Finger flexor contractile properties and hemodynamics following a sustained submaximal contraction: A study using electrical stimulation and near-infrared spectroscopy

We examined the effect of a low-level sustained contraction on the muscle contractile properties, hemodynamics and oxygenation of the flexor digitorum superficialis muscle (FDS) of the finger. We tested the hypothesis that hemodynamics and oxygenation, reflecting the muscle metabolic characteristics, would recover more quickly than the muscle contractile properties. Eleven subjects (26 +/- 4 years) were equipped with electrodes for electrical stimulation and a near-infrared spectroscopy (NIRS) probe on the forearm over the FDS. The experimental protocol consisted of three baselines measurements (-60, -30 min and pre-exercise), immediately after a sustained 15-min contraction of the FDS at 10% maximal voluntary contraction (post-exercise), and after 30, 60 and 120 min of recovery. For each time point, participants were subjected to a battery of test that included upper arm venous occlusion (at rest), a computer-mouse point and click task (standardized voluntary task), and electrical stimulation. For venous occlusion (50 mmHg, 1 min), slopes were calculated for NIRS-derived total hemoglobin (HbTslope) and deoxyhemoglobin (HHbslope) as estimates of blood flow and oxygen consumption, respectively. The computer-mouse task entailed using the mouse to point and click on targets presented on the screen during which NIRS signals were monitored for determination of change in total hemoglobin (Delta HbT) and oxygen saturation (Delta StO(2%)). Electrical stimulation (2 Hz, 5 trains of 15 twitches) provided twitch force (Tw-force), contraction time (CT) and one-half relaxation time (1/2RT) data. Statistical analysis revealed significant changes over time for all contractile parameters as well as for HHbslope (P < 0.05 for each). Post-hoc testing demonstrated significant decreases for Tw-force post-exercise and at 60 min: for CT at post-exercise, 30 and 60 min: and for 1/2RT at post-exercise and at 30 min. HHbslope was significantly higher post-exercise as compared to pre-exercise. During the computer-mouse point and click task, no significant differences were detected for Delta HbT, however, Delta StO(2%) showed a tendency to decrease, albeit not significant (P = 0.11). Further testing showed Delta StO(2%) was significantly lower post-exercise and at 30 min as compared to pre-exercise. The present study shows that NIRS provides insight into muscle hemodynamics and oxygenation for low-level sustained activity to fatigue. The overall quick recovery of hemodynamic and oxygenation responses, and a more prolonged recovery of contractile responses confirms our hypothesis, and this may fit well with the classical definition of low-frequency fatigue. Relevance to industry: Understanding changes in tissue hemodynamics, oxygenation and muscle contractile properties in response to sustained contractions may provide insight into mechanisms behind work-related musculoskeletal disorders. Detecting changes in the underlying physiology of the muscle before the onset and development of an injury, may lead to primary prevention methods to reduce the occurrence of such injuries. (c) 2009 Elsevier B.V. All rights reserved.