Repetition Maximum Testing: Single Leg (SL) Leg Curl (Flexion)

The Single-Leg Leg Curl Repetition Maximum Test assesses machine-based knee-flexion strength on one leg at a time. It is useful for tracking hamstring strength, comparing sides and monitoring progress across lower-limb training programmes.

Introduction

A field sport athlete may show a clear difference between hamstring strength on the left and right. A gym client may be returning to hamstring loading after a period of reduced training. The Single-Leg Leg Curl Repetition Maximum Test provides a controlled way to assess knee-flexion strength using a standardised machine setup.

The result is useful for strength tracking, but it should not be interpreted as a complete measure of sprint performance, hamstring injury risk or return-to-sport readiness.

Quick Summary

Test name: Single-Leg Leg Curl Repetition Maximum Test
Purpose: Assess single-leg knee-flexion strength
What it assesses: Machine-based hamstring knee-flexion performance
Equipment: Leg curl machine, load stack or plates, Measurz for recording
Key finding: Maximum load completed for the target repetition standard
Best used with: Hamstring bridge endurance, deadlift, sprint testing, hop tests and lower-limb strength tests
Key limitation: It measures machine-based knee flexion, not full hamstring function

What Is the Single-Leg Leg Curl Repetition Maximum Test?

The Single-Leg Leg Curl Repetition Maximum Test measures the maximum load a client can move through a defined knee-flexion range for a target number of repetitions on one leg.

Depending on the protocol, this may be a 1RM, 3RM, 5RM or another repetition maximum. The repetition target should be clearly recorded.

Why It Is Used

This test is used to assess unilateral hamstring knee-flexion strength, compare sides and track strength changes over time.

It may be useful for athletes, gym clients and lower-limb strength programmes where hamstring strength is a key performance or monitoring variable.

What It Measures

The test measures single-leg leg curl performance under the tested machine setup.

It may reflect:

Hamstring knee-flexion strength
Side-to-side strength difference
Load tolerance
Control through the selected ROM
Pain or symptom response
Strength progress across training blocks

It does not measure hip-extension strength, sprint performance, hamstring fascicle length, eccentric capacity or injury risk on its own.

Who It Is Useful For

This test may be useful for field sport athletes, sprinters, gym clients, lower-limb strength clients and anyone completing hamstring-focused training.

It may not be appropriate when machine setup causes pain, the client cannot isolate the limb safely, or the test cannot be performed through a repeatable ROM.

Equipment Required

Single-leg leg curl machine
Load stack or plates
Adjustable seat/bench and lever arm
Measurz for recording
Optional metronome
Optional video

Step-by-Step Protocol

1. Set the machine

Adjust the seat, hip position, knee axis and lever pad. Record these settings so the setup can be repeated.

2. Define the repetition target

Decide whether the test is 1RM, 3RM, 5RM or another RM. Use the same target for retesting.

3. Define valid ROM

Set a clear start and finish range. A valid rep should use the same ROM each time without hip lift or trunk compensation.

4. Warm up

Complete light warm-up sets on each side. Increase load gradually before testing.

5. Test one side at a time

Choose the testing order and keep it consistent. Allow adequate rest between attempts and sides.

6. Record the maximum successful load

The score is the heaviest load completed for the target rep range with acceptable technique and ROM.

Scoring and Interpretation

Record the load, reps and side tested. If using an estimated 1RM equation from a submaximal test, clearly label the result as estimated rather than directly measured.

Interpretation should include absolute load, side-to-side comparison, symptoms, ROM, machine setup and previous baseline.

A lower score on one side may suggest reduced machine-based knee-flexion strength under the tested setup, but it does not identify the cause.

Normative Data, Benchmarks or Reference Values

No universal normative value should be applied across machines and populations. Leg curl results vary by machine design, lever arm, body position, ROM and training history.

Use baseline comparison, limb symmetry and progress across sessions as the primary benchmarks.

Reliability and Validity

Repetition maximum testing can be reliable when setup, ROM, repetition criteria, familiarisation and loading progression are standardised. Broader 1RM evidence supports high reliability for maximal strength testing under consistent procedures.  

For machine-based unilateral tests, reliability depends heavily on identical machine settings and ROM standards.

Common Errors and Testing Limitations

Common errors include changing seat settings, using inconsistent ROM, lifting the hips, rotating the pelvis, using momentum, changing tempo, testing one side while fatigued and comparing results across different machines.

The main limitation is that the test measures machine-based knee-flexion strength only.

Practical Applications

Use this test to monitor hamstring strength, compare sides, guide training loads and track response to hamstring strengthening programmes.

It is most useful when paired with hip-extension strength, sprint exposure, bridge testing, jump/hop tests and symptom monitoring.

How to Record This in Measurz

Record side tested, machine type, seat setting, lever setting, pad position, ROM, load, reps, RM target, RPE, pain score, symptom location, compensation notes, comparison side and retest date.

Useful notes include hip lift, shortened ROM, cramping, pain-limited rep, momentum, poor eccentric control or side-to-side difference.

Related Tests or Internal Linking Suggestions

Hamstring Bridge Endurance Test
Deadlift 1RM Test
Single-Leg Leg Press Test
Sprint Test
Triple Hop Test
90/90 Active Knee Extension Test

FAQs

What does the Single-Leg Leg Curl Test measure?

It measures single-leg machine-based knee-flexion strength.

Is this a hamstring strength test?

It can provide useful hamstring knee-flexion strength information, but it does not measure all hamstring functions.

Should left and right sides be compared?

Yes. Side-to-side comparison is one of the most useful parts of this test.

Can results be compared across different machines?

Only cautiously. Machine design and setup can change the score.

Should pain be recorded?

Yes. Pain score, symptom location and ROM should be recorded.

Key Takeaways

The test measures unilateral leg curl performance.
Machine setup and ROM must be repeated.
Do not treat the result as a complete hamstring profile.
Record load, reps, side and symptoms in Measurz.
Use with sprint, bridge and lower-limb strength tests.

References

Grgic, J., Lazinica, B., Schoenfeld, B. J., & Pedisic, Z. (2020). Test-retest reliability of the one-repetition maximum strength assessment: A systematic review. Sports Medicine - Open, 6, 31.

Kellis, E., Galanis, N., & Natsis, K. (2021). Hamstring muscle activation during strength exercises: Implications for exercise selection. Journal of Functional Morphology and Kinesiology, 6(4), 83.

Suchomel, T. J., Nimphius, S., Bellon, C. R., & Stone, M. H. (2018). The importance of muscular strength: Training considerations. Sports Medicine, 48(4), 765–785.