Evaluation Of ES-induced Fatigue In Relation To Muscle Size And Fiber Recruitment

Abstract

Electrical stimulation (ES) has been used to strengthen muscles in athletic training and rehabilitation programs. ES is an effective stimulus to increase muscle force by evoking contraction via the application of electrical current. However, fast neuromuscular fatigue development limits its application in the clinical setting. The rapid fatigue onset has been attributed to differences in ES parameters and motor unit (MU) recruitment pattern compared to voluntary contractions. PURPOSE: To evaluate the ES intensity needed to generate predetermined force in two muscles of varying size and fiber type (proportion of type I and type II fibers) during isometric contractions, as well as to investigate the fatigue characteristics and time to fatigue under these conditions. METHODS: ES-induced fatigue was initiated in the small, type I abductor pollicis brevis (APB) and large, type II vastus lateralis (VL) muscles of three healthy individuals (mean age: 22.33±2.36 years). ES was delivered at three frequencies (10, 30, 50Hz) in randomized order, with a duty cycle of 4s on/4s off, pulse duration of 300μs, and stimulation intensity level required to achieve an initial tetanic force equal to 25% of the pre-fatigue MVC. ΔMVC values, required intensity, and %drop in force during ES were compared between APB and VL. RESULTS: ΔMVC values were relatively low, though higher in the VL with no apparent association to frequency (VL mean ΔMVC: 10Hz = 16.8N; 35Hz = 28.3N; 50Hz = 6.0N, APB mean ΔMVC: 10Hz = -4.0N; 35Hz = 0.32N; 50Hz = 2.38N). ES at 10Hz requires a higher intensity to achieve 25%MVC force than at 35Hz or 50Hz for both the APB (mean intensity: 10Hz = 20.3mA; 35Hz = 17mA; 50Hz = 15.3mA) and VL (mean intensity: 10Hz = 54mA; 35Hz = 46.7mA; 50Hz = 46.7mA). The %drop in force was greatest at 50Hz (APB = 21% drop; VL = 18% drop), then 35Hz (APB = 17.5% drop; VL = 19.9% drop), then 10Hz (APB = 14.75% drop; VL = 16.35% drop) at the first quartile of time to fatigue, while the second quartile values were consistent among frequencies. CONCLUSIONS: Fatigue is a major limiting factor for application of ES in sports injury and rehabilitation. Muscles with different fiber type composition respond differently to stimulation frequencies and fatigue development. These differences should be considered when developing treatment plans. Supported by NSF EFRI 1332329.