Compensatory strategies that reduce knee extensor demand during a bilateral squat change from 3 to 5 months following anterior cruciate ligament reconstruction

Key points

1. 01Knee extensor moment deficits in the surgical limb averaged 38% at 3 months and approximately 30% at 5 months, but this improvement was not statistically significant
2. 02At 3 months, both inter-limb (weight shift to non-surgical leg) and intra-limb (hip-dominant loading) compensations together explained 85% of the variance in knee extensor moment deficits
3. 03At 5 months, only the intra-limb hip-dominant strategy predicted knee extensor moment deficits, accounting for 58% of variance
4. 04The vertical ground reaction force was on average 13% lower under the surgical limb, and the hip-to-knee extensor moment ratio was 40% larger in the surgical limb compared to the non-surgical limb
5. 05Differences in knee angle between limbs were small (about 2.4 degrees), making clinical detection challenging

How it was conducted

Design

Controlled laboratory study, longitudinal observational

Participants

11 individuals post-ACLr (7 females, 4 males; mean age 22.9 years; mean weight 67.5 kg)

Time points

3 months (T3) and 5 months (T5) post-surgery

Task

Bilateral bodyweight squat to self-selected depth, each foot on a separate force platform

Instrumentation

11-camera 3D motion capture at 250 Hz plus dual force platforms at 1500 Hz

Primary analysis

2-way repeated measures MANOVA (limb x time) and stepwise linear regression of inter- and intra-limb compensations on knee extensor moment ratio

What they found

• Surgical limb showed significantly less peak knee extensor moment compared to non-surgical limb (0.367 plus or minus 0.18 Nm/kg; P < 0.001; effect size 2.16)
• Surgical limb showed significantly less peak knee flexion angle (2.4 plus or minus 2.6 degrees; P = 0.013; effect size 0.95)
• Surgical limb showed significantly less peak vertical ground reaction force (0.090 plus or minus 0.06 N/kg; P < 0.001; effect size 1.69)
• Surgical limb hip-to-knee extensor moment ratio was significantly greater than non-surgical limb (0.677 plus or minus 0.55; P = 0.002; effect size 1.28)
• At 3 months, vGRF ratio and hip/knee ratio together predicted knee extensor moment ratio (R2 = 0.854, P < 0.001); vGRF ratio alone explained R2 = 0.624 (P = 0.004, Beta = 0.790), hip/knee ratio added R2 = 0.230 (P = 0.007, Beta = -0.492)
• At 5 months, only hip/knee ratio predicted knee extensor moment ratio (R2 = 0.584, P = 0.006, Beta = -0.765)
• No significant limb-by-time interaction was observed (main effect of limb: F = 6.59, P = 0.028), indicating deficits did not significantly change between 3 and 5 months

Limitations

• Very small sample size (n = 11) limits generalizability to broader clinical populations
• No strength or EMG data were collected, so the role of quadriceps weakness or hamstring co-contraction in driving compensations cannot be determined
• Surgical techniques, graft types, surgeons, and treating therapists varied across participants, preventing attribution of findings to any specific intervention
• Two participants had previous contralateral ACLr, which may have affected ratio calculations by making those individuals appear less impaired

Why it matters

For patients

Patients after ACL reconstruction should be aware that even during simple bodyweight squats they may be significantly under-loading the surgical knee for at least 5 months, and targeted rehabilitation cues may be needed to correct this.

For clinicians

Clinicians should actively monitor and correct both weight distribution between limbs and hip-dominant compensations during bilateral squats throughout early-to-mid rehabilitation, as visual assessment of knee angle alone is insufficient to detect these deficits.

For readers

This study provides longitudinal biomechanical evidence that compensatory loading strategies after ACLr are not static and evolve over time, highlighting specific mechanical targets for rehabilitation programming.