INTRODUCTION: It is important for combat athletes to understand effects of rapid changes of body weight on muscular functions. The purpose of this case study was to investigate time-course changes in muscle strength, muscle size and neural input during rapid weight losing in a professional boxer. METHODS: One professional men boxer (26 years old) participated in two matches during measurements, welter weight (66.6kg; WW) and super welter weight (69.85kg; SWW). He needed to lose weight substantially for WW and slightly for SWW. The measurement was performed at 6 week (baseline), 3 week (Pre3wk), 2 week (Pre2wk), 1 week (Pre1wk) and 0 week (Pre0wk) before and 1 week (Post1wk) after the matches. Body composition was measured using a bioelectrical impedance spectroscopy. Maximal voluntary isometric knee extension torque was measured using a dynamometer. Cross sectional area of vastus lateralis muscle (mCSA) were evaluated using an ultrasonography. High density surface electromyography on vastus lateralis was also recorded during submaximal isometric contraction based on the maximal strength at baseline (MVC-base) and individual motor units were identified. Motor units discharge rate was evaluated at 60-70% MVC-base during 70% MVC-base ramp-up task. RESULTS: His baseline body weights were 70.80 kg and 71.42 kg for matches of WW and SWW, respectively. At Pre0wk, his body weight decreased to 68.75 and 71.36 kg at WW and SWW, respectively. For WW match, skeletal muscle mass and mCSA were decreased a lot from baseline to the match (36.1 to 35.8 kg and 21.1 to 20.3 cm2, respectively). On the other hand, for SWW match, skeletal muscle mass and muscle cross sectional did not decrease a lot (36.4 to 36.5 kg and 23.2 to 22.7 cm2, respectively). The number of detected motor units was 15.7±5.2 at each section. As for recruitment threshold of motor units, two-way ANOVA revealed no interaction. As for motor unit discharge rate, two-way ANOVA revealed a significant interaction between condition (WW vs SWW) and time. Post-hoc tests indicated that discharge rate in WW was significantly greater at Pre0wk (16.2±2.7 pps) than at baseline (13.5±1.6 pps), Pre3wk (12.6±2.0 pps), Pre2wk (13.3±2.1 pps) and Post1wk (13.8±2.9 pps), but not change in SWW. A correlation coefficient between motor unit discharge rate and body weight at 6 time points in WW was -0.951. CONCLUSION: Skeletal muscle mass decreased but motor units discharge rate increased as body weight losing. It is suggested that an excitability as neural input to muscle is activated as body weight losing and muscle mass decrease to maintain the muscle strength. When considering the effects of rapid body weight losing, we should keep in mind that not only body compositions but also neural factor is adapted.
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