Vastus lateralis surface and single motor unit EMG following submaximal shortening and lengthening contractions

Abstract

A single shortening contraction reduces the force capacity of muscle fibers, whereas force capacity is enhanced following lengthening. However, how motor unit recruitment and discharge rate (muscle activation) are adapted to such changes in force capacity during submaximal contractions remains unknown. Additionally, there is limited evidence for force enhancement in larger muscles. We therefore investigated lengthening- and shortening-induced changes in activation of the knee extensors. We hypothesized that when the same submaximal torque had to be generated following shortening, muscle activation had to be increased, whereas a lower activation would suffice to produce the same torque following lengthening. Muscle activation following shortening and lengthening (20 degrees at 10 degrees /s) was determined using rectified surface electromyography (rsEMG) in a 1st session (at 10% and 50% maximal voluntary contraction (MVC)) and additionally with EMG of 42 vastus lateralis motor units recorded in a 2nd session (at 4%-47%MVC). rsEMG and motor unit discharge rates following shortening and lengthening were normalized to isometric reference contractions. As expected, normalized rsEMG (1.15 +/- 0.19) and discharge rate (1.11 +/- 0.09) were higher following shortening (p < 0.05). Following lengthening, normalized rsEMG (0.91 +/- 0.10) was, as expected, lower than 1.0 (p < 0.05), but normalized discharge rate (0.99 +/- 0.08) was not (p > 0.05). Thus, muscle activation was increased to compensate for a reduced force capacity following shortening by increasing the discharge rate of the active motor units (rate coding). In contrast, following lengthening, rsEMG decreased while the discharge rates of active motor units remained similar, suggesting that derecruitment of units might have occurred.

Feedback controlled force enhancement and activation reduction of voluntarily activated quadriceps femoris during sub-maximal muscle action

“…Stretch of the activated QF led to big deviations from the target torque, and comparable torque conditions for the isometric reference and the experimental stretch-contractions could only be found beyond 8 s and 6 s after stretch for the 30% and 60% MVC trials, respectively. As described by Altenburg et al (2008), in the time intervals where there were no differences in steady-state torque control, there was a distinct average AR for the QF, but this AR was not found necessarily in all parts of the muscle simultaneously.…”

“…Altenburg et al (2008) could not find consistent AR, as different parts of QF showed inconsistent behavior after stretch. However, we speculated that if there is consistent RFE, as observed previously in human QF for maximal contractions, there should also be AR across the entire QF group.…”

“…Since RFE or AR for voluntary contractions was more consistently observed in muscles of increasing size and with increasing intensity (Pinniger and Cresswell, 2007;Herzog, 2005, 2006b), the results of Altenburg et al (2008) were quite unexpected. Assuming that the mechanisms underlying RFE and AR are the same, inconsistent AR in QF muscle parts is likely not caused by an inability of QF to produce RFE, as RFE was clearly observed in this muscle group for MVC conditions Seiberl et al, 2010).…”

“…Specifically, there is only one study about RFE for submaximal effort contractions in large muscles such as the human knee extensors (Altenburg et al, 2008). With constant torque at 10% and 50% MVC, Altenburg et al (2008) found reduced activation in quadriceps femoris (QF) after stretch but only for 10% MVC conditions and not in all measured parts of QF.…”

“…We hypothesized that QF shows consistent AR when accounting for the size and relative activation of the different parts comprising QF. In addition to Altenburg et al (2008), RFE was further analyzed for sub-maximal voluntary contractions when controlled via muscle activation.…”