Eccentric knee flexor weakness in elite female footballers 1-10 years following anterior cruciate ligament reconstruction

Key points

1. 01Reconstructed limbs were on average 53.77 N weaker than the uninjured contralateral limb (95% CI 85.0 to 24.2, d = 0.51)
2. 02Reconstructed limbs were on average 46.32 N weaker than limbs of uninjured control players (95% CI 86.65 to 5.99, d = 0.73)
3. 03Strength deficits persisted at a median of 4.5 years post-surgery (range 1-10 years) and showed no correlation with time since reconstruction (r = 0.10)
4. 0425% of players with prior ACL reconstruction had also suffered a hamstring strain in the previous 12 months, compared with 2.8% of uninjured players
5. 05No weakness was found in the uninjured contralateral limbs of ACLR players versus controls, suggesting the deficit is limb-specific

How it was conducted

Design

Case-control study

Setting

Elite Australian Football League Women's (AFLW) pre-season screening, three clubs

Participants

84 elite female AFLW players: 12 with prior unilateral ACLR (ipsilateral semitendinosus graft), 72 with no ACLR history

Primary outcome

Peak eccentric knee flexor force (Newtons) during the Nordic Hamstring Exercise (NHE) measured with load cells

Time since surgery

Median 4.5 years post-ACLR (range 1-10 years)

Analysis

Bootstrapped mean differences with 95% CIs, Cohen's d effect sizes, and linear mixed effects model

What they found

• Reconstructed limb vs. uninjured contralateral limb: mean difference 53.77 N (95% CI 85.0 to 24.2, d = 0.51)
• Reconstructed limb vs. control limbs (no injury history): mean difference 46.32 N (95% CI 86.65 to 5.99, d = 0.73)
• When players with concurrent hamstring strain history were excluded, the deficit increased: mean difference 84.0 N (95% CI 6.25 to 151.00, d = 1.25)
• No difference between uninjured contralateral limbs of ACLR players and control limbs: mean difference 6.1 N (95% CI -33.3 to 46.62, d = 0.10)
• Dominant kicking limb was approximately 18 N stronger than the non-dominant limb in uninjured controls (d = 0.21)
• Effect of previous ACLR on eccentric strength estimated at -49.96 N (95% CI -70.1 to -29.81) in the mixed model
• No correlation between time since ACLR and peak eccentric strength (r = 0.10, 95% CI -0.60 to 0.48) or between-limb imbalance (r = 0.03, 95% CI -0.60 to 0.55)
• Moderate correlation between peak eccentric force and body mass (r = 0.40, 95% CI 0.21 to 0.52)

Limitations

• Retrospective design cannot determine whether eccentric knee flexor weakness preceded or resulted from ACL reconstruction
• Small ACLR group (n = 12) limits statistical precision, particularly for the hamstring injury subgroup analysis
• Rehabilitation program details were not available, so differences in post-operative care could not be controlled for
• The NHE does not capture angle-specific torque production or allow comparison with quadriceps or hip flexor strength, limiting interpretation of functional recovery

Why it matters

For patients

Female football players who have had ACL reconstruction should be aware that significant hamstring weakness can persist for many years after surgery and return to sport, even at elite level.

For clinicians

Eccentric knee flexor strength should be formally assessed in female athletes long after ACLR, as deficits around 20% may remain and could contribute to re-injury risk; targeted Nordic hamstring strengthening warrants inclusion in long-term rehabilitation.

For readers

This case-control study extends existing findings from male footballers to elite women, showing durable post-ACLR hamstring deficits, but its small sample and cross-sectional design mean prospective studies are needed to establish whether these deficits directly predict re-injury.