(Golden Oldie) Fibril morphology and tendon mechanical properties in patellar tendinopathy: effects of heavy slow resistance training

Key points

1. 01Patellar tendinopathy tendons had lower fibril density and a tendency toward larger mean fibril area compared with healthy tendons at baseline
2. 02Heavy slow resistance training increased fibril density by 70% and reduced mean fibril area by 26% in tendinopathic tendons, reaching levels similar to healthy controls
3. 03Clinical symptoms improved significantly, with VISA-p scores rising 27% and pain on VAS improving 36%
4. 04Tendon stiffness was similar between groups at baseline but decreased in the tendinopathy group after training, possibly reflecting increased matrix turnover
5. 05Fibril volume fraction was equal between groups at all time points, which may explain why tendon mechanical properties were not impaired by tendinopathy

How it was conducted

Design

Cohort study with a healthy control group (Level of evidence 2)

Participants

8 male patients with chronic patellar tendinopathy (mean age 32.9 years, symptoms >3 months) and 9 age- and activity-matched healthy male controls

Intervention

12 weeks of heavy slow resistance training (squat, leg press, hack squat; 3 sessions/week; progressive load from 15 RM to 6 RM)

Primary outcomes

Tendon fibril density, fibril volume fraction, and mean fibril area via transmission electron microscopy of biopsy samples; tendon stiffness and modulus via ultrasound and force measurement

Clinical outcomes

VISA-p questionnaire score and maximal tendon pain during activity on a 100-mm VAS

Assessments

Baseline and 12 weeks in both groups

What they found

• VISA-p score improved 27% +/- 7% in the tendinopathy group (57 +/- 3 to 82 +/- 7, P = .02)
• VAS pain score improved 36% +/- 5% (59 +/- 6 to 23 +/- 5, P = .008); all 8 patients improved on VAS
• Fibril density at baseline was significantly lower in tendinopathy versus controls (50.0 +/- 4.8 vs 79.5 +/- 10.2 fibrils/um2, P = .03)
• Mean fibril area tended to be higher in tendinopathy at baseline (9352 +/- 1289 vs 7709 +/- 1114 nm2, P = .07)
• After 12 weeks of HSR, fibril density in the tendinopathy group increased 70% +/- 18% (P = .008) and mean fibril area decreased 26% +/- 2% (P = .04)
• At 12 weeks, no significant differences in fibril density (P = .7), volume fraction (P = .37), or mean fibril area (P = .29) remained between groups
• Tendon stiffness decreased significantly in the tendinopathy group (3185 +/- 187 to 2701 +/- 201 N/mm, P = .04) but remained unchanged in controls (3487 +/- 392 to 3157 +/- 327 N/mm, P = .57)
• Quadriceps cross-sectional area increased 7% +/- 1% (P = .008) and peak knee extension moment increased 10% +/- 2% (P = .03) in the tendinopathy group

Limitations

• Very small sample of 8 patients and 9 controls limits statistical power and generalizability
• All participants were male, so findings may not apply to women
• Biopsy sampling may occasionally have captured healthy rather than pathological tissue, despite standardized targeting of the abnormal proximal region
• Mechanical measurements were performed at subfailure loads only, so conclusions about maximal or failure tendon strength cannot be drawn

Why it matters

For patients

People with chronic patellar tendinopathy who follow a structured heavy slow resistance program for 12 weeks can expect meaningful pain relief and functional improvement alongside biological repair of tendon fibril structure.

For clinicians

HSR appears to normalize collagen fibril morphology (increased density, reduced fibril size) in tendinopathic patellar tendons, providing a plausible tissue-level mechanism for its clinical effectiveness, though the concomitant reduction in stiffness warrants further investigation.

For readers

This is a small mechanistic cohort study offering the first human biopsy evidence that exercise-induced fibril remodeling accompanies clinical recovery in patellar tendinopathy, supporting collagen turnover as a key therapeutic target.