Real-time detection of fatigue effect on active muscle hemodynamics using diffuse correlation spectroscopy

The effect of fatigue on the hemodynamic responses of active skeletal muscle has yet to be fully understood due to the limited evaluation techniques of deep tissues hemodynamics We adopted diffuse correlation spectroscopy (DCS), a promising optical imaging technique of tissue blood flow velocity, to determine fatigue-related changes in blood flow and vascular conductance of active muscle. Hemodynamic and electrophysiological responses of the flexor digitorum superficialis were continuously monitored by DCS and electromyogram (EMG) from the non-dominant forearm of young-adult participants (n=12). Systemic blood pressure was monitored from the other side of upper arm every minute. Participants performed 2 minutes of dynamic handgrip exercise (with a duty cycle of 2-s contraction and 2-s relaxation) with the load of 10% or 30% of maximal voluntary contraction (MVC) twice. To induce muscle fatigue, static handgrip (40% MVC for 2 minutes) was performed between the two sessions of dynamic handgrip exercise. Induction of fatigue was confirmed by significant decreases of EMG mean frequency in both load conditions. Muscle fatigue induced significant increase of active muscle blood flow and mean blood pressure in both load conditions. Vascular conductance of active muscle showed tendency of (10% MVC) or significant (30% MVC) increase after fatigue induction. These results suggest the enhanced vasodilatation along with fatigue in active muscles, possibly derived by accumulated metabolic and vasodilator substances. Our results demonstrate the possible application of DCS to detect and evaluate the fatigue effect of active skeletal muscle.