Which muscle strength tests for knee extensors and flexors should we use after an ACL or meniscal injury?

Keywords: Anterior cruciate ligament, Muscle function, Psychometric properties

Anterior Cruciate Ligament (ACL) tears and meniscal injuries lead to knee extensor and flexor muscle weakness. If these strength deficits persist, they increase the risk of re-injury and/or early development of knee osteoarthritis. For this reason, it is important to monitor muscle strength after these injuries. Currently, there are no recommendations for what is the best way to do this. To explore this topic, the international OPTIKNEE team synthesized the current evidence and conducted a consensus exercise. As part of this process, we conducted a systematic review that assessed the measurement properties of existing muscle strength tests that are used with persons that have an ACL or meniscal injury.1

Why is this study important?

There are many ways to test knee extensor and flexor strength after an ACL or meniscal injury. The ‘best’ choice depends on type or mode of test (e.g., isometric, isotonic, isokinetic), available equipment (e.g., dynamometer or free weights), outcome being measured (e.g., Newtons, Newton-meter, Newton-meter/kilogram), and  measurement properties – the one that gives the most precise and accurate results. To assist with choosing tests our systematic review assessed the measurement properties of existing strength test to provide the highest level of evidence.

What did we do?

We searched four databases for studies that evaluated measurement properties of knee extensor and flexor strength tests in people with ACL and/or meniscal injuries with a mean injury age of ≤30 years. The measurement properties we were interest in included: reliability (i.e., produces the same every time and across testers), measurement error (i.e., error not attributed to true change), validity (i.e., measures what it intends to), and responsiveness (i.e., detects change over time).2

We followed guidelines developed by the international COSMIN initiative3 to assess the risk of bias of each study that assessed at least one measurement property for a knee extensor and flexor strength test. For each measurement property of each test, we synthesised values, graded the evidence, and reached a conclusion (including evidence level).4


What did we find?

Across the 36 studies included in our review we found a large variability in the type or mode of strength test, equipment used, and outcome (e.g., Newtons, Newton-meters, Newton-meters/kilogram) reported. High-quality studies were scarce. Table 1 summarises the measurement properties per strength test category.

  • Concentric isokinetic tests (computerised dynamometry) of the knee extensors and flexors showed good reliability across testing sessions and good construct validity (i.e., strongly or moderately correlated to hop performance and patient-reported knee function, respectively).
  • Concentric isotonic one-repetition maximum (1RM) tests in a seated knee extension or prone knee curl machine showed good criterion validity (i.e., strongly correlated to the gold standard of isokinetic computerised dynamometry). 
  • Isometric knee extensor strength tests (handheld dynamometry) showed good reliability when an experienced rater assessed consecutive contractions in a standardized setting, but had poor criterion validity (i.e., weak correlation to the gold standard of isokinetic computerised dynamometry).

 Table 1. Measurement properties per strength test category

Strength tests Instrument Reliability Measurement error Validity
Intrarater Interrater Criterion Construct
Isokinetic concentric extension Computerised dynamometry good NA 10.5% LSI§ good good
Isokinetic concentric flexion Computerised dynamometry good NA 3.4%* NA poor
Isokinetic eccentric extension Computerised dynamometry NA NA NA NA good
Isokinetic eccentric flexion Computerised dynamometry NA NA NA NA good
Isotonic extension Leg extension NA NA NA good NA
Isotonic extension Leg press NA NA NA poor poor
Isotonic flexion, prone Leg curl NA NA NA good NA
Isometric extension  Computerised dynamometry NA NA NA NA good
Isometric extensor  Handheld dynamometry good poor 1.7% PT/BW§ poor poor
Isometric flexion Computerised dynamometry NA NA NA NA poor
Isometric flexion, prone Handheld dynamometry NA NA NA NA poor

LSI = limb symmetry index; NA = not available; PT/BW = peak torque per body weight.

§Smallest detectable change. *Coefficient of variation (CV = (standard deviation/mean)*100).


What are the key take-home messages?

