Can adaptation to a stressor happen without injury that requires healing in the total absence of the stressor?
Or can adaptation be a continual process.
These seem to be important questions relevant to exercise frequency. It may be argued that all adaptation stems from some level of injury. It is my belief that there is a threshold between desirable adaptation and injury.
I read a lot of proponents of the “micro tear” theory of PE. I believe these can and do happen at a certain level of mechanical stretch in the ligaments or/and in the ligament like tissues of the tunica. , but I am not at all sure this is what we want.
Ligaments under increasing load undergo a phase of elasticity as component fibers align in the direction of stress. This results in a small elongation of the ligament until these fibers become fully aligned in the direction of pull, then the ligament becomes rigid (1). Even low amounts of mechanical stretch modifies the composition of extracellular matrix of human ligament fibroblasts (2).
I believe this is the level of “stress” we want to achieve during PE, well below the stress that would cause micro tearing thus require a longer “healing” process, but high enough to cause an adaptation response at a cellular level. This level of stress must be quite low however because cellular damage is induced at ligament strains significantly below the structural damage threshold (3). Injuries to ligaments induce a healing response that is characterized by the formation of a scar tissue that is weaker, larger and biomechanically inferior to the tissue it replaces (4),(5).
It is unsettling to think that by using too high a level of stress and causing “micro tears”, one may be slowly replacing his tunica with inflexible scar tissue.
References;
(1) Biomechanics Journal of Acta Ortopédica Brasileira vol.11-2 72-78, 2003
(2) Hua Xi Kou Qiang Yi Xue Za Zhi. 2001 Jun;19(3):155-7
(3) J Appl Physiol. 2002 Jan;92(1):362-71
(4) Can J Surg. 1998 Dec;41(6):425-9.
(5) J Musculoskelet Neuronal Interact. 2004 Jun;4(2):199-201.