Cellular mechanisms underlying overreaching of skeletal muscle following excessive high-intensity interval training

Introduction :Overtraining is a significant concern for individuals who incorporate exercise into their routines such as athletes.Overreaching (OR) can be defined as a decline in physical performance caused by excessive exercise training and requires days to weeks recovery.The impairment of skeletal muscle contractile function is believed to be a major cause of OR.However, a cellular mechanism triggering OR remains unclear.Therefore, this study aimed to elucidate mechanism(s) underlying OR.Methods : Plantar flexor muscles of rats were subjected to repeated electrical stimulations mimicking excessive high-intensity interval training (HIIT) every single day for 13 days.One day after HIIT, the torques at various stimulation frequencies were measured and, subsequently, a physiological function was analyzed by using mechanically-skinned-fiber technique.Results : Eleven out of 17 rats exhibited torque decline but others did not during training period, and therefore the rats were subdivided into OR and non-overreaching (NOR) groups.Skinned fibers from the gastrocnemius (GAS) muscles of both groups showed decreased depolarization-induced force [an indicator of Ca 2+ release function of sarcoplasmic reticulum (SR)] and increased myofibrillar Ca 2+ sensitivity.In contrast, the fibers from the OR group, but not the NOR group, displayed decreased myofibrillar maximal force.Biochemical analyses of a superficial region of GAS muscles indicated increased -actinin 2 content in the NOR group and increased calpain-3 autolysis in the OR group. Conclusions :These results provide insights into the cellular mechanism underlying OR : Excessive HIIT training promotes a protein degradation pathway, such as calpains, destabilizing the sarcomere structure leading to depression in myofibrillar maximal force.Furthermore, the increased myofibrillar Ca 2+ sensitivity likely compensates for the decreased SR Ca 2+ release in trained fibers.