Limb stiffness or musculotendinous stiffness (MTS) has previously been examined in relation to performance and characterized using a number of different methods. However, the fact that MTS shows only low to moderate correlations to performances may indicate a lack of understanding of this parameter. In addition to this, variation is seen between studies examining the same factors. To date, our understanding of MTS and its components are not complete and thus it is unclear which characteristic value represents the ideal index of stiffness as it relates to performance. Moreover, it is uncertain how MTS stiffness as a functional measure relates to performance, and also if there is an optimal amount of MTS stiffness for specific functions or tasks. The knowledge of the interplay of MTU stiffness as it relates to performance and injury risk is also poorly understood in that there is likely a disparity between levels of stiffness required to optimize performance and those required to minimize injury risk. The aim of this article is to review the literature as it describes the components of MTS and to discuss these in terms of their relationship to functional performance; consider adaptations of the MTU with training along with associated performance changes; highlight and discuss how stiffness may affect loading of the soft and bony tissues in terms of the MTU components and gender, with respect to risk of injury; discuss the apparent differences in the literature regarding associations of the various forms of stiffness index to function; suggest recommendations for training in light of adaptation of the muscle and tendon and injury risk in context of gender; and, finally, to highlight potential limitations of current methodologies and suggest further work to gain insight into the mechanisms of stiffness. It is hoped that by suggesting future work, a more detailed and comprehensive understanding of MTS will be gained, thus enabling appropriate interventions to optimally modify this parameter for specific requirements.
