followed 35 TKA patients and 23 controls before surgery and at one and six months. RTD measures included peak derivative and slopes from onset to 25% and 50% of maximum, selecting the trial with the steepest initial rise. Those percentage-of-maximum endpoints change when maximum torque changes; they are not equivalent to fixed 50- or 200-ms windows. The retrieved methods also describe a torque-onset criterion using a force unit, a reporting ambiguity that should be clarified before exact replication. [4]

RTD fell markedly at one month and returned toward preoperative levels by six months, while remaining below controls. Adding RTD improved explanation of concurrent TUG and stair performance beyond maximum strength at selected visits. The additional variance was endpoint- and time-specific: a variable useful at one month did not necessarily add information at six. These are repeated concurrent regressions embedded in a longitudinal study, not demonstrated prediction of a future visit from an earlier RTD measurement. [4]

Relationship with gait mechanics

The 2019 quadriceps-avoidance gait study included 24 independently walking unilateral OA-TKA patients at three and six months. It measured operative-limb knee and hip muscle performance and treadmill knee-flexion excursion. RTD was the mean slope over 200 ms of a visually selected linear portion after the initial nonlinear region, sampled at 100 Hz and normalized to body mass. This is not a conventional fixed interval beginning at a reproducible force-onset threshold. [6]

RTD and pain contributed to the three-month model; RTD and hip external-rotation strength contributed at six months. However, each model related variables measured at the same visit. Sixteen candidate predictors in a sample of 24, stepwise selection and lack of independent validation make estimates vulnerable to selection instability. The title’s “primary contributor” should not be interpreted as established causality or proof that an RTD intervention will normalize gait. The study usefully motivates including rapid-force assessment when maximal strength and gait recovery diverge, but it does not provide an RTD clearance threshold. [6]

What is missing for routine RFD implementation

A clinical RFD protocol needs the same hardware settings, sampling and filters, onset rule, contraction instruction, pretension policy, trial rejection rule, rest, joint angle, selected trial summary and normalization at every visit. Preserve force-time traces when possible. An apparently improved peak derivative can reflect noise or onset processing, and a different trial may supply the best peak force and the best RFD. Recording only the final number prevents adequate quality review.

Within the postoperative TKA originals appraised here, no directly applicable patient-anchored RFD/RTD MIC or externally validated future-function/falls threshold was verified. This is a bounded evidence statement: it does not claim that all reliability research is absent, and it does not erase relevant preoperative OA or methodological research. Preoperative and healthy-sample estimates may inform protocol development, but they do not validate an individual postoperative threshold. [4–6]

Table 3 Rapid force and power protocols

[4–8]. Keep contraction instructions, normalization, onset and trial-selection rule with every result.

Dynamic power and contraction velocity

Marmon and colleagues directly measured knee-extensor power in 24 patients six months after unilateral TKA and 22 controls. The protocol included isokinetic velocities of 60, 90 and 120°/s and isotonic resistances corresponding to 20%, 30% and 40% of body weight. Torque and angular velocity were used to derive power, with 1000-Hz recording and gravity correction. Power at a 30%-body-weight resistance added explanatory information for concurrent TUG, stair climbing and six-minute walk performance after isometric strength was considered. [7]

The construct is genuinely mechanical power, but the design is cross-sectional despite the article’s “prospective cohort” level-of-evidence label. Some isotonic observations were missing because of load adjustments or inability to move higher resistance and were imputed. Performance at a relative load based on maximum strength would pose a different question from performance against a body-weight-based load. These results support a complementary assessment, not a validated future-outcome predictor, treatment-response formula or universal wattage target.

Iwata and colleagues assessed 186 unilateral TKA patients before surgery and at two and three weeks using maximal unloaded knee-extension angular velocity and quadriceps strength. Operated-side velocity had strong concurrent relationships with gait measures in multivariable models. Unloaded angular velocity may be feasible and informative, but it is not measured power without corresponding torque, and repeated contemporaneous relationships do not show that early velocity predicts later gait. A recommendation to train a variable is also not established by that association alone. [8]

For clinic software, chair-rise-derived estimates, stair-climb power and isolated knee-extension power need separate labels and evidence. A task time reflects multiple joints and movement strategies. A formula-derived whole-body power estimate should not appear in the same longitudinal series as direct dynamometer power. Nor should faster unloaded motion be called a recovered ability to generate high power against a meaningful load.

