Abstract
Differences in muscle and tendon responsiveness to mechanical stimuli and time courses of adaptive changes may disrupt the interaction of the musculotendinous unit (MTU), increasing the risk for overuse injuries. We monitored training-induced alterations in muscle and tendon biomechanical properties in elite jumpers over 4 years of athletic training to detect potential non-synchronized adaptations within the triceps surae MTU. A combined cross-sectional and longitudinal investigation over 4 years was conducted by analyzing triceps surae MTU mechanical properties in both legs via dynamometry and ultrasonography in 67 elite track and field jumpers and 24 age-matched controls. Fluctuations in muscle and tendon adaptive changes over time were quantified by calculating individual residuals. The cosine similarity of the relative changes of muscle strength and tendon stiffness between sessions served as a measure of uniformity of adaptive changes. Our cross-sectional study was unable to detect clear non-concurrent differences in muscle strength and tendon stiffness in elite jumpers. However, when considering the longitudinal data over several years of training most of the jumpers demonstrated greater fluctuations in muscle strength and tendon stiffness and hence tendon strain compared to controls, irrespective of training period (preparation vs. competition). Moreover, two monitored athletes with chronic Achilles tendinopathy showed in their affected limb lower uniformity in MTU adaptation as well as higher fluctuations in tendon strain over time. Habitual mechanical loading can affect the MTU uniformity in elite jumpers, leading to increased mechanical demand on the tendon over an athletic season and potentially increased risk for overuse injuries.
Highlights
Tendons are an integral part of musculotendinous units (MTUs) and their primary function is to transmit forces from muscles to rigid bone levers to produce joint motion
Irrespective of the analyzed limb, elite jumpers demonstrated in comparison to the control group significantly higher triceps surae (TS) muscle strength (average values of both legs: athletes 303 ± 85 vs. controls 241 ± 65 Nm; subject group effect: F(1, 89) = 10.415, P = 0.002, ηp2 = 0.105) as well as greater tendon stiffness (653 ± 187 vs. 570 ± 131 N/mm; F(1, 89) = 4.004, P = 0.048, ηp2 = 0.043) with similar relative difference between groups (∼20% in TS muscle strength vs. ∼15% in tendon stiffness)
Concerning the between leg analysis, we found limb-specific differences in TS MTU mechanical properties in jumpers but not in controls
Summary
Tendons are an integral part of musculotendinous units (MTUs) and their primary function is to transmit forces from muscles to rigid bone levers to produce joint motion. When a MTU is constantly experiencing increased mechanical loading (e.g., by resistance training) it is generally observed that the associated adaptive enhancements in muscle strength lead to a markedly greater tendon stiffness (Kubo et al, 2001, 2012; Arampatzis et al, 2007a,b; Bohm et al, 2014). The reported modifications in tendon stiffness may exhibit a protective homeostatic process to withstand greater functional demand due to muscle strength gains. The muscle strength gains seem to be merely in a moderate association with modifications in tendon mechanical properties (Arampatzis et al, 2007a,b). An imbalance between the generated muscle forces and the strength of the tendon may occur over the course of training process, placing the tendon under greater mechanical demand (i.e., higher strain)
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