No Strain, No Gain? The Role of Strain and Load Magnitude in Human Tendon Responses and Adaptation to Loading

McMahon, G. No strain, no gain? The role of strain and load magnitude in human tendon responses and adaptation to loading. J Strength Cond Res 36(10): 2950-2956, 2022-Transmission of force and energy storage and release are affected by the mechanical properties and morphology of tendons. Therefore, understanding the modulation of mechanical properties through training is key as part of optimizing athletic task performance and rehabilitation. In vivo human tendon adaptation to exercise occurs in a nongraded manner, that is, there seems to be a threshold to which a tendon responds and adapts to mechanical stimuli, whereas below this threshold, minimal or no adaptation is observed. However, this remains controversial as such findings have not always been universal. Modulation of strain magnitude (change in tendon length relative to its original length) or loading intensity (e.g., % 1 repetition maximum or % maximal voluntary contraction) therefore may play a fundamental role in enhancing tendon adaptation. This review outlines the key evidence of these phenomena through direct comparative studies of higher vs. lower strain/load magnitude and indirect noncomparative studies and also explores some of the potential mechanobiological underpinnings of these adaptations. Furthermore, this review outlines practical considerations for exercise prescription using a strain magnitude-based approach and why previous non-strain magnitude-based approaches may have been a confounding factor in load magnitude studies investigating tendon adaptation.