Luke and Andrew break down the key things you need to know when assessing the knee including:
- Why knee extensor strength is so important and what happens when you don't have it?
- What are some important strength tests for assessing strength around the knee?
- What numbers are normal for knee strength?
- Ways in which you can improve knee strength with exercises.
Reducing Injury Risk in Athletes
Common Metrics and Usage
Athletes are often screened using force-plate jump tests (e.g. vertical countermovement jumps) and isometric strength tests to identify imbalances or deficits in how they produce or absorb force that might predispose to injury. Key metrics include vertical jump height, ground reaction force asymmetries on take-off or landing, and explosive force measures (e.g. rate of force development). On dynamometers, peak force of the quadriceps and hamstrings and their ratio (H:Q) are commonly measured. The H:Q ratio has been theorized to indicate muscle imbalances (low values meaning relatively weak hamstrings) that could elevate risk of hamstring strain or ACL injury, especially in sports like soccer and rugby.1,2 Force-plate tests like the drop vertical jump are also used to observe landing mechanics, as poor landing technique (e.g. high impact forces or asymmetric loading) has been linked to knee injury risk.1
- Hamstring:Quadriceps (H:Q) Ratio
Kellis et al.1 (2023) conducted a systematic review on H:Q ratios as a risk factor and found little consistent evidence that it independently predicts knee or hamstring injuries. A few studies did find extremely low H:Q ratios associated with hamstring strain risk—for example, one reported players with H:Q < 0.50 had about 3× higher odds of hamstring injury (OR ≈ 3.14).2 However, overall the review concluded that H:Q ratio is not a reliable standalone predictor for ACL or hamstring injuries and should be considered amongst other testing.1 - Force-Plate Jump Tests
Force plates can detect subtle deficits in how an athlete jumps or lands, potentially indicating elevated injury risk, but recent human studies show mixed results. In a Division I athlete cohort, Padua et al.3 (2021) found that certain vertical jump kinetics modestly predicted ACL injury risk within ~2 months. Athletes who went on to tear their ACL showed significantly lower force production scores (concentric force output and impulse) and an altered eccentric loading profiles.3 A logistic model using those force-plate variables significantly distinguished injured vs. uninjured athletes (p=0.03). Another prospective study in elite ballet dancers reported that greater inter-limb asymmetry during jump take-off and landing correlated with higher injury incidence.4
In summary, vertical jump testing is widely used and can reveal deficits (like low explosive power or asymmetry) that logically relate to knee stability, but evidence for direct injury prediction is limited.3,5,6 It appears more useful as one component of a multifactor screening battery rather than a definitive crystal ball for injury.
Improving Return-to-Sport (RTS)
Common Metrics and Criteria
After knee injury or surgery (especially ACL reconstruction), clinicians routinely track quadriceps and hamstring strength (often via isometric or isokinetic dynamometry) and functional jump/hop performance (often via force plates). Key RTS criteria typically include a Limb Symmetry Index (LSI) ≥ 90% for quadriceps peak torque and hop distances—i.e., the injured limb is at least 90% as strong/powerful as the uninjured side.7 Force-plate testing can provide finer detail (e.g., measuring asymmetries in ground reaction force, jump height, or time to peak force between legs). Rate of force development (RFD) of the quadriceps is gaining attention as well, since athletes may regain maximal strength but still have slower explosive force—a possible gap in recovery.8
- Strength Symmetry and Reinjury Risk
Many protocols insist on ≥90% quad strength symmetry before clearance, and this was the most common criterion in a 2019 review of RTS decision-making.7 However, simply hitting that threshold does not guarantee success. Losciale et al. (2019)8 found that athletes who passed RTS test batteries (strength and hop LSIs ≥90%) had a 14% second ACL injury rate vs 20% in those who failed—a nonsignificant difference (risk difference ~3%, p>0.05). Meanwhile, Wellsandt et al. (2022)9 showed that athletes with high quad symmetry as early as 6 months post-op were actually more likely to suffer a second ACL rupture, especially if they returned to sport before 8 months. Every 1% increase in quadriceps LSI at 6 months raised reinjury risk by ~2% in these early-return athletes.9 This paradox may reflect how symmetry is sometimes achieved—not by fully restoring strength to the injured leg, but by detraining of the uninjured side or compensatory strategies. Clinicians now caution that LSI ≥90% is necessary but not sufficient—it must be interpreted with healing time and absolute force values, not in isolation, a possible highlight of the importance of having comparable normative data for the those being tested.7,9 - Quadriceps Strength and Function
Robust evidence links stronger quadriceps with better knee function after injury. A systematic meta-analysis showed that even years post-ACL reconstruction, the reconstructed knees often exhibit a persistent quad strength deficit compared to healthy controls (standardized mean difference >0.8, a large effect).10 These deficits correlate with poorer self-reported function and lower hop performance. Ithurburn et al. (2019)11 reported that quadriceps strength index at ~6 months explained ~45–50% of the variance in hop distance and certain gait measures at 1–2 years. Clinically, this means someone with weak quads is likely to have difficulty with dynamic tasks (stairs, jumping, cutting), underscoring the importance of restoring peak torque. Additionally, better quad strength is associated with higher activity levels and less knee instability, while large strength deficits align with subjective knee “giving way” and inferior knee scores.10,11 - Rate of Force Development (RFD) and Power
A growing body of research advocates measuring explosive strength (not just max strength) during rehab. Buckthorpe & Roi (2018)12 observed that athletes can achieve normal peak torque but still have significant deficits in RFD at the initiation of contraction. One longitudinal study found RFD still ~26–28% lower on the surgical leg at RTS, even when max strength was near-normal.8 These deficits matter functionally—early-phase RFD at 5 months explained ~25% of the variance in single-leg hop distance at 9 months, independently of max strength.11 In practical terms, an athlete who can eventually exert 300 N of force may still be disadvantaged if it takes too long to reach that force. RFD also correlates strongly with patient-reported knee function. Clinicians are thus advised to include explosive strength tests (e.g., a rapid isometric contraction test) as an adjunct criterion for RTS.12 - Force-Plate Jump/Hop Tests in RTS
Besides lab strength tests, functional hop tests measured via force plates are common. They quantify not just distance but also landing forces, side-to-side asymmetry, and jump height more precisely than tape-measure methods. A 2022 meta-analysis of force-plate jump studies (33 studies, >1100 ACL patients) found consistent deficits in jump performance post-ACL.10 On average, at 6–12+ months post-surgery, athletes jump about 3–4 cm lower than healthy peers, with ~8–15% higher asymmetry. Jump height (or concentric impulse) was the most commonly reported, discriminative metric. Athletes may offload the surgical limb during take-off or landing, producing subtle asymmetries that standard hop tests might miss.10 Although direct correlation with reinjury risk remains uncertain, many clinics integrate these jump tests into RTS exams to detect persistent compensations and refine rehab if asymmetries persist.
