Endurance training alters motor unit activation strategies for the vastus lateralis, yet sex-related differences and relationships with muscle size remain

Purpose To examine the effects of 10 weeks of endurance cycling training on mechanomyographic amplitude (MMG(RMS))-torque relationships and muscle cross-sectional area (mCSA) of the vastus lateralis (VL) for 10 sedentary males (Age +/- SD; 20.2 +/- 1.9 years) and 14 sedentary females (21.9 +/- 5.3 years). Methods Participants performed maximal voluntary contractions (MVCs) and an isometric ramp up muscle action to 70% MVC of the knee extensors before (PRE) and after training at the same absolute pre-treatment submaximal torque (POSTABS). MMG was recorded from the VL and b terms were calculated from the natural log-transformed MMG(RMS)-torque relationships for each subject. mCSA was determined with ultrasonography. Results Cycling decreased MVCs from pre- (168.10 +/- 58.49 Nm) to post-training (160.78 +/- 58.39 Nm; p = 0.005) without changes in mCSA. The b terms were greater for POSTABS (0.623 +/- 0.204) than PRE (0.540 +/- 0.226; p = 0.012) and for males (0.717 +/- 0.171) than females (0.484 +/- 0.168; p = 0.003). mCSA was correlated with the b terms for PRE (p < 0.001, r = 0.674) and POSTABS (p = 0.020, r = 0.471). Conclusion The decrease in MVC and increase in MMG(RMS) (b terms) post-training suggests increased motor unit (MU) recruitment to match pre-training torques. The greater acceleration in the b terms by males may reflect sex-related differences in fiber-type area. MMG(RMS)-torque relationships during a high-intensity contraction provided insight on MU activation strategies following endurance training and between sexes. Furthermore, the findings suggest a relationship between MMG(RMS) and muscle size.