The Relationship Among Muscle Mass, Contractile Properties, and Critical Torque

Abstract

Critical Torque (CT) is defined as the asymptote of the torque-duration relationship, representing the maximal force a muscle can sustain with wholly aerobic metabolism. In this way, CT serves as a determinant of pacing in individual muscles. Impulse Above Critical Torque (IACT) represents a finite amount of work produced above CT before fatigue accumulates. IACT is often linked to anaerobic energy stores, such as creatine phosphate, and represents anaerobic work capacity within the muscle. It has been suggested that both CT and IACT are determined, in part, by muscle mass and fiber type. Increased lean muscle mass has been associated with greater anaerobic work capacity, like IACT, and the whole-body analog of CT, Critical Power (CP). Muscle fiber biopsies have revealed significant relationships among fiber type, CT, %CT (CT relative to maximum voluntary contraction [MVC]), and IACT. Alternatives to muscle biopsy have been demonstrated using maximal neuromuscular stimulation (to derive contractile properties) and the Thorstensson protocol. These noninvasive measures of fiber type have yet to be examined in relation to CT and IACT. Therefore, the primary purpose of this study was to assess the relationship among lean muscle mass, contractile properties, and CT. A secondary purpose was to assess the intersession reliability of the Thorstensson protocol. Methods. Thirty-one participants (18 males, 13 females) took part in this study, with thirty participants completing all three visits across one week. One female participant dropped out of the CT protocol and was subsequently omitted from the correlations among lean thigh mass, contractile properties, and CT. Visit 1 consisted of electrically stimulating the right knee extensor (KE) muscle for contractile properties, familiarization with MVC and CT protocols, and the Thorstensson protocol. Visit 2 involved a DXA scan, followed by the MVC and CT protocols. Electrical stimulation was provided on the 1st and every 6th contraction during the CT protocol. Visit 3 consisted of the Thorstensson protocol only. Results. Pearson correlations revealed a weak correlation between TLM and CT (r = 0.389, p = 0.05). No significant correlation was found between TLM and %CT (r= -0.334, p = 0.071 Positive correlations were found between Pt and CT (r = 0.362, p = 0.05; Table 4), as well as RTR and CT (r = 0.434, p < 0.05). No significant correlations were found among TPT and CT (r =0.163, p = 0.389), 1/2RT and CT (r = -0.238, p = 0.206), or RTD and CT (r = 0.299, p = 0.109). No significant correlations were found among contractile properties and %CT (p 0.091). Significant negative correlations were found among %Type-II fibers and CT (r = -0.422, p = 0.02) and %CT (r = -0.57m p < 0.001). ICCs for %Type-II Fiber demonstrated moderate reliability and variability (ICC = 0.616, p = 0.05, CV= 12.9%). Conclusion. Findings of this study are consistent with previous studies suggesting that CT is positively associated with greater lean thigh mass and negatively associated with increased distributions of type-II fibers. A novel finding of this study is the moderate intersession reliability of the Thorstensson predicted fiber type—suggesting that it is fairly consistent between trials.