Chronic sequelae after muscle strain injuries: influence of heavy resistance training on functional and structural characteristics in a randomized controlled trial

Key points

1. 01Eccentric heavy resistance training improved concentric hamstring strength deficit by 23 percentage points and eccentric strength by 17 percentage points after 3 months
2. 02Medial gastrocnemius cross-sectional area increased by 12% and semitendinosus by 7% with heavy resistance training, compared to no change or decline in the control group
3. 03Both training groups reported improved pain and function scores, but scores remained worse than the healthy leg after intervention
4. 04Scar tissue biopsies revealed substantial fatty infiltration (mean 11.6% of biopsy area), hypercontracted sarcomeres, and sparse collagen - none of which improved with training
5. 05Fatty infiltration and structural scar changes persisted after heavy resistance training, suggesting these may be permanent features of chronic strain injury

How it was conducted

Design

2-arm randomized controlled trial (Level of evidence 1)

Participants

30 amateur athletes with chronic thigh or calf muscle strain injury (symptoms for at least 6 months), mean injury duration approximately 43-44 months

Groups

Heavy resistance training (HRT, eccentric focus) vs control core exercises (CORE, back and abdominal muscles only)

Duration

3 months of supervised training

Primary outcomes

Isokinetic or isometric muscle strength, muscle cross-sectional area on MRI, patient-reported outcome measures, and scar tissue ultrastructure from biopsy

Setting

Institute of Sports Medicine Copenhagen, Denmark; recruitment May 2014 to May 2016

What they found

• Concentric hamstring MVC deficit improved by 23 percentage points in HRT group (P = .002; Cohen d = 1.2); no change in CORE group; group difference post-intervention P = .03
• Eccentric hamstring MVC improved by 17 percentage points in HRT group (P = .001; Cohen d = 0.9); no change in CORE group (P = .3); group x time interaction P = .04
• Before intervention, concentric MVC was 17.5% lower in injured vs healthy leg in HRT group and 18% lower in CORE group; eccentric MVC was 27% lower in HRT and 25% lower in CORE
• Semitendinosus CSA increased 7% in HRT group (P = .03; Cohen d = 0.6); no change in CORE (group difference P = .046)
• Medial gastrocnemius CSA increased 12% in HRT group (P = .003; Cohen d = 1.1); decreased 8% in CORE group (P = .06; Cohen d = 0.7); group difference P = .001 (Cohen d = 1.8)
• Mean scar tissue fat area was 11.6% across all participants; adipocytes ranged from 1.5% to 63.2% of biopsy area; no statistically significant change with either intervention
• Soreness score showed group x time interaction (P = .02) with improvement in HRT (P < .001); pain, symptoms, function, and quality of life improved over time in both groups (P < .001 to P = .007) with no group difference
• Calf isometric MVC and heel-rise performance did not differ between groups before or after intervention

Limitations

• Small sample size (n=30) with no formal power calculation for this chronic injury arm; the trial was powered for an acute injury cohort
• No healthy-leg MRI scans were obtained, so relative changes in the uninjured limb cannot be assessed
• Isokinetic calf strength could not be measured due to equipment limitations, reducing sensitivity for detecting calf-specific functional gains
• Scar tissue biopsies were not taken from healthy muscle for comparison, and formal statistical analysis of fat and vessel content was avoided due to high variability

Why it matters

For patients

People with persistent weakness or pain years after a muscle strain can expect meaningful strength and functional gains from a 3-month eccentric resistance training program, though some impairment may remain.

For clinicians

Eccentric heavy resistance training is an effective rehabilitation option for chronic strain sequelae and normalizes concentric strength deficits, but clinicians should counsel patients that the underlying scar tissue and fatty infiltration are unlikely to resolve with training alone.

For readers

This RCT provides the first direct biopsy evidence of fatty infiltration and sarcomere disruption in chronic human strain injuries, and shows the functional benefits of training are dissociated from structural tissue repair.