Prognosis and prediction models

Genuinely ordered predictors and later outcomes

Mizner and colleagues’ 2005 cohort assessed 40 patients two weeks before and one year after TKA. Strength was maximal volitional knee-extension force at 75° knee flexion, normalized to BMI in N/BMI, not torque in Nm/kg. Hierarchical models included age, preoperative flexion range, SF-36 bodily pain and then involved-limb strength. Full-model R² was 0.414 for one-year TUG and 0.539 for stairs; adding strength accounted for ΔR² of 0.361 and 0.471. However, baseline TUG or stair performance was not included, and no held-out validation was performed. This is genuine temporal association beyond age, range and pain, not demonstrated added prediction beyond baseline function or a validated clinical rule. [23]

Zeni and Snyder-Mackler followed 155 unilateral primary OA-TKA recipients, with 125 returning at two years. Predictors from the initial outpatient evaluation included baseline functional performance, age, BMI, pain, flexion and both limbs’ quadriceps strength. The nonoperated limb added information about later TUG and stairs. The study itself cautions that a visually suggested strength threshold was not established using a predefined success/failure outcome. No externally validated risk rule follows from this regression, and attrition or not-yet-reached follow-up must remain distinct from complete ascertainment. [22]

The clinical Quadriceps Activation Battery (QAB) offers a practical early marker. In 162 trial participants, its three tasks were scored from zero to two each: quadriceps contraction, straight-leg raise and extension-lag testing. A day-four median split separated scores of three or less from four or more. Lower early scores were associated with worse activation, strength and function during early recovery after adjustment for baseline outcome, treatment, sex and site; most differences did not persist at three or twelve months. The threshold was a sample split, not a validated diagnostic or risk cutoff. Intensive rehabilitation and exclusion of patients discharged away from home limit transportability. [17]

A 2024 regression-tree study of 264 patients ordered preoperative and three-week predictors before two-year knee-extension strength. Its abstract reports splits involving acute operated-side strength, TUG, preoperative walking and nonoperated strength. This is relevant direct prognostic research, but the full tree construction, pruning, validation, calibration and stability could not be inspected. Its 1.00 and 0.90 Nm/kg splits should not become universal rehabilitation targets or deterministic prognosis categories. [24]

A 2026 prospective muscle-health cohort provides a useful boundary case. It included 1,056 knee-arthroplasty patients, only 68% of whom received TKA, with the remainder undergoing unicompartmental replacement. Preoperative quadriceps strength and phase angle were associated with membership in one-year patient-reported functional trajectories. The trajectory definitions included the preoperative functional score, so membership can reflect initial status as well as subsequent recovery. The complete article and full adjustment/validation methods were not recovered. This is promising mixed-procedure prognostic research, but neither a TKA-only prediction model nor a basis for interpreting phase angle as measured muscle power or voluntary activation. [25]

Outcome information masquerading as early prediction

The 2024 secondary analysis of 22 patients reports a model explaining 77% of three-month strength variance using preoperative strength, early postoperative percentage loss and percentage recovery from day four to month three. The last predictor contains the three-month outcome itself. Once that future gain is known, the clinical forecasting question has already changed. Mathematical coupling also links early loss to its component measurements. This model cannot establish early predictive performance, even though its regression diagnostics and coefficient tests appear acceptable. Its serial strength measurements remain informative as a small, selected rehabilitation cohort. [15]

The 2026 Hiyama study is larger and more sophisticated. It analyzed 1,341 patients and 1,475 knees, with a 564-knee development cohort and a 911-knee later temporal cohort from the same centre. Strength was belt-stabilized at 90° knee flexion, based on the stronger of two three-second efforts, with measured lever arm and normalization to mass. A sex-specific tenth percentile from 120 reference adults defined a benchmark: 0.97 Nm/kg for women and 1.17 Nm/kg for men. These are normative reference thresholds, not MICs. [16]

