Recurrent Exercise-Induced Muscle Damage and Overtraining Syndrome: Is There Any Relationship?

Abstract

Strenuous and unusual physical exercise, with predominance of eccentric contractions, constitutes a vehicle that potentially induces muscular damage. It is believed that this successive muscle damage accumulation due to the training overload is the origin of a chronic condition designated by overtraining syndrome (OTS). However, there is no scientific direct evidences associating the chronic structural muscle damage induced by training overload with the occurrence of OTS. Thus, this study aim is to test the assumption that successive muscle damage accumulation due to the training overload is in the origin of the OTS, using the soleus (SOL) and tibialis anterior (TA) muscles of Wistar rats exposed to a demanding exercise training protocol. Animals were randomized distributed to a control group (CG, n=5) or to an exercised training group (EE, n=10), which performed a treadmill running training (-20º; from 25m/min, with progressive increase of 1.25m/min per day; 60 min) for 6 times/week being animals sacrificed 1 (EE1, n=5) and 3 weeks (EE3, n=5) after the beginning of the training program. Body weight, food intake, hair appearance, ability to perform work, and animals' behavior were measured during the protocol. After sacrifice, SOL and TA muscles were collected for histological analysis. Both showed muscle damage signs in EE1 and EE3 through the increase of cell degeneration, tissue necrosis, and inflammatory activity. However, the exercise training protocol was not able to induce OTS. In parallel to the occurrence of muscle injury, adaptative signs in the exercised muscles were found, such as enhanced collagen content and cross-sectional area were also observed. In conclusion, the great amount of chronic muscular damage in SOL and TA is not associated with the OTS in the short and medium-term. Further, muscle damage demonstrates different behaviors according to the type of work that each muscle performs, questioning the use of systemic markers of muscle damage as reliable methods to study the relationship between skeletal muscle damage and OTS.