Shear elastic modulus can be used to estimate an index of individual muscle force during a submaximal isometric fatiguing contraction

Abstract

The present study was designed to determine whether fatigue alters the ability to estimate an index of individual muscle force from shear elastic modulus measurements ( experiment I), and to test the ability of this technique to highlight changes in load sharing within a redundant muscle group during an isometric fatiguing task ( experiment II). Twelve subjects participated in experiment I, which consisted of smooth linear torque ramps from 0 to 80% of maximal voluntary contraction (MVC) performed before and after an isometric fatigue protocol, beginning at 40% of MVC and stopped when the force production dropped below 30% of MVC. Although the relationships between modulus and torque were very similar for pre- and postfatigue [root mean square deviation (RMSdeviation) = 3.7 ± 2.6% of MVC], the relationships between electromyography activity level and torque were greatly altered by fatigue (RMSdeviation = 10.3 ± 2.6% of MVC). During the fatiguing contraction, shear elastic modulus provided a significantly lower RMSdeviation between measured torque and estimated torque than electromyography activity level (5.7 ± 0.9 vs. 15.3 ± 3.8% of MVC). Experiment II performed with eight participants consisted of an isometric knee extension at 25% of MVC sustained until exhaustion. Opposite changes in shear elastic modulus were observed between synergists (vastus medialis, vastus lateralis, and rectus femoris) of some participants, reflecting changes in load sharing. In conclusion, despite the fact that we did not directly estimate muscle force (in Newtons), this is the first demonstration of an experimental technique to accurately quantify relative changes in force in an individual human muscle during a fatiguing contraction.

Effect of vastus lateralis fatigue on load sharing between quadriceps femoris muscles during isometric knee extensions

“…2). In light of previous work (Bouillard et al 2012a), the saturation level of the device (266 kPa) was not expected to be reached. However, we checked that the saturation level was never reached for any ROI of any participant.…”

“…Finally, this inability to use EMG to accurately estimate muscle force is confounded further during fatiguing tasks, where the presence of neuromuscular fatigue impairs the relationship between surface EMG activity and force (Dideriksen et al 2011;Edwards and Lippold 1956;Enoka and Stuart 1992;De Luca 1984), leading to unacceptable errors. For instance, an error of Ϸ20% of the maximal voluntary contraction (MVC) was observed when muscle force was estimated from the EMG during an isometric fatiguing contraction performed at 40% of MVC (Bouillard et al 2012a). These limitations may explain the lack of reliable information regarding the effect of neuromuscular fatigue on load sharing strategies.…”

“…A strong linear relationship exists between muscle shear elastic modulus measured by a shear wave elastographic technique (supersonic shear imaging, SSI) and individual muscle force (Bouillard et al 2011), and this relationship is not altered by fatigue (Bouillard et al 2012a). Consequently, SSI can be used to quantify relative changes in force for an individual muscle during isometric fatiguing contractions (Bouillard et al 2012a).…”

“…Consequently, SSI can be used to quantify relative changes in force for an individual muscle during isometric fatiguing contractions (Bouillard et al 2012a). Taking advantage of this experimental technique, Bouillard et al (2012a) reported evidence of changes in load sharing among the knee extensors during a sustained isometric fatiguing contraction. However, this was not observed in all participants, and when it was observed participants did not exhibit the same strategies, i.e., compensations did not systematically occur between the same muscles.…”

“…Second, we investigated the effect of prefatiguing VL on the changes of load sharing that occurred throughout a sustained contraction performed until task failure. Shear wave elastography was used to measure muscle shear elastic modulus, as this is considered to be an index of individual muscle force (Bouillard et al 2011(Bouillard et al , 2012a(Bouillard et al , 2012b. We hypothesized that 1) localized fatigue on VL would induce a systematic decrease in the force produced by this muscle, compensated by an increase in the force produced by the other extensors, and 2) fatigue on VL would lead to similar changes in load sharing between individuals during the sustained fatiguing contraction, suggesting a common compensation strategy.…”