Comparison of forearm muscle oxygenation dynamics during isometric and isotonic contractions monitored by near-infrared spectroscopy

Introduction: By elevating intramuscular pressure and increasing muscle oxygen consumption, a sustained contraction of a limb muscle can compromise muscle metabolic status and function. In this study, we aimed to investigate the effect of isometric forearm muscle contractions at different intensities and isotonic contractions on muscle oxygenation dynamics using near-infrared spectroscopy (NIRS). Methods: The maximum voluntary contraction (MVC) force of forearm flexor muscles in the dominant arm was measured in ten healthy adult volunteers. A NIRS sensor was placed and fixed over the common flexor muscles of the forearm to monitor muscle rSO(2). A reflectance pulse oximeter sensor was placed over the common extensor muscles, and a tourniquet cuff was placed loosely around the upper arm on the same side. Following a three-minute baseline measurement, each subject was instructed to perform a series of 30-second sustained isometric flexor muscle contractions at 10%, 30% and 50% MVC using a handgrip dynamometer. A 30-second isotonic muscle contraction followed by a 30-second episode of tourniquet-induced ischemia completed the experiment. Three minutes of recovery time were allowed after each episode. Results: Similar patterns of rSO(2) changes were seen in all subjects during episodes of isometric contractions (30% MVC and 50% MVC), isotonic contractions, and ischemia. Isometric muscle contraction at 50% MVC induced the lowest level of muscle rSO(2) (-16% +/- 2.7%, p<0.0001). Conclusions: Isometric muscle contraction at 50% MVC induces a higher level of muscle hypoxia, in comparison to isotonic muscle contraction and limb muscle ischemia. Sustained isometric muscle contraction can compromise muscle oxygenation dynamics, which may expedite muscle fatigue and dysfunction.