Effect of the fatiguing exercise on VAMMG
In the exercising limb, VA
MMG was decreased in all muscle heads but more so in the
VM muscle. The changes were accompanied by a decrease in MMGp-p
MVC, MMGp-p
POT, MMG RMS, MMG MF, sEMG RMS, and sEMG RMS/M-wave in all muscle heads, with a greater decrease observed in the
VM muscle. These decrements were also accompanied by reductions in M-wave amplitude occurring with a similar extent in all the three muscle heads. MMGp-p
MVC, MMG RMS, and sEMG RMS are the gross sum of central and peripheral mechanisms underlying exercise-induced fatigability (Orizio et al.
2003). To explore these aspects, we added MMG MF, which reflects the central mechanisms of exercise-induced fatigability, since it indirectly monitors the firing rate of activation of the motor neurons responsible for muscle contraction (Orizio
1993; Cè et al.
2015,
2020b), and the M-wave and MMGp-p
POT, which are more influenced by peripheral mechanisms related to exercise-induced fatigability (Orizio et al.
2003). The changes in MMG are consistent with the changes observed using the interpolated twitch technique. However, while the interpolated twitch technique cannot provide any further information about the synergistic muscle heads involved in a fatiguing task, the MMG can distinguish it. Furthermore, greater fatigue was noted in the
VM muscle than the other superficial muscle heads.
The present outcomes seem to suggest that the larger decrease in VA
MMG in the
VM could be induced by both central and peripheral fatigue mechanisms. The between-muscle difference in sEMG RMS/M-wave and MMG MF (more related to central mechanisms), together with the MMGp-p
POT, seems to suggest a role is played by both the central and peripheral mechanisms linked to fatigue. Using a similar single-leg knee-extension exercise, a previous study found a trend of
VM exhibiting larger glucose uptake than
VL or
RF muscle, as detected by positron emission tomography (Kalliokoski et al.
2011). This occurrence could have induced two different possible mechanisms, one more “central” and the other more “peripheral” in nature: (i) the greater metabolic involvement of the
VM might have generated greater afferent feedback from group III/IV fibers (which are sensitive to the changes in the metabolic milieu). This in turn may have induced a greater inhibition of the descending drive in
VM compared to
VL and
RF; (ii) the greater metabolic involvement of the
VM muscle during the fatiguing exercise could have likely impaired the cross-bridge cycle efficiency by a higher extent in
VM compared to
VL and
RF. Further studies are needed to explore these two possible mechanisms. Though no direct comparison with the literature can be made, our data suggest the need to distinguish the effects of exercise-induced fatigability in synergistic muscles, since they may respond differently to a fatiguing task.
In the contralateral non-exercising limb, the VA
MMG decreased similarly in all muscle heads, as observed in the MMGp-p
MVC, MMG RMS, MMG MF, sEMG RMS, and sEMG RMS/M-wave, with smaller reductions than the exercising limb. Regarding VA, the crossover effect for VA
MMG may be ascribed to central mechanisms, since the contralateral non-exercising muscle was not involved in the fatiguing exercise (as also confirmed by the similar M-waves and potentiated force values in POST compared to PRE). Although this approach is novel in the literature, a previous study showed that the contralateral non-exercising stretch-induced effects in the MMG signal were related to the central but not the peripheral components (Cè et al.
2020a). This may also account for the lack of between-muscle difference observed in the contralateral non-exercising muscle, suggesting that the greater fatigue-induced decrease in VA
MMG in
VM of the exercising limb could be due mainly by peripheral mechanisms.
The baseline measurements for VA and VA
MMG and the subsequent changes showed
moderate correlations irrespective of the muscle in both the exercising and the contralateral non-exercising limb. VA and VA
MMG are the expression of voluntary activation mechanisms, including the motor drive, the Ca
2+ transient, and the level of muscle activation (Orizio
1993; Orizio et al.
2003; Gobbo et al.
2006; de Haan et al.
2009). However, the previous studies have found that MMGp-p
SUP amplitude has a greater correlation with the level of fascicle shortening during contraction than with superimposed twitch amplitude (Ohta et al.
2007,
2009). Such a discrepancy between the two variables fits the equation for calculating VA
MMG and VA, and could explain the difference between the percentage of exercise-induced decrease in VA
MMG and VA reported here. Finally, the lack of correlation between VA
MMG and the sEMG variables is likely due to the different nature of the mechanisms of their occurrence; while VA
MMG is also affected by the excitation–contraction coupling and the muscle mechanical properties, the sEMG variables are influenced by the excitability of the motoneurons and the sarcolemma’s action potential transmission properties (Orizio
1993; Merletti et al.
2003; Cè et al.
2015).
The VA
MMG data resulted in
very high inter-session reliability, accompanied by small SEM%. Concurrently, the low levels of MDC
95% observed for the VA
MMG highlight adequate sensitivity to detect exercise-induced variations in the muscle heads in both limbs. The observed reliability of VA is shared by a previous study (Cè et al.
2020a), whereas the reliability of VA
MMG is novel and cannot be compared. Moreover, the MMG signal crosstalk values spanned from 4 to 11%, in line or lower than the data reported in the previous studies (Beck et al.
2010; Islam et al.
2014; Ismail et al.
2021). These results suggest that the outcomes regarding VA
MMG behavior were minimally influenced by possible methodological biases derived from this new approach.
The present study has several limitations. First, since the between-muscle difference was not determined by any metabolic assessment, no deeper mechanistic explanation for the observed results can be given. Second, our observations are related to the muscles investigated and to the fatiguing task used here; other synergistic muscles or other types of exercise may yield different findings. Similarly, the present results refer to the present population and should not be generalized (e.g., in women). Moreover, as recently confirmed by a review (Dotan et al.
2021), there are several limitations related to the use of the twitch interpolation technique in assessing VA. The use of other approaches such as the transcranial magnetic stimulation could have provided a more accurate evaluation of VA. Finally, as reported in a previous investigation (Mira et al.
2017), the 2 min delay between the end of exercise and the post-fatigue assessments could have possibly introduced an underestimation of the decrease in VA, as central fatigue has been shown to recover quickly after exercise.