Muscle Activity During Running At Reduced Body Weight.

Abstract

Running at reduced body weight (BW) may have rehabilitation advantages in that it may be possible to modulate the stress applied to the lower extremity. A unique treadmill has been designed to apply an upward directed force on the runner such that effective BW can be reduced. For example, the treadmill can be set to have the runner run at 60% of his/her BW. Recently, it has been reported that ground reaction forces (GRFs) are influenced by the variation of BW (Grabowski and Kram, JAB, 2008). The change in GRFs may be an indication that running style changes as BW is varied and we sought to understand the activity of lower extremity muscles as BW is manipulated. PURPOSE: To determine if knee flexor and extensor muscle activity is influenced by 1) magnitude of effective BW and 2) intensity of exercise across BW conditions. METHODS: Subjects (n=5, 26.4±10.6 yrs; 71±14.2 kg, 176 ± 15.4 cm) completed two minutes of running at 100, 90, 75 and 60% of BW at Rating of Perceived Exertion (RPE) levels of 11 (fairly light), 13 (somewhat hard) and 15 (hard). Order of conditions was RPE 11, 13 and 15 from highest (100%) to lowest (60%) BW. Prior to testing, subjects completed a familiarization period where they ran at different speeds and effective BWs. Measurement began by placing electromyography (EMG) electrodes on the belly of the rectus femoris (RF) and biceps femoris (BF). Data were collected (1000 Hz) during the final 30 seconds of each 2 minute run. EMG data were reduced by removing any zero offset, full wave rectifying, and then averaging muscle activity across 30 seconds. Dependent variables (average RF and BF muscle activity) were analyzed using a 3 (RPE: 11, 13, 15) × 4 (BW: 100, 90, 75, 60%) repeated measures analysis of variance. RESULTS: Neither muscle was influenced by the interaction of BW and RPE (p>0.05). RF and BF muscle activity increased across RPE levels (p0.05). CONCLUSIONS: Muscle activity was not influenced by changes in BW when subjects were allowed to adjust treadmill speed to select a specific RPE. In general, subjects selected a faster running speed as BW was reduced and/or RPE increased. The faster running speed per reduction in effective BW offset any potential reduction of muscle demand. The treadmill used in the study was provided by Alter-G, Inc.