To test muscle strength for knee extensors and flexors after ACL injuries, we recommend:

1 – Isokinetic concentric tests if a computerised dynamometer is available. Although functional tests such as the hop test may assess similar constructs, they should not be used interchangeably but instead alongside the isokinetic tests to evaluate knee function.5, 6

2 – Isotonic 1RM tests using conventional weight machines (such as leg extension or leg curl) if computerised dynamometry is not available in a clinical setting.7

3 – Isometric tests using handheld dynamometry with standardised procedures and a single rater if a computerised dynamometer or conventional weight machines are not available (particularly if the patient is young with high muscle strength).8 

Unfortunately, we are unable to recommend strength tests for people with isolated meniscal injury because there is a lack of evidence.


Authors and Affiliations:

Anouk P. Urhausen (1), Bjørnar Berg (2), Britt Elin Øiestad (2&3), Jackie Whittaker (4&5), Alison Hoens (4&5), May Arna Risberg (1&6)

Department of Sports Medicine, Norwegian School of Sport Sciences, Oslo, Norway (1)

Centre for Intelligent Musculoskeletal Health, Faculty of Health Sciences, Oslo Metropolitan University, Oslo, Norway (2)

Department of Rehabilitation Science and Health Technology, Oslo Metropolitan

University, Oslo, Norway (3)

Department of Physical Therapy, Faculty of Medicine, University of British Columbia, Vancouver, British Columbia, Canada (4)

Arthritis Research Canada, Vancouver, British Columbia, Canada (5)

Division of Orthopaedic Surgery, Oslo University Hospital, Oslo, Norway (6)




1 Urhausen AP, Berg B, Øiestad BE, et al. Measurement properties for muscle strength tests following anterior cruciate ligament and/or meniscus injury: What tests to use and where do we need to go? A systematic review with meta-analyses for the OPTIKNEE consensus. British Journal of Sports Medicine 2022:bjsports-2022-105498 doi:10.1136/bjsports-2022-105498 [published Online First: 

2 Mokkink LB, Terwee CB, Patrick DL, et al. The COSMIN study reached international consensus on taxonomy, terminology, and definitions of measurement properties for health-related patient-reported outcomes. J Clin Epidemiol 2010;63:737-45 doi:10.1016/j.jclinepi.2010.02.006 [published Online First: 2010/05/25]

3 Mokkink LB, Terwee CB, Patrick DL, et al. The COSMIN checklist for assessing the methodological quality of studies on measurement properties of health status measurement instruments: an international Delphi study. Qual Life Res 2010;19:539-49 doi:10.1007/s11136-010-9606-8 [published Online First: 2010/02/20]

4 Buttner F, Winters M, Delahunt E, et al. Identifying the ‘incredible’! Part 1: assessing the risk of bias in outcomes included in systematic reviews. Br J Sports Med 2020;54:798-800 doi:10.1136/bjsports-2019-100806 [published Online First: 2019/12/25]

5 Wisloff U, Castagna C, Helgerud J, et al. Strong correlation of maximal squat strength with sprint performance and vertical jump height in elite soccer players. Br J Sports Med 2004;38:285-8 doi:10.1136/bjsm.2002.002071 [published Online First: 2004/05/25]

6 Berg B, Urhausen AP, Øiestad BE, et al. What tests should be used to assess functional performance in youth and young adults following anterior cruciate ligament or meniscal injury? A systematic review of measurement properties for the OPTIKNEE consensus. Br J Sports Med 2022 doi:10.1136/bjsports-2022-105510 [published Online First: 2022/06/14]

7 Pua YH, Ho JY, Chan SA, et al. Associations of isokinetic and isotonic knee strength with knee function and activity level after anterior cruciate ligament reconstruction: a prospective cohort study. Knee 2017;24:1067-74 doi:10.1016/j.knee.2017.06.014 [published Online First: 2017/07/26]

8 Hirano M, Katoh M, Gomi M, et al. Validity and reliability of isometric knee extension muscle strength measurements using a belt-stabilized hand-held dynamometer: a comparison with the measurement using an isokinetic dynamometer in a sitting posture. J Phys Ther Sci 2020;32:120-24 doi:10.1589/jpts.32.120 [published Online First: 2020/03/12]

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