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

Overreaching (OR) can be defined as a decline in physical performance resulting from excessive exercise training, necessitating days to weeks recovery. Impairments in the contractile function of skeletal muscle are believed to be a primary factor contributing to OR. However, the cellular mechanism triggering OR remains unclear. The purpose of this study was to elucidate the mechanisms underlying OR. Rats' plantar flexor muscles were subjected to repeated electrical stimulations mimicking excessive high-intensity interval training (HIIT) daily for 13 consecutive days, and isometric torques were monitored. The torque was measured one day after HIIT, and subsequently, the physiological function of type II fibers was analyzed by using mechanically skinned-fiber technique. Eleven of 17 rats exhibited torque decline, whereas others did not. Thus, the rats were divided into OR and nonoverreaching (NOR) groups. Skinned fibers from the gastrocnemius (GAS) muscles of both groups showed decreased depolarization-induced force and increased myofibrillar Ca²⁺ sensitivity. However, the fibers from the OR group, but not the NOR group, exhibited a decrease in myofibrillar maximal force. Biochemical analyses of a superficial region of GAS muscle revealed that α-actinin 2 content was increased in the NOR group, but not in the OR group, whereas calpain-3 autolysis was increased in the OR group, but not in the NOR group. These findings shed light on the cellular mechanism underlying OR: OR following excessive HIIT was induced by a decreased myofibrillar maximal force, whereas Ca²⁺ sensitivity was increased.NEW & NOTEWORTHY An early sign of overtraining is a performance impairment known as overreaching (OR). This study revealed the cellular mechanism underlying OR by combining in vivo fatiguing contractions with mechanically skinned-fiber technique. Thirteen consecutive days of intense training result in myofibrillar force depression in OR. This study provides valuable insights not only for athletes and coaches but also for nonathletes who incorporate exercise into their daily activity.