Faster movement speed results in greater tendon strain during the loaded squat exercise

Introduction: Tendon dynamics influence movement performance and provide the stimulus for long-term tendon adaptation. As tendon strain increases with load magnitude and decreases with loading rate, changes in movement speed during exercise should influence tendon strain. Methods: Ten resistance-trained men [squat one repetition maximum (1RM) to body mass ratio: 1.65 ± 0.12] performed parallel-depth back squat lifts with 60% of 1RM load at three different speeds: slow fixed-tempo (TS: 2-s eccentric, 1-s pause, 2-s concentric), volitional-speed without a pause (VS) and maximum-speed jump (JS). In each condition joint kinetics, quadriceps tendon length (L_T), patellar tendon force (F_T), and rate of force development (RFD_T) were estimated using integrated ultrasonography, motion-capture, and force platform recordings. Results: Peak L_T, F_T, and RFD_T were greater in JS than TS (p < 0.05), however no differences were observed between VS and TS. Thus, moving at faster speeds resulted in both greater tendon stress and strain despite an increased RFD_T, as would be predicted of an elastic, but not a viscous, structure. Temporal comparisons showed that L_T was greater in TS than JS during the early eccentric phase (10-14% movement duration) where peak RFD_T occurred, demonstrating that the tendon's viscous properties predominated during initial eccentric loading. However, during the concentric phase (61-70 and 76-83% movement duration) differing F_T and similar RFD_T between conditions allowed for the tendon's elastic properties to predominate such that peak tendon strain was greater in JS than TS. Conclusions: Based on our current understanding, there may be an additional mechanical stimulus for tendon adaptation when performing large range-of-motion isoinertial exercises at faster movement speeds.