The nomogram reported temporal-validation AUC 0.888, calibration intercept −0.107, slope 0.885 and Brier score 0.135. Those are substantive validation data and should not be dismissed. However, the model includes postoperative contralateral strength measured at one year to classify the operated limb’s one-year benchmark achievement. It is therefore a temporally validated model of a contemporaneous/conditional strength state, not a preoperative or early-recovery forecast. The same centre, local reference distribution, selected age range, exclusions and contralateral arthroplasty history further restrict transportability. Replacing the one-year input with an earlier measurement would create an unvalidated new model. [16]

The associated satisfaction and function differences are also contemporaneous. They cannot demonstrate that making a patient cross the tenth percentile will cause the observed benefit. Multiple knees per patient and the relationship between development and validation membership need attention when reproducing the model; the primary association analyses used generalized estimating equations. A normative target can support discussion, but cannot stand alone as a treatment decision, surgical success definition or promise of recovery.

Table 4 Separating prognosis from concurrent explanation

[15–17, 22–24]. Later temporal validation of a dataset does not make an outcome-time input available at an earlier clinical decision.

Meaningful change and clinical decisions

MDC/SDD estimate change beyond measurement noise under the specified design. MIC estimates change considered important by an external anchor. A normative threshold compares a person with a reference distribution. A prediction-tree split separates observations in a development sample. They answer different questions and should not share a generic “clinically significant” label.

In the direct postoperative strength, power and rapid-force sources appraised here, several error estimates and distribution-based response measures were verified, but a robust protocol-matched patient-anchored MIC for the central dynamometry or RFD measures was not established. Do not import an OA strength MIC, a neurological symmetry criterion or a healthy-athlete return-to-sport threshold. A measured change can matter to a patient while remaining uncertain relative to measurement error; conversely a large real gain can coexist with inadequate function or persistent pain.

Ratios require special care. Record operated and nonoperated values independently before computing limb symmetry, and specify whether the displayed quantity is operated/nonoperated, an asymmetry deficit, or another denominator. A ratio near 100% can conceal bilateral weakness. Mass normalization can also change because weight changes, even when absolute force is stable. Display absolute and normalized results together where the protocol supports both. Never infer recovery solely from a ratio or percentile.

Practical assessment pathway

Before surgery, establish both limbs’ symptoms, prior arthroplasty, expected procedure, relevant comorbidity, baseline function and a tolerable dynamometry protocol. Record the angle the patient can actually reach rather than substituting an intended angle. During inpatient and early outpatient recovery, readiness and the surgical team’s restrictions govern maximal testing. Capture pain, swelling context, analgesia or nerve-block context when known, fatigue, assistance and inability. If full dynamometry is unsuitable, clinical activation tasks can describe performance, but do not convert their ordinal score into an activation percentage. [1–3, 9, 10, 17]

During subsequent outpatient recovery, use the same device, lever arm, fixation, angle, instruction and trial summary. Assess the other limb and relevant hip groups when clinically indicated. At later visits, dynamic power or rapid-force testing can help characterize residual limitations that a maximal value misses. The clinical question should determine whether the additional signal is useful; collecting a sophisticated variable without interpretable error or a consequence for care adds burden without assured value.

The minimum dynamometry record contains device and calibration; muscle action; tested side; surgical side and dates; joint angles; body and limb stabilization; force-contact location; measured moment arm; gravity correction; unit; mass and normalization; practice and scored trials; contraction duration; rest; instruction; feedback; pain and completion status. For power add external load or controlled velocity and range of motion. For RFD/RTD add sampling, filter, baseline and onset rules, analysis interval, pretension policy and trace-quality decisions. For activation add stimulation method and the exact calculation.

A useful dashboard separates raw performance, comparable within-person change, protocol-matched error, any verified important-change evidence, and genuine future-outcome evidence. It should flag a device or protocol change and begin a new series when comparability is lost. It should withhold an individualized forecast when an input is measured after the decision time, when validation was not established, or when the patient lies outside the population. Rehabilitation recommendations then remain grounded in the patient’s limitations and goals rather than an unsupported cutoff.

Table 5 Minimum muscle assessment protocol record

Protocol synthesis from the appraised studies; record changes explicitly rather than assuming longitudinal interchangeability.

