Achilles tendon adaptation during strength training in young adults.

PURPOSE Tendons are specialized musculoskeletal structures responsible for transferring forces between muscles and bones. To maintain joint mechanics and structural integrity, tendons must adapt to changes in mechanical loading, but little is known about the interaction between muscle and tendon adaptations in vivo. We tested the hypothesis that tendons adapt to changes in muscle strength to maintain strains within a preferred operating range. METHODS The right lower leg of 10 male subjects, age 24.9 +/- 3.4 yr (mean +/- SD), mass 78.1 +/- 9.7 kg, and height 176.5 +/- 7.2 cm, were tested before and during weeks 1, 2, 4, 6, and 8 of an 8-wk ankle plantar flexion strength training program. Subjects performed isometric plantar flexion efforts slowly ramping up from rest to a maximum effort. Plantar flexion force, Achilles tendon deformation, and cross-sectional area were measured. Triceps surae muscle force (assumed equal to Achilles tendon force), normalized force, and Achilles tendon stress and strain were calculated. Achilles tendon strains during maximum plantar flexion efforts (epsilon(max)) were compared between weeks to test the hypothesis. Repeated-measures ANOVA was used to test for significant changes during the 8 wk, with alpha = 0.05 used as the criterion for significance. RESULTS There were no significant differences in the group's mean percent or absolute change in epsilon(max) (P = 0.607 and 0.351, respectively) despite a 21.4% average increase in muscle strength during the 8 wk. CONCLUSIONS This is the first study that quantifies Achilles tendon strain throughout a strength training program. Findings indicate that the Achilles tendon adapts quickly to muscle strength training and experiences relatively little change in peak strain despite large increases in muscle strength.