## 1 Introduction  
Surface mechanomyography (MMG) records low-frequency lateral oscillations produced from muscle fibers during a contraction [1]. These oscillations are believed to result from (1) the overall movement at the start of a muscle action, (2) ensuing oscillations at the resonant frequency of the active muscle, and (3) active fiber dimensional changes [1, 2]. MMG represents the mechanical output of input excitation [3], and the frequency of the signal provides insight on the overall firing frequencies for the untetanized active motor units (MUs) [3, 4, 5, 6, 7]. Despite substantial interest regarding adaptations in neuromuscular behavior resulting from training, information regarding the effects of endurance cycling on MU firing frequencies is limited. Previously, [PERSON] et al. [8] reported 6 weeks of primarily moderate-intensity continuous cycling (50-75% heart rate reserve [HRR]) significantly decreased MU firing frequencies of the vasti muscles and resulted in additional MU recruitment to complete a contraction at 30% of maximal strength for sedentary males. In addition, [PERSON] et al. [9] reported higher-threshold MUs of the VL exhibited a decrease in average firing frequencies following 5 weeks of vigorous-intensity continuous cycling training (70-75% HHR) in sedentary females during contractions with targeted torques at 40% of pre- and post-training strength. Conversely, [PERSON] et al. [10] indicated firing frequencies of the VL were unchanged during various submaximal MVCs following 2 weeks of prolonged (90-120 min) cycling (\(\sim\)65% VO2 PEAK) in males. Despite the MMG signal being a relatively inexpensive measurement tool that is easy to interpret and that requires minimal signal processing compared to advanced single MU techniques, only one study has utilized MMG to examine the possible adjustments in MU behavior of the VL as a result of cycling. Recently, our group [11] reported that 10 weeks of high-intensity (70-90% HRR) continuous cycling training significantly increased maximal aerobic capacity by \(\sim\)15% yet significantly reduced knee extensor MVC by \(\sim\)4% for sedentary males and females. Following training, MMG amplitude (MMGRMS)-torque relationships of the VL indicated increased MU recruitment was necessary to complete a linearly increasing muscle action to 70% of pre-training MVC, therefore suggesting MMG is sensitive to adjustments in MU activation strategies following short-term endurance training. However, it has yet to be investigated if short-term endurance training would alter MU deactivation strategies (linearly decreasing muscle action). In addition, although median power frequency (MDF) of the MMG signal may present information on MU firing rates [12], no study has examined possible changes in MMGMDF-torque relationships for the VL due to continuous cycling training. Furthermore, it is plausible that adjustments in MU behavior following endurance training differ between moderate- and high-intensity contractions, as the former would primarily result in the recruitment of MUs that possess type I fiber characteristics, whereas the latter would activate MUs that possess characteristics of both fiber types (I and II). Consequently, more research is necessary. It is suggested that the MMG-torque relationship should be examined with a subject-to-subject comparison [13; 14; 15], as analyzing composite means may not obscure changes in the linearity of the individual patterns due to large inter-subject variability [6]. Thus, [PERSON] et al. [5] suggested log-transforming the MMG-force relationships for each contraction and utilizing linear regression to calculate \(b\) terms (slopes) and investigate potential changes in the subject patterns. Further insight concerning the linearity of the original relationships is also provided by the 95% confidence intervals (CIs) calculated for the \(b\) terms [16], such as \(b\) terms with 95% CIs that include 1 indicate a linear relationship between MMGMDF and torque.