Notable Meta-analyses: A 2022 synthesis by Webster & Hewett concluded that no single strength or hop measure perfectly predicted who would succeed or reinjure. Instead, a combination of factors (including psychological readiness) plus longer recovery times (>9 months) generally led to lower reinjury risk. This supports combining robust strength, power, and timeline considerations for RTS decisions.
Long-Term Knee Health
Focus
In older adults and those with chronic knee conditions (e.g., osteoarthritis), the emphasis shifts to preserving joint function, reducing pain, and preventing disability. Muscle strength and power around the knee are critical: they influence shock absorption, joint loading, balance, and mobility. Force plates can be used to measure gait, static balance or sit-to-stand kinetics (asymmetries, push-off forces), while isometric dynamometers track peak force or H:Q ratios in a clinical setting.
- Strength/Power vs. Functional Decline
It is well-established that quadriceps strength declines with age and more so in knee OA, and this weakness is linked to worse outcomes. Older adults with knee OA typically have 20–40% lower knee extensor torque than age-matched healthy peers.2 Low quadriceps strength is associated with slower gait and higher disability risk. Muscle contraction RFD declines even faster—making it a better predictor of functional decline in older adults. Studies show that seniors with low knee extensor power are much more likely to develop mobility limitations over time. Encouragingly, power training (fast, moderate-load resistance exercises) appears to yield better improvements in functional tasks (e.g., chair-rise speed) than slow-velocity strength training alone.10,12 - Correlation With Outcomes
Strong evidence links better knee strength/power to favorable outcomes:- Falls and Instability: Knee weakness contributes to balance deficits and falls in older adults with knee OA. One prospective study reported that stronger knee extensors/flexors significantly reduced the risk of falls over 6 months.9
- Osteoarthritis Progression: Very high quadriceps strength in a malaligned knee can theoretically accelerate joint wear, but in general populations, more strength tends to protect function without necessarily altering X-ray progression. However, a 2018 study showed that quadriceps RFD—the ability to generate force quickly—was a stronger predictor of functional independence in knee OA than maximal torque.2
- Healthy Aging: Knee extensor power correlates with the ability to live independently in seniors. Every standard deviation increase in knee extension power has been linked to ~30% lower risk of mobility disability.12
- Interventions and Guidelines
Based on this evidence, clinical guidelines for knee OA strongly recommend exercise therapy focusing on progressive strength and power training. Some physical therapy clinics use force plates for biofeedback (e.g., balance training, guiding symmetrical sit-to-stand), whereas isometric dynamometry helps quantify knee extensor force changes over time. For healthy aging, senior fitness programs now increasingly incorporate explosive movement drills (e.g., fast chair rises, low box step-ups) to preserve power and reduce functional decline.
Practical Recommendations
Force-plate and isometric dynamometer metrics are well-established indicators of knee function correlating with injury risk, rehab success, and long-term joint health. The most validated metrics remain straightforward (peak force, jump height), but advanced measures (e.g., RFD, impulse metrics) are gaining traction. Clinicians should employ these tools to uncover deficits and guide interventions, recognizing that no single measure is foolproof. A multi-faceted, data-driven approach—encompassing strength, power, movement quality, and adequate healing time—offers the best path to robust knee health for athletes and older adults alike. Key considerations should include:
- Sports Settings: Practitioners should use a test battery (e.g., bilateral countermovement jump on force plates, single-leg hop and landing, dynamometer H:Q ratio) rather than relying on one metric.1
- Rehab: Set ≥90% symmetry as a baseline but combine it with adequate recovery time (≥8–9 months) and also track explosive strength (RTD). Force plates help detect subtle offloading or slower force generation in the post-op limb.8,9
- Older Adults / OA: Incorporate periodic tests of knee extensor torque and functional power (e.g., sit-to-stand, knee extension dynamometry) to guide exercise prescriptions. Handheld dynamometers and single or dual force platforms can give objective feedback that fosters more targeted interventions and motivation.2,10