Reduced Active Muscle Stiffness after Intermittent Submaximal Isometric Contractions

Abstract

Whether muscle stiffness is influenced by fatigue remains unclear. Classical methods used to assess muscle stiffness provide a global measure at the joint level. As fatigue may selectively affect specific muscles, a joint-level approach may not be sensitive enough to detect potential changes in muscle stiffness. Taking advantage of ultrasound shear wave elastography, this study aimed to determine the influence of a fatiguing protocol involving intermittent submaximal isometric contractions on muscle shear modulus (an index of stiffness). Methods: Shear modulus was measured on either the vastus lateralis (VL; n=9) or the abductor digiti minimi (ADM; n=10) before and after 15-mins of intermittent submaximal isometric contractions at 60% of maximal voluntary contraction (MVC; 4 s ON, 4 s OFF). An index of active muscle stiffness was estimated PRE and POST-fatigue as the slope of the linear regression established between shear modulus and absolute joint force up to 60% MVC. Results: After the fatiguing exercise, MVC was significantly decreased by 22 ± 7 % and 32 ± 15 % for knee extension and little finger abduction, respectively (p<0.001). When compared to PRE-fatigue, the index of active muscle stiffness was 12 ± 15 % lower for the VL (p < 0.031) and 44 ± 19 % lower for the ADM (p < 0.001) POST-fatigue. Conclusion: Although the present results cannot clearly determine the involved mechanisms, they demonstrate a decreased active muscle stiffness after a fatiguing task involving intermittent submaximal isometric contractions. Further studies should now determine whether this change in stiffness affects performance and risk of injury.

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

“…The VL active muscle stiffness index (slope) was also reduced with fatigue (−27.0 ± 10.4% relative to baseline). These results are in accordance with those showing ∼12% reduction of active stiffness of the VL after sustained isometric (Morel et al, 2019), dynamic (Vigreux et al, 1980;Hunter and Kearney, 1983), and intermittent isometric (submaximal) contractions (Zhang and Rymer, 2001). There are several possible explanations for these phenomena.…”

“…It has been recently reported that VL µ is significantly reduced after maximal (−34.7 ± 6.7% at rest; Siracusa et al, 2019) and submaximal (−22 ± 7% during contractions; Morel et al, 2019) voluntary fatiguing isometric contractions of the KE muscles. Andonian et al (2016) showed a decrease (∼−8%) in resting µ of the quadriceps muscle (VL, RF, and VM) after ultraendurance running, which was still reduced more than 48 h (∼−5%) after exercise.…”

“…VL muscle was chosen because previous studies (Alkner et al, 2000;Place et al, 2007) showed that this muscle is representative of the muscle quadriceps. Moreover, studies showing decreases in muscle stiffness by assessing aponeurosis or muscle tendon junction displacements or by SWE after repeated isometric contractions were made exclusively in the VL muscle (Kubo et al, 2001a;Morel et al, 2019;Siracusa et al, 2019), allowing comparisons with the present study.…”

“…However, conflicting results have been reported. Indeed, decreases in active stiffness with fatigue have been confirmed after dynamic and intermittent submaximal isometric contractions (Vigreux et al, 1980;Morel et al, 2019), but not after sustained isometric protocols (Bouillard et al, 2012a;Akagi et al, 2019). 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.…”

Medial Gastrocnemius Muscle Architecture Is Altered After Exhaustive Stretch-Shortening Cycle Exercise

“…Whereas passive stiffness may have increased, active stiffness likely decreased, as evidenced by reduced joint stiffness (Kuitunen et al, 2002;Kubo and Ikebukuro, 2019), greater peak force reduction (Avela et al, 1999b), and MVC output (Morel et al, 2019), all of which were found in the current study. Myoelectric activity, which is a major factor in active stiffness (Aura and Komi, 1986), has previously been shown to be decreased during MVC and maximal DJs after exhaustive SSC exercise (Avela et al, 1999b;Kuitunen et al, 2002;Ishikawa et al, 2006;Dousset et al, 2007).…”