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
= 9) or the abductor digiti minimi
= 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
< 0.031) and 44% ± 19% lower for the abductor digiti minimi
< 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.