A personalized muscle-tendon assessment and exercise prescription concept reduces muscle-tendon injuries

Key points

1. 01Prevalence of muscle-tendon imbalances fell from 32% to 6% at final measurement in the intervention group, while the control group showed a temporary increase during the season.
2. 02Intervention group showed significantly lower fluctuations in maximum patellar tendon strain over time (p = 0.005) and an overall decrease in tendon strain (p = 0.010).
3. 03Athletes with initially high tendon strain (at or above 9%) showed the greatest reductions in strain, driven mainly by increases in tendon stiffness.
4. 04Tendon microstructural organization (PSF) decreased significantly in the intervention group (p = 0.013, d = -0.89), though values stayed within the normal asymptomatic range.
5. 05No significant changes in muscle strength were observed at the group level in either group, suggesting the muscle adaptation component of the concept was less effective in these female athletes than previously seen in males.

How it was conducted

Design

Two-season prospective controlled study; first season as control period, second as intervention period in female adolescent elite handball athletes aged 13-16 years

Participants

Control group: n = 15 (after exclusions); Intervention group: n = 22 at baseline (after exclusions), with 4 measurement time points over approximately 32 weeks

Intervention

Personalized knee extension exercises 3 times per week for 32 weeks; load individualized to achieve approximately 5.5% tendon strain (target 4.5-6.5%) for those with balanced or deficient tendon stiffness; high-volume low-load training to failure for the one athlete with muscle strength deficit

Primary outcome

Maximum patellar tendon strain during maximal voluntary contractions, measured by dynamometry and ultrasonography

Secondary outcomes

Knee extensor muscle strength, patellar tendon stiffness, tendon micromorphology (peak spatial frequency analysis), and VISA-P tendon pain score

What they found

• Fluctuations of maximum patellar tendon strain were significantly lower in the intervention group than the control group (p = 0.005).
• Maximum patellar tendon strain decreased significantly over time in the intervention group (p = 0.010, d = -0.45) but not in the control group (p = 0.727, d = -0.10).
• Prevalence of muscle-tendon imbalances (strain at or below 4.5% or at or above 9%) fell from 32% at M1 to 6% at M4 in the intervention group; the control group showed a temporary increase during the season.
• Frequency of athletes with low- or high-level patellar tendon strain at any measurement was 14% in the intervention group versus 38% in the control group.
• Among athletes with a tendon stiffness deficit at baseline (n = 6), the decrease in tendon strain was caused by a predominant increase in tendon stiffness, confirmed by significant increases in tendon force needed to induce a given strain magnitude across the full force-strain curve.
• Peak spatial frequency (tendon microstructure) decreased significantly in the intervention group (p = 0.013, d = -0.89) with a significant time-by-group interaction (p = 0.041); values remained within the interquartile range reported for healthy asymptomatic athletes (1.7-2.0 mm-1).
• No significant changes in maximum knee joint moment, normalized knee joint moment, or tendon force at the group level in either group (all p > 0.05).
• Strong negative correlation between initial tendon strain and change in tendon strain from M1 to M4 in the intervention group (r = -0.675, p = 0.002) but not in the control group (r = -0.265, p = 0.404).
• Maximum tendon elongation decreased significantly in the intervention group (p = 0.020, d = -0.39) with lower fluctuations compared to controls (p = 0.013).

Limitations

• Training adherence was monitored only through coach reports rather than direct tracking; individual participation rates could not be confirmed.
• The intervention group was significantly older and biologically more mature than the control group at baseline (p = 0.004 and p = 0.044), although age-matched subgroup analysis produced similar results.
• Only one athlete in the intervention group had a muscle strength deficit, making it impossible to draw conclusions about the effectiveness of the muscle-targeted training component.
• The study was retrospectively registered and used a non-randomized two-season design, limiting causal inference and generalizability beyond elite female handball athletes.

Why it matters

For patients

Young female handball players with high tendon strain may benefit from individualized tendon loading exercises to reduce injury risk, though the long-term effect on actual tendon injuries was not measured.

For clinicians

Measuring patellar tendon strain during maximal contractions can guide personalized exercise prescription to address specific muscle-tendon imbalances, with load targets around 5.5% tendon strain appearing effective for tendon stiffness adaptation in female adolescent athletes.

For readers

This study extends previous findings in male athletes to female adolescents, showing that a strain-guided personalized training concept can stabilize tendon mechanics during a competitive season, though muscle strength gains were not achieved as expected.