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).
Shear modulus was measured on either the vastus lateralis (n = 9) or the abductor digiti minimi (n = 10) before and after 15 min 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.
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 vastus lateralis (P < 0.031) and 44% ± 19% lower for the abductor digiti minimi (P < 0.001) POST-fatigue.
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.
1University of Saint-Etienne, Inter-university Laboratory of Human Movement Science, University of Lyon, Saint-Etienne, FRANCE
2Laboratory “Movement, Interactions, Performance,” Faculty of Sciences and Technologies, Department of Sport Sciences, Le Mans University, Le Mans, FRANCE
3Laboratory “Movement, Interactions, Performance,” Faculty of Sport Sciences, University of Nantes, Nantes, FRANCE
4Institut Universitaire de France, Paris, FRANCE
5Health and Rehabilitation Research Institute, Faculty of Health and Environmental Sciences, Auckland University of Technology, Auckland, NEW ZEALAND
Address for correspondence: Thomas Lapole, Ph.D., Laboratoire Interuniversitaire de Biologie de la Motricité, Bâtiment IRMIS, 10 rue de la Marandière, 42270 Saint Priest en Jarez; E-mail: email@example.com.
Submitted for publication February 2019.
Accepted for publication June 2019.
Online date: July 2, 2019