Abstract

This study aimed to test the assumption that successive muscle damage accumulation due to the training overload is in the origin of the overtraining syndrome (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, progressive increase of 1.25m/min per day; 60min) for 6 times/week being 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, no OTS signs were observed. In parallel to the occurrence of muscle injury, adaptative signs in the exercised muscles were found, such as enhanced collagen content and muscle fibers cross-sectional area. The great amount of chronic muscular damage in SOL and TA was not associated with the OTS in the short (1 week) and medium-term (3 weeks). Further, muscle damage demonstrated different behaviors according to the type of work that each muscle has performed, questioning the use of systemic markers of muscle damage as reliable methods to study the relationship between skeletal muscle damage and OTS.

Keywords: Training overload; Muscle injury; Eccentric exercise; Soleus muscle; Tibialis anterior muscle

Abbreviations: BW: Body Weight; CD: Cellular Degeneration; CG: Control Group; CK: Creatine Kinase; CSA: Cross-Sectional Area; DPX: Dibutyl Xylene Phthalate; EE: Exercised Training Group; EE1: 1-Week Exercised Training Subgroup; EE3: 3-Week Exercised Training Subgroup; EM: Exercise Myopathy; FI: Food Intake; H&E: Hematoxylin-Eosin; IA: Inflammatory Activity; N: Necrosis Extent; NP: Nuclei Position; OTS: OvertraingSyndrome; PE: Physical Exercise; rTAW: Relative Tibialis Anterior Weight; rSW: Relative Soleus Weight; SOL: Soleus; SOLCC: Soleus Collagen Content; SOLCSA: Soleus Cross-Sectional Area; SOLMD: Soleus Muscle Damage; SOLNP: Soleus Nuclei Position; SW: Soleus Weight; TA: Tibialis Anterior; TACC: Tibialis Anterior Collagen Content; TACSA: Tibialis Anterior Cross-Sectional Area; TAMD: Tibialis Anterior Muscle Damage; TANP: Tibialis Anterior Nuclei Position; TAW: Tibialis Anterior Weight; TD: Total Damage; TO: Tissue Organization