Effects of eccentric-oriented strength training on return to sport criteria in late-stage rehabilitation

Key points

1. 01Eccentric group improved isometric strength on the injured leg by 27.1% vs. 18.1% for the traditional group over 6 weeks
2. 02Countermovement jump improved 12.9% (eccentric) vs. 6.7% (traditional)
3. 03Single-leg hop on the injured leg improved 23.9% (eccentric) vs. 8.1% (traditional)
4. 04No significant between-group difference was found for limb symmetry indexes, possibly due to low reliability of those measures
5. 05Flywheel devices were used to deliver eccentric overload at a moderate inertia load of 0.075 kg m2

How it was conducted

Design

Randomized controlled trial, between-subject longitudinal, 6-week intervention

Participants

22 professional team sport athletes (14 male, 8 female; age 19.9 +/- 4.4 years) with unilateral BTB-graft ACL reconstruction, tested 5-6 months post-surgery

Groups

Eccentric-oriented (ECC, n=11): flywheel training; Traditional (CON, n=11): free-weight isotonic training at ~80% 1RM; equivolumed sets and reps

Training dose

6 weeks, 2-3 lower-body strength sessions per week (15 total sessions), 6 training days per week overall

Primary outcomes

Isometric semi-squat peak force (injured and uninjured legs), countermovement jump, single-leg vertical jump, single-leg hop, and triple hop tests with limb symmetry indexes

Analysis

Two-way ANOVA (group x time) with effect sizes reported as Cohen's d

What they found

• Significant group-by-time interaction for uninjured-leg isometric strength (p < 0.05, ES = 2.51, very large): ECC +28.1% vs. CON +15.1%
• Significant group-by-time interaction for injured-leg isometric strength (p < 0.05, ES = 1.78, large): ECC +27.1% vs. CON +18.1%
• Significant group-by-time interaction for countermovement jump (p < 0.05, ES = 2.23, very large): ECC +12.9% vs. CON +6.7%
• Significant group-by-time interaction for single-leg jump - injured leg (p < 0.05, ES = 1.48, large): ECC +23.8% vs. CON +13.7%
• Significant group-by-time interaction for single-leg hop - injured leg (p < 0.05, ES = 1.83, large): ECC +23.9% vs. CON +8.1%
• Significant group-by-time interaction for triple hop - injured leg (p < 0.05, ES = 1.78, large): ECC +14.3% vs. CON +5.3%
• No significant group-by-time interaction for limb symmetry indexes (all p > 0.05): ISOSLSI ES = 0.156 trivial, SLJLSI ES = 0.48 small, SLHLSI ES = 0.64 moderate, THLLSI ES = 0.55 moderate
• Main effects of time significant for all dependent variables (posttest > pretest, p < 0.05)

Limitations

• Small sample size (n = 11 per group) limits statistical power and generalizability
• Limb symmetry index measures showed poor to acceptable reliability (ICC 0.50-0.78), which may have masked true between-group differences
• The strength and conditioning coach delivering training could not be blinded to group allocation
• Hamstring strength and rate of force development were not assessed, limiting the completeness of return-to-sport profiling

Why it matters

For patients

Athletes recovering from ACL surgery may regain injured-leg strength and hopping ability faster by adding flywheel eccentric training to their late-stage rehab program compared to conventional weight training.

For clinicians

Substituting or supplementing traditional resistance exercises with flywheel eccentric-overload drills during months 5-6 post ACL-BTB reconstruction appears to accelerate performance recovery in professional team sport athletes, though evidence is still limited by small samples.

For readers

This small RCT supports eccentric-oriented flywheel training as a superior option over traditional strength training for professional athletes nearing return to sport after ACL reconstruction, but replication with larger and more diverse populations is needed.