# The frog semitendinosus muscle: Contractile and metabolic studies of muscle fatigue

#### Abstract

Fatigue, defined as a reduction in muscle force and power, likely results from multiple factors including alterations in excitation contraction coupling, cross-bridge events, and cell metabolism. In this thesis, the frog semitendinosus (ST) muscle was employed as a model for the study of skeletal muscle fatigue. Following the determination of the functional and histochemical properties of the ST, a stimulation protocol was developed to study the contractile and biochemical properties during recovery from fatigue. The 5 min stimulation protocol reduced peak twitch (P$\sb{\rm t}$) and tetanic tension (P$\sb{\rm o}$) to 32% and 8.5% of initial tension, respectively. The decline in P$\sb{\rm t}$ was less than P$\sb{\rm o}$ in part due to a prolongation in the isometric contraction time (CT), which increased to 300% of the initial value. The isometric twitch duration was prolonged as reflected by the lengthened CT and the 800% increase in the one-half relaxation time (1/2RT). Both P$\sb{\rm t}$ and P$\sb{\rm o}$ showed a biphasic recovery, a rapid initial phase followed by a slower return to the pre-fatigue tension. The maximal shortening velocity was significantly reduced at fatigue. A detailed investigation of the force-frequency relationship demonstrated a fatigue-induced shift to the left, such that optimal frequency for generating P$\sb{\rm o}$ was reduced from 150 to 60 Hz. To investigate H$\sp+$ involvement in fatigue, glass microelectrodes were constructed to directly assess the extent of fatigue-induced acidosis. The slow phase of P$\sb{\rm o}$ recovery, CT and 1/2RT following fatigue were highly correlated to pH$\sb{\rm i}$ recovery. These data support the hypothesis that H$\sp+$ may, in part, mediate fatigue. Finally, this work employed single fiber biochemical techniques to investigate the hypothesis that the high energy phosphates and/or inorganic phosphate (P$\sb{\rm i}$) mediate fatigue. ATP and phosphocreatine (PC) decreased, whereas lactate and P$\sb{\rm i}$ increased in the fatigued single cells. It was concluded that the decline in ATP could not limit force production, as even the lowest post-fatigue cell ATP was 100-fold higher than required for full cross-bridge activation. The significant correlation between PC and P$\sb{\rm o}$ during recovery was likely not causative but rather directly related to the H$\sp+$ content which simultaneously depressed P$\sb{\rm o}$ and the rate of PC resynthesis. The P$\sb{\rm i}$ concentration increased 3-fold with stimulation. In conclusion, the results are consistent with the hypothesis that muscle fatigue during high intensity exercise is caused, in part, by an elevated cell H$\sp+$ and P$\sb{\rm i}$ concentrations.

#### Recommended Citation

Thompson, LaDora Vaughan, "The frog semitendinosus muscle: Contractile and metabolic studies of muscle fatigue" (1991). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. AAI9133813.
https://epublications.marquette.edu/dissertations/AAI9133813

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