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 youngadult 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.
We investigated blood flow response for reactive hyperemia test in type1 diabetic and control rats using diffuse correlation spectroscopy (DCS). The baseline blood flow, peak amplitude and time to peak blood flow responses after releasing from ischemia were significantly decreased after induction of hyperglycemic state in diabetic rats. Furthermore, the baseline blood flow and the peak amplitude were significantly decreased from 1st week to 10th week in diabetic rats, suggesting the progress of vascular dysfunction caused by sustained glycemic stress. This research showed a potential of DCS to diagnose vascular function in diabetes that was altered by acute and chronic hyperglycemic stress.
Diffuse correlation spectroscopy (DCS) has a potential to noninvasively and quantitatively measure the blood flow in the exercising muscle that could contribute to the fields of sports physiology and medicine. However, the blood flow index (BFI) measured from skin surface by DCS reflects hemodynamic signals from both superficial tissue and muscle layer. Thus, an appropriate calibration technology is required to quantify the absolute blood flow in the muscle layer. We therefore fabricated a realistic two-layer phantom model consisted of a static silicon layer imitating superficial tissue and a dynamic flow layer imitating the muscle blood flow and investigated the relationship between the simulated blood flow rate in the muscle layer and the BFI measured from the surface of the phantom. The absorption coefficient and the reduced scattering coefficient of the forearm were measured from 25 healthy young adults using a time-resolved nearinfrared spectroscopy. The depths of the superficial and muscle layers of forearm were also determined by ultrasound tomography images from 25 healthy young adults. The phantoms were fabricated to satisfy these optical coefficients and anatomical constraints. The simulated blood flow rate were set from 0 mL/ min to 68.7 mL/ min in ten steps, which is considered to cover a physiological range of mean blood flow of the forearm between per 100g of muscle tissue at rest to heavy dynamic handgrip exercise. We found a proportional relationship between the flow rates and BFIs with significant correlation coefficient of R = 0.986. Our results suggest that the absolute exercising muscle blood flow could be estimated by DCS with optimal calibration using phantom models.
We studied blood flow dynamics of active skeletal muscle using diffuse correlation spectroscopy (DCS), an emerging optical modality that is suitable for noninvasive quantification of microcirculation level in deep tissue. Seven healthy subjects conducted 0.5 Hz dynamic handgrip exercise for 3 minutes at intensities of 10, 20, 30, and 50 % of maximal voluntary contraction (MVC). DCS could detect the time-dependent increase of the blood flow response of the forearm muscle for continuous exercises, and the increase ratios of the mean blood flow through the exercise periods showed good correlation with the exercise intensities. We also compared blood flow responses detected from DCS with two different photon sampling rates and found that an appropriate photon sampling rates should be selected to follow the wide-ranged increase in the muscle blood flow with dynamic exercise. Our results demonstrate the possibility for utilizing DCS in a field of sports medicine to noninvasively evaluate the dynamics of blood flow in the active muscles.
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