Conclusions

Quantitative strength assessment is central to describing recovery after TKA, but numerical precision does not guarantee physiological or prognostic validity. Stabilized dynamometry can be useful while still having clinically important error and poor agreement with another device. Hip strength, activation, dynamic power and rapid force provide complementary information; their protocols, units and evidence must remain distinct. The most defensible clinical strategy combines serial bilateral measurements with function and symptoms, and reserves prediction claims for temporally ordered, adequately validated models. The accompanying tables and study matrix make those boundaries explicit.

Primary study characteristics

Primary studies are grouped by measurement or prognostic question. Population, protocol, endpoint and source access constrain interpretation. The bibliography identifies access limitations. Related publications from one cohort are not independent replications.

Table 6 Strength and activation measurement

Table 7 Power and rapid force production

Table 8 Prognosis and related longitudinal findings

Search appendix

This is an auditable critical narrative assessment and prognosis review, with targeted original-study appraisal, not a formal systematic review. Search date and cutoff: 2 October 2026. No independent duplicate screening or exhaustive eligibility count is claimed. Domain searches overlap, as do the targeted supplements; their totals must not be summed as unique eligible studies. Source matrices prioritize measurement properties, protocol definitions, recovery and prospective outcomes; treatment-only studies and off-population measurement studies can inform discovery without becoming primary evidence.

Principal PubMed search

("Arthroplasty, Replacement, Knee"[MeSH Terms] OR "total knee arthroplast*"[Title/Abstract] OR "total knee replacement*"[Title/Abstract] OR TKA[Title/Abstract]) AND (strength[Title/Abstract] OR dynamomet*[Title/Abstract] OR "muscle power"[Title/Abstract] OR "leg power"[Title/Abstract] OR "force development"[Title/Abstract] OR "torque development"[Title/Abstract] OR "voluntary activation"[Title/Abstract]) AND (reliab*[Title/Abstract] OR valid*[Title/Abstract] OR responsiv*[Title/Abstract] OR "measurement error"[Title/Abstract] OR "minimal detectable"[Title/Abstract] OR "minimal important"[Title/Abstract] OR prognos*[Title/Abstract] OR predict*[Title/Abstract] OR longitudinal[Title/Abstract] OR recovery[Title/Abstract] OR "time course"[Title/Abstract]) AND ("1800/01/01"[Date - Publication] : "2026/10/02"[Date - Publication])

Provider total: 638. Unique connector PMIDs: 434. Official NCBI ESearch identified 638 IDs and EFetch recovered 638 records, with 0 missing from the official set. This reconciliation was necessary because successive connector pages repeated records. A returned final page was not treated as evidence of complete unique coverage.

Scopus search

TITLE-ABS-KEY(("total knee arthroplast*" OR "total knee replacement*" OR TKA) AND (strength OR dynamomet* OR "muscle power" OR "leg power" OR "force development" OR "torque development" OR "voluntary activation") AND (reliab* OR valid* OR responsiv* OR "measurement error" OR "minimal detectable" OR "minimal important" OR prognos* OR predict* OR longitudinal OR recovery OR "time course")) AND PUBYEAR < 2027

Provider total 1031; 42 archived pages; 1031 unique Scopus source IDs. No missing records were apparent from the returned-page unique-count comparison, but this is not an independent database replay. The publication-year filter through 2026 is broader than the October 2 cutoff. Publication timing was checked for retained current sources where necessary; discovery results are not all treated as eligible. Individual Scopus detail access was not presumed from successful search access.

Targeted supplemental PubMed search

("total knee arthroplasty" OR "total knee replacement") AND ("muscle power" OR "rate of force development" OR "rate of torque development" OR "voluntary activation" OR "hip abductor strength") AND ("1800/01/01"[Date - Publication] : "2026/10/02"[Date - Publication])

Two connector pages were archived. Connector unique PMIDs 51 versus official ESearch total 63; official EFetch recovered 63 with no missing IDs. Supplemental records overlap with the principal query. For balance the supplement broadens reactive/perturbation/falls terminology; for strength it broadens power, activation, hip-abduction and rapid-force terminology without a measurement-property filter.

Citation chasing and source expansion

Selection boundaries

Primary elective OA-TKA is the main population. Preoperative OA reliability is labeled separately from postoperative reliability. Simultaneous bilateral, revision, UKA and mixed THA/TKA studies are not silently generalized to unilateral primary TKA. Operative ligament or gap balancing is not postural balance. Cross-sectional or repeated contemporaneous regressions are not future prognosis. Reviews and guidelines are contextual sources, not independent primary-study replications.

