Single leg vertical jump performance identifies knee function deficits at return to sport after ACL reconstruction in male athletes

Key points

1. 01Athletes achieved 97% symmetry in horizontal hop distance but only 83% symmetry in single-leg vertical jump height and 77% symmetry in single-leg drop jump height at return to sport.
2. 02Knee work generation symmetry was even lower: 75% during the single-leg vertical jump and 70% during the single-leg drop jump.
3. 03The involved knee produced significantly less work and had lower flexion moments than the uninvolved limb and healthy controls across both jump tasks.
4. 04Athletes compensated by shifting load to the hip and using greater hamstrings co-contraction in the involved limb, a pattern linked to elevated reinjury risk.
5. 05Jump height and Reactive Strength Index can be measured with low-cost tools (contact mats, phone apps) and should replace hop distance as primary return-to-sport metrics.

How it was conducted

Design

Cross-sectional biomechanical study with between-limb and between-group comparisons

Participants

26 male athletes cleared for return to sport after primary ACL reconstruction and 22 healthy male controls; all tested within 2 weeks of clearance at a mean 9.5 +/- 2.7 months post-surgery

Tasks

Single-leg vertical jump (SLJ) and single-leg drop jump (SLDJ) from a 15 cm step, assessed bilaterally

Primary outcomes

Jump height, Reactive Strength Index (RSI), sagittal plane kinematics, joint moments, joint work, and individual muscle force contributions via EMG-constrained musculoskeletal modelling

Return-to-sport criteria

Surgeon and physiotherapist clearance, quadriceps strength LSI >90%, and hop battery LSI >90%

Analysis

Mixed-effects models adjusted for age, Tegner score, and BMI; effect sizes calculated with Cohen's d

What they found

• Jump height (SLJ): involved limb 11.0 +/- 3.5 cm vs uninvolved 13.3 +/- 3.3 cm vs controls 13.7 +/- 2.1 cm; involved vs uninvolved p<0.001, d=0.67; involved vs controls p=0.016, d=0.87.
• Jump height LSI: 83% during SLJ and 77% during SLDJ in the ACLR group; controls showed 98% and 100% symmetry respectively.
• SLDJ jump height: involved 10.6 +/- 3.4 cm vs uninvolved 13.8 +/- 3.5 cm vs controls 14.4 +/- 3.1 cm; involved vs uninvolved p<0.001, d=0.92; involved vs controls p=0.001, d=1.12.
• RSI (SLDJ): involved 0.30 +/- 0.10 cm/s vs uninvolved 0.43 +/- 0.12 cm/s vs controls 0.43 +/- 0.12 cm/s; involved vs uninvolved p<0.001, d=1.18; involved vs controls p=0.001, d=1.16.
• Horizontal hop distance LSI: 97 +/- 4% in ACLR group vs 100 +/- 5% in controls (p=0.011).
• Knee work generation (SLJ propulsion): involved 1.2 +/- 0.4 J/kg vs uninvolved 1.6 +/- 0.3 J/kg vs controls 1.5 +/- 0.4 J/kg; involved vs uninvolved p<0.001, d=1.19; involved vs controls p=0.003, d=0.91.
• Knee work generation (SLDJ): involved 1.0 +/- 0.3 J/kg vs uninvolved 1.5 +/- 0.3 J/kg vs controls 1.3 +/- 0.3 J/kg; involved vs uninvolved p<0.001, d=1.54; involved vs controls p=0.002, d=1.05.
• Knee work generation LSI: 75% during SLJ and 70% during SLDJ in the ACLR group; controls showed 100% and 99% respectively.
• Knee flexion moment (SLJ propulsion): involved vs uninvolved p<0.001, d=1.23; knee flexion moment (SLJ landing): p<0.001, d=0.85; knee flexion moment (SLDJ): p<0.001, d=1.44.
• Hamstrings contribution was significantly greater in the involved limb than the uninvolved and controls during both jump tasks; soleus contribution was bilaterally lower in the ACLR group compared with controls.

Limitations

• All participants were male athletes from a single centre; findings cannot be generalised to female athletes or other populations.
• Small sample size (26 ACLR, 22 controls) prevented subgroup analysis by graft type.
• Cross-sectional design provides only a single time-point snapshot; no longitudinal data on reinjury rates or later performance.
• Musculoskeletal model used a generic rather than subject-specific anatomy, and the foot was modelled as a rigid segment which may have overestimated ankle work calculations.

Why it matters

For patients

If you have been cleared to return to sport after ACL reconstruction, standard hop tests may overestimate your recovery; asking your physiotherapist to also check how high you can jump on one leg gives a more accurate picture of whether your knee is truly ready.

For clinicians

Single-leg vertical jump height and RSI should be added to return-to-sport test batteries because they expose knee work deficits and load-shifting compensations that hop distance symmetry alone misses, and they are measurable with inexpensive equipment.

For readers

This study demonstrates that the 90% hop-distance threshold commonly used to clear athletes after ACL reconstruction is an inadequate proxy for knee function, and that vertical jump metrics with large effect sizes provide a more sensitive and clinically accessible alternative.