History-Dependence of Muscle Excitation and Oxygenation During Isometric Knee Extension Force Production

Over the years, the mechanisms of the change in physiological variables and their mechanical consequences have been extensively studied. The mechanical outcome forces demonstrate the proportional relationship with the parameters of the electromyography (EMG) and Near-infrared spectroscopy (NIRS) [1-3]. The NIRS is a non-invasive technique that has been applied to quantify the local oxidative metabolism of the tissues (e.g., muscle) during exercise. In particular, the NIRS measures the total hemoglobin (tHb) and tissue oxygen saturation index (TSI) during voluntary activities, which are indicative of regional blood volume [4] and dynamic balance between oxygen supply and consumption [5], respectively. Skeletal muscles are physiological structure whose primary function is to convert chemical and electrical resources to the mechanical outcome. The changes in force demand, therefore, yield the adjusted levels of physiological parameters of muscle activation [6]. The process of muscle activation requires metabolic energy, and the mechanism of oxidative phosphorylation is responsible for the generation of the muscular energy source for operating the cross-bridge cycle [7,8]. The EMG amplitude and patterns were linearly related to the steady-state levels of outcome forces during isometric contraction [9-11], and the studies that employed PURPOSE: This study examined the issue of history-dependence on muscle excitation and oxygenation by using surface electromyography (EMG) and near-infrared spectroscopy (NIRS). METHODS: Eight male participants performed isometric knee extension force production tasks that started at four levels of initial force production (30, 40, 60, 70% of maximal voluntary contraction, MVC) and commonly converged to the production of 50% of MVC by increasing or decreasing knee extension forces. We quantified the integrated electromyogram (iEMG), total hemoglobin (tHb), and the desaturation rate of muscle tissue (TSISLOPE) of the vastus lateralis and compared the differences in the quantified variables between the experimental conditions. RESULTS: For the four levels of initial forces production, all the variables showed significant linear relationships with outcome forces. However, the magnitudes of the variables were varied depending on the contraction history at the terminal phase where the identical level of force production (50% of MVC) was required. At the terminal phase, the iEMG was affected by the time history of both mode (e.g., increment or decrement) and magnitude (e.g., 10% or 20% change of MVC), while the tHb and TSISLOPE showed significant differences between the time history of mode only. CONCLUSIONS: The main result of the current study demonstrates the history-dependence of the changes in physiological demands in muscle activation. In particular, the indices of muscle excitation and oxygenation at the same level of force were varied depending on the time history of contraction, which implies the indices of muscle excitation and oxygenation may not be invariant component to specify the levels of outcome forces.