Access and numerical verification

The bibliography distinguishes complete articles, partially available tables, abstract-only evidence and unavailable full articles. Original articles were sought through bibliographic databases, official PMC and NCBI records, publishers and public accepted-manuscript repositories. Selected consequential estimates were checked against original tables. This does not constitute independent appraisal of every paper.

Remaining access gaps are material. Abstract-level estimates with unverified stage, positioning, ICC model or trial aggregation stay non-deployable. A complete article body can omit image-only tables or supplements; that limitation is preserved rather than inferred away. All delivered DOI/PMID references were reconciled to verified metadata; unsuccessful retrievals were not treated as source evidence. Publication year follows the journal issue when verified, with online dates retained in archived metadata.

Shared cohorts and interpretation

Chan 2018 responsiveness, Chan 2020 important change and Chan 2018 falls have matching recruitment, hospital and 134-person complete samples, consistent with a shared cohort. They do not provide three independent replications. QAB 2018, Hip 2017 and the gait report's Kittelson four-metre analysis arise from NCT01537328. Cross-report outcomes remain useful but are not independent participant samples. No neurological, healthy-athlete, nonsurgical OA or THA threshold is imported as a postoperative TKA clinical boundary.

References

References are numbered in first citation order. Source descriptions identify the material examined and do not constitute a study quality rating. Links identify the original publication or the named primary source version.

1. Jette, Diane U, Hunter, Stephen J, Burkett, Lynn, Langham, Bud, Logerstedt, David S, Piuzzi, Nicolas S, et al. Physical Therapist Management of Total Knee Arthroplasty. Physical therapy. 2020. DOI 10.1093/ptj/pzaa099 Source examined: Complete guideline; context.

Source note: SRC-27ba0555a48c Jette 2020

2. Mizner RL, Petterson SC, Stevens JE, Vandenborne K, Snyder-Mackler L. Early quadriceps strength loss after total knee arthroplasty. The contributions of muscle atrophy and failure of voluntary muscle activation. The Journal of bone and joint surgery. American volume. 2005;87(5):1047-53. DOI 10.2106/jbjs.d.01992 Source examined: Complete original article.

Source note: SRC-cfe7e22fa152 Mizner RL 2005

3. Kittelson AJ, Christensen JC, Loyd BJ, Burrows KL, Iannitto J, Stevens-Lapsley JE. Reliability, responsiveness, and validity of handheld dynamometry for assessing quadriceps strength in total knee arthroplasty. Disability and rehabilitation. 2021;43(21):3070-3077. DOI 10.1080/09638288.2020.1730454 Source examined: Complete article and tables.

Source note: SRC-f1c1162cce91 Kittelson AJ 2021

4. Winters JD, Christiansen CL, Stevens-Lapsley JE. Preliminary investigation of rate of torque development deficits following total knee arthroplasty. The Knee. 2014;21(2):382-6. DOI 10.1016/j.knee.2013.10.003 Source examined: Complete original article; force-unit onset wording retained.

Source note: SRC-943b62411d16 Winters JD 2014

5. Maffiuletti NA, Bizzini M, Widler K, Munzinger U. Asymmetry in quadriceps rate of force development as a functional outcome measure in TKA. Clinical orthopaedics and related research. 2010;468(1):191-8. DOI 10.1007/s11999-009-0978-4 Source examined: Complete original article and tables.

Source note: SRC-2d3a5edb2520 Maffiuletti NA 2010

6. Kline PW, Jacobs CA, Duncan ST, Noehren B. Rate of torque development is the primary contributor to quadriceps avoidance gait following total knee arthroplasty. Gait & posture. 2019;68:397-402. DOI 10.1016/j.gaitpost.2018.12.019 Source examined: Complete article and tables.

Source note: SRC-b2e50f266f12 Kline PW 2019

7. Marmon AR, Milcarek BI, Snyder-Mackler L. Associations between knee extensor power and functional performance in patients after total knee arthroplasty and normal controls without knee pain. International journal of sports physical therapy. 2014;9(2):168-78. PubMed. Source examined: Complete original article.

