BACKGROUND: Skeletal muscle (SkM) is a dynamic tissue that contracts when stimulated, but also adapts to repetitive use. One such adaptation improves SkM Endurance. Daily activities living employ different Skm’s differently. For example, walking uses the SkM’s of the quadriceps extensively but only minimally activates the bicep brachii. The Endurance Index (EI) is a tool to assess SkM endurance independent of perceived effort. Therefore, the purpose of this study was to determine whether daily physical activity patterns influence the Endurance Index in SkM’s of the upper and lower appendages. METHODS: Study participants were assessed for voluntary maximal torque with an isokinetic dynamometer and SkM endurance by accelerometer-based mechanomyography (aMMG) before and after fatiguing exercise for their dominant arm and leg. Muscle fatigue was induced in the elbow flexors and knee extensors via 50 maximal voluntary concentric contractions at 60°/sec. aMMG was assessed for the biceps brachii (BB) and vastus lateralis (VL) of the dominant limb. Surface electrodes delivered a 20 to 25 mAmps current at 6 Hz for 5 min. A 3-dimensional accelerometer fixed to the belly of the BB or VL recorded the contractions in units of gravity. These units were normalized to baseline. Data were analyzed by repeated measures ANOVA. RESULTS: Maximal voluntary torque decreased by 44±11% in leg extensors and by 38±7% in elbow flexors following the fatigue test (P<.05). A significant interaction (P<.05) between LimbsXPre/PostXTime indicates that the BB fatigue similarly to the VL at baseline, but contraction-induced accelerations remain depressed in the BB after fatiguing exercise when compared to the VL (BB Pre: .66±.2; BB Post: .33±1; VL PRE: .8±.2; VL Post: .74±.4). CONCLUSION: These data support the use of EI to assess a SkM’s resistance to fatigue or endurance. Furthermore, these data provide support to determine whether the EI can detect changes in a SkM’s endurance with either resistance or endurance training.

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