Abstract
This study aimed to examine the relationship between muscle output and action potential amplitude measured with an electromyogram (EMG) to clarify the influence of instruction conditions on the evaluation of rhythmic repeated muscular endurance. Twelve healthy young men performed repeated maximal rhythmic gripping for 6 min. (180 times) under the following two instruction conditions: 1) “Squeeze as hard as you can quickly at the cue of a beep every two sec (Exercise 1: Ex1)” and 2) “Squeeze as hard as you can quickly at the cue of a beep every two sec, and then hold the exertion for one sec (Exercise 2: Ex2)”. EMG was recorded during the muscular endurance measurement in both conditions.
The relative value of the action potential amplitude in each condition to the maximal action potential amplitude at the time of maximal voluntary contraction (MVC) showed a significant and lower value in the middle or latter half of Ex1 compared with the first half of Ex1. However, such a change with time was not observed in Ex2. The relative value of output (%Peak) based on MVC and the integrated area of force (Int F) of muscular output of each condition significantly decreased with time in Ex1 and Ex2. After the middle stage of measurement, the %Peak of Ex2 showed a lower value in comparison with Ex1. %EMG showed a significant and moderate correlation with Int F (r=0.60) only in the first half of Ex1.
Muscle outputs were related to EMG closely in the first half of measurement of Ex1; however, muscle outputs were not related to EMG through the entire time of Ex2. Therefore, evaluated muscular endurance may be different when instructions conditions are different.