Source note: SRC-c9e3fd9287e8 Marmon AR 2014

8. Iwata A, Sano Y, Wanaka H, Kobayashi S, Okamoto K, Yamahara J, et al. Maximum knee extension velocity without external load is a stronger determinant of gait function than quadriceps strength in the early postoperative period following total knee arthroplasty. PloS one. 2022;17(11):e0276219. DOI 10.1371/journal.pone.0276219 Source examined: Complete article and tables.

Source note: SRC-f9a9e87177aa Iwata A 2022

9. Eymir M, Yuksel E, Unver B, Karatosun V. Hand-Held Dynamometry in the Inpatient Care Setting After Total Knee Arthroplasty: Reliability of Static Knee Strength Measurements. American journal of physical medicine & rehabilitation. 2021;100(6):570-575. DOI 10.1097/phm.0000000000001592 Source examined: Abstract verified; full article unavailable.

Source note: SRC-958d628e0b21 Eymir M 2021

10. Koblbauer IF, Lambrecht Y, van der Hulst ML, Neeter C, Engelbert RH, Poolman RW, et al. Reliability of maximal isometric knee strength testing with modified hand-held dynamometry in patients awaiting total knee arthroplasty: useful in research and individual patient settings? A reliability study. BMC musculoskeletal disorders. 2011;12:249. DOI 10.1186/1471-2474-12-249 Source examined: Complete article and tables; preoperative subgroup.

Source note: SRC-ff8074992423 Koblbauer IF 2011

11. Alnahdi AH, Zeni JA, Snyder-Mackler L. Hip abductor strength reliability and association with physical function after unilateral total knee arthroplasty: a cross-sectional study. Physical therapy. 2014;94(8):1154-62. DOI 10.2522/ptj.20130335 Source examined: Complete original article; key ICC/MDC in body; image-only tables not fully re-extracted.

Source note: SRC-03fa9ef960f1 Alnahdi AH 2014

12. Schache MB, McClelland JA, Webster KE. Reliability of measuring hip abductor strength following total knee arthroplasty using a hand-held dynamometer. Disability and rehabilitation. 2016;38(6):597-600. DOI 10.3109/09638288.2015.1046565 Source examined: Abstract verified; full article unavailable.

Source note: SRC-e229a5d7c777 Schache MB 2016

13. Lienhard K, Lauermann SP, Schneider D, Item-Glatthorn JF, Casartelli NC, Maffiuletti NA. Validity and reliability of isometric, isokinetic and isoinertial modalities for the assessment of quadriceps muscle strength in patients with total knee arthroplasty. Journal of electromyography and kinesiology : official journal of the International Society of Electrophysiological Kinesiology. 2013;23(6):1283-8. DOI 10.1016/j.jelekin.2013.09.004 Source examined: Abstract and publisher preview; full article unavailable.

Source note: SRC-427ce84b80b1 Lienhard K 2013

14. Kuwahara K, Makino Y, Kato T, Fukuda A, Asada K. Reliability of a portable device for measuring lower-limb muscle strength in patients who underwent total knee arthroplasty. Journal of physical therapy science. 2025;37(12):613-618. DOI 10.1589/jpts.37.613 Source examined: Complete article and tables.

Source note: SRC-f509bd7a5f14 Kuwahara K 2025

15. Kubo Y, Fujita D, Sugiyama S, Takachu R, Sugiura T, Sawada M, et al. Quadriceps Strength Loss Following Total Knee Arthroplasty as a Predictor of Three-Month Strength Recovery: A Secondary Analysis of a Randomized Controlled Trial. Cureus. 2024;16(8):e68244. DOI 10.7759/cureus.68244 Source examined: Complete article and tables.

Source note: SRC-5b343225dc90 Kubo Y 2024

16. Hiyama Y, Sawa R, Yokoyama M, Ohtera S, Wada O, Mizuno K. Achieving normative quadriceps strength after total knee arthroplasty : associations with satisfaction, function, and a predictive nomogram. The bone & joint journal. 2026;108-B(4):469-477. DOI 10.1302/0301-620x.108b4.bjj-2025-0859.r2 Source examined: Complete publisher PDF; p 473 numerical results visually checked; supplement not retrieved.

