Mechanisms Influencing Maximum Joint Range of Motion and Stiffness: A Narrative Review

Joint 'flexibility' is an encompassing term influenced by both a joint's maximum range of motion (ROM_max) and its resistance to rotation, i.e. its stiffness. A critical amount of flexibility is thought to be important for both sports performance and daily and occupational tasks. Improvements in joint ROM_max, specifically, are often assumed to result from material and mechanical changes in the joint's soft tissue structures; however, they also may result from changes in sensory information relayed from peripheral receptors to the brain, or the interpretation of that information. Changes in psychological state mediated by pain, state arousal and other psychological factors can therefore influence the voluntary capacity of individuals to tolerate a stretch, and thus their ROM_max. Stiffness, on the other hand, may be affected by both muscular and non-muscular tissues, although the most important sources of stiffness are yet to be agreed upon. In this narrative review, we define important terms as well as the factors impacting stiffness and ROM_max. We assert that joint stiffness is affected by the extracellular matrix, intramuscular fat and water as well as fascicle rotation, and that these factors also by extension may affect ROM_max. While sarcomere (and fibre) stiffness is largely regulated by the giant protein titin, it plays a lesser role at the whole muscle level. Alternatively, ROM_max can be influenced by our processing of sensory information and the decisions we make (consciously or subconsciously) about our willingness to stretch further. Thus, cognitive and attention processes can influence an individual's decision to stop a stretch.