Reactive strength index and knee extension strength characteristics are predictive of single-leg hop performance after anterior cruciate ligament reconstruction

Key points

1. 01Jump height, peak knee extension torque, and RTD 200-200 ms explained 60.9% of variance in single-leg hop distance (p < 0.001)
2. 02RSI, peak torque, RTD 200, and RTD 100 explained 61.8% of variance in triple hop distance (p < 0.001)
3. 03RSI correlated strongly with triple hop (r = 0.689) and single hop (r = 0.613), but only predicted triple hop in regression models
4. 04Amortization time was not significantly correlated with any strength or hop variable and did not predict hop performance
5. 05Single hop and triple hop provide distinct information: single hop reflects maximal strength; triple hop reflects reactive and rate-based force production

How it was conducted

Design

Cross-sectional descriptive laboratory study with multivariable linear regression

Participants

52 individuals aged 18-36 with unilateral ACLR (35 female, 17 male); mean 37.6 months post-surgery

Plyometric assessment

Single-leg drop vertical jump (SLDVJ) using 10-camera 3D motion capture and embedded force plate to measure RSI, amortization time, and jump height

Strength assessment

Isometric knee extension MVIC torque via Biodex dynamometer; peak torque, average RTD, RTD 0-100 ms, and RTD 100-200 ms

Hop tests

Single-leg hop for distance and triple hop for distance on the ACLR limb; distances normalized to leg length

Primary outcome

Proportion of variance in normalized hop distance explained by strength and plyometric predictors

What they found

• Jump height, peak torque, and RTD 200 explained 60.9% of variance in single-leg hop distance (R2 = 0.609, p < 0.001)
• RSI, peak torque, RTD 200, and RTD 100 explained 61.8% of variance in triple hop distance (R2 = 0.618, p < 0.001)
• RSI correlated strongly with jump height (r = 0.928, p < 0.010), triple hop (r = 0.689, p < 0.010), single-leg hop (r = 0.613, p < 0.010), and peak torque (r = 0.609, p < 0.010)
• Jump height correlated strongly with triple hop (r = 0.803, p < 0.010) and single hop (r = 0.711, p < 0.010)
• RTD 200 showed a moderate positive correlation with RSI (r = 0.473, p < 0.010) and jump height (r = 0.473, p < 0.010)
• Amortization time was not significantly correlated with any strength or hop variable (all p > 0.05)
• Mean single-leg hop distance was 141.1 +/- 35.4 cm; mean triple hop distance was 456.1 +/- 107.9 cm

Limitations

• No sex-based subgroup analyses were performed, despite known sex differences in landing kinematics after ACLR
• Wide range in time since surgery (mean 37.6 +/- 23.7 months) may have introduced heterogeneity in strength and hop outcomes
• RSI was measured during a separate drop jump task, not during the triple hop itself, limiting direct mechanistic inference
• Sample drawn from a single Midwestern university and associated clinic, limiting generalizability

Why it matters

For patients

Patients recovering from ACL reconstruction should work on both maximal strength and explosive reactive strength, as different hop tests reflect different aspects of recovery readiness.

For clinicians

Using both the single-leg hop and triple hop provides complementary information: the single hop captures quadriceps strength, while the triple hop captures rate of force development and reactive strength, which may be important targets for late-stage rehabilitation.

For readers

This study shows that passing a single hop distance threshold alone does not capture the full picture of functional recovery; plyometric capacity measured by RSI adds predictive value for more demanding hop tasks.