Source note: SRC-dfec30e1e735 Hiyama Y 2026

17. Bade M, Struessel T, Paxton R, Winters J, Baym C, Stevens-Lapsley J. Performance on a Clinical Quadriceps Activation Battery Is Related to a Laboratory Measure of Activation and Recovery After Total Knee Arthroplasty. Archives of physical medicine and rehabilitation. 2018;99(1):99-106. DOI 10.1016/j.apmr.2017.07.013 Source examined: Complete article; not all original figures and tables examined.

Source note: SRC-c37e6befbe7d Bade M 2018

18. Loyd BJ, Jennings JM, Judd DL, Kim RH, Wolfe P, Dennis DA, et al. Influence of Hip Abductor Strength on Functional Outcomes Before and After Total Knee Arthroplasty: Post Hoc Analysis of a Randomized Controlled Trial. Physical therapy. 2017;97(9):896-903. DOI 10.1093/ptj/pzx066 Source examined: Complete original article.

Source note: SRC-784a6b2914c9 Loyd BJ 2017

19. Loyd BJ, Stackhouse SK, Hogan C, Dayton MR, Stevens-Lapsley JE, Kittelson AJ. Peripheral Nociception Is Associated with Voluntary Activation Deficits and Quadriceps Weakness Following Total Knee Arthroplasty. The Journal of bone and joint surgery. American volume. 2019;101(17):1539-1545. DOI 10.2106/jbjs.18.01457 Source examined: Abstract verified; full article unavailable.

Source note: SRC-9a3148637b6f Loyd BJ 2019

20. Singla R, Niederer D, Franz A, Happ K, Zilkens C, Wahl P, et al. The course of knee extensor strength after total knee arthroplasty: a systematic review with meta-analysis and -regression. Archives of orthopaedic and trauma surgery. 2023;143(8):5303-5322. DOI 10.1007/s00402-022-04750-5 Source examined: Complete review; context and citation discovery.

Source note: SRC-948789a8e2e9 Singla R 2023

21. Ravi Singla, Daniel Niederer, Alexander Franz, Kevin Happ, Christoph Zilkens, Patrick Wahl, et al. Correction to: The course of knee extensor strength after total knee arthroplasty: a systematic review with meta-analysis and -regression. Archives of Orthopaedic and Trauma Surgery. 2023. DOI 10.1007/s00402-023-04808-y Source examined: Complete correction; study country changed to Türkiye.

Source note: SRC-27ef44ae501e Ravi Singla 2023

22. Zeni JA Jr, Snyder-Mackler L. Early postoperative measures predict 1- and 2-year outcomes after unilateral total knee arthroplasty: importance of contralateral limb strength. Physical therapy. 2010;90(1):43-54. DOI 10.2522/ptj.20090089 Source examined: Complete original article; image-only tables not separately examined.

Source note: SRC-95a9e043b7c1 Zeni JA Jr 2010

23. Mizner RL, Petterson SC, Stevens JE, Axe MJ, Snyder-Mackler L. Preoperative quadriceps strength predicts functional ability one year after total knee arthroplasty. The Journal of rheumatology. 2005;32(8):1533-9. PubMed. Source examined: Complete original article; protocol and model details checked.

Source note: SRC-2fcb591c8c3e Mizner RL 2005

24. Kitamura G, Nankaku M, Yuri T, Kuriyama S, Nakamura S, Nishitani K, et al. Predictors for the Knee Extension Strength at 2 Yrs After Total Knee Arthroplasty Using Regression Tree Analysis. American journal of physical medicine & rehabilitation. 2024;103(6):518-524. DOI 10.1097/phm.0000000000002398 Source examined: Abstract verified; full article unavailable.

Source note: SRC-d2c3768bb209 Kitamura G 2024

25. Kamitani T, Wada O, Mizuno K, Kurita N. Preoperative muscle health can predict distinct recovery patterns of patient-reported outcomes during 1 year after knee arthroplasty. Clinical nutrition ESPEN. 2026;74:103334. DOI 10.1016/j.clnesp.2026.103334 Source examined: Official abstract verified; mixed TKA/UKA cohort; full article unavailable.

Source note: SRC-14089cce8e1b Kamitani T 2026