Muscle Shear Moduli Changes and Frequency of Alternate Muscle Activity of Plantar Flexor Synergists Induced by Prolonged Low-Level Contraction

Abstract

During prolonged low-level contractions, synergist muscles are activated in an alternating pattern of activity and silence called as alternate muscle activity. Resting muscle stiffness is considered to increase due to muscle fatigue. Thus, we investigated whether the difference in the extent of fatigue of each plantar flexor synergist corresponded to the difference in the frequency of alternate muscle activity between the synergists using muscle shear modulus as an index of muscle stiffness. Nineteen young men voluntarily participated in this study. The shear moduli of the resting medial and lateral gastrocnemius muscles (MG and LG) and soleus muscle (SOL) were measured using shear wave ultrasound elastography before and after a 1-h sustained contraction at 10% peak torque during maximal voluntary contraction of isometric plantar flexion. One subject did not accomplish the task and the alternate muscle activity for MG was not found in 2 subjects; therefore, data for 16 subjects were used for further analyses. The magnitude of muscle activation during the fatiguing task was similar in MG and SOL. The percent change in shear modulus before and after the fatiguing task (MG: 16.7 ± 12.0%, SOL: −4.1 ± 13.9%; mean ± standard deviation) and the alternate muscle activity during the fatiguing task (MG: 33 [20–51] times, SOL: 30 [17–36] times; median [25th–75th percentile]) were significantly higher in MG than in SOL. The contraction-induced change in shear modulus (7.4 ± 20.3%) and the alternate muscle activity (37 [20–45] times) of LG with the lowest magnitude of muscle activation during the fatiguing task among the plantar flexors were not significantly different from those of the other muscles. These results suggest that the degree of increase in muscle shear modulus induced by prolonged contraction corresponds to the frequency of alternate muscle activity between MG and SOL during prolonged contraction. Thus, it is likely that, compared with SOL, the alternate muscle activity of MG occurs more frequently during prolonged contraction due to the greater increase in fatigue of MG induced by the progression of a fatiguing task.

Changes in the Viscoelastic Properties of the Vastus Lateralis Muscle With Fatigue

“…However, our results are in contrast to a previous study (Akagi et al, 2019), which showed an increase in resting VL µ (21.7 ± 32.4%) after a low-intensity prolonged fatiguing task of the KE muscles. Akagi et al (2017) suggested that resting muscle stiffness could be higher after prolonged fatiguing task due to muscle failure to fully relax and cramps. The well-known task dependency of fatigue needs to be considered to better understand discrepancies concerning changes in muscle stiffness and fatigue.…”

“…Akagi et al (2019) showed an increase in resting VL µ (21.7 ± 32.4%) after a low-intensity prolonged fatiguing task of the KE muscles. Akagi et al (2017) suggested that resting muscle stiffness could be higher after prolonged fatiguing task due to muscle failure to fully relax and cramps. Moreover, it is not known whether the changes in viscoelastic properties of muscles contribute to restoration of force transmission capacities during the recovery period.…”

Ultrasound Elastography in Musculoskeletal Radiology: Past, Present, and Future

“…[35][36][37] There is no consensus in the literature on the effect of age on muscle stiffness, with some authors reporting increasing and others decreasing muscle stiffness. [38][39][40] A few important factors may be debated regarding this disagreement. First, the difference in the evaluation protocol resulted in varied USE values based on the type of functional muscle assessment.…”

Modelling indentation of human lower-limb soft tissue: simulation parameters and their effects

“…The comparison of the obtained numerical results with the experimental response suggested that many modelling assumptions resulted in overestimation of the soft-tissue stiffness. Considering more recent studies focusing on measuring mechanical properties [57][58][59][60][61][62][63][64][65], a lower range of elastic moduli was reported (see Table 2). It is thus important to consider lower values for material parameters that correspond to Although the indentation experiments were used extensively to characterise the in vivo soft tissue behaviours of the lower limb, there were only a few studies that investigated the distributions of stress and strain in the tissue directly beneath the indenter.…”