Influence of instruction conditions on the evaluation of muscular endurance based on muscle oxygenation

Proper instruction is essential for evaluating muscular endurance with high reliability. This study aimed to examine the influence of instruction conditions on the evaluation of rhythmic repeated muscular endurance based on muscle oxygenation kinetics. Fifteen healthy young men performed repeated maximal rhythmic grips for 6 minutes under the following 3 instruction conditions: (1) "Squeeze as hard as you can during 2 seconds freely (Ex1)"; (2) "Squeeze as hard as you can quickly at the cue of a beep every 2 seconds (Ex2)"; and (3) "Squeeze as hard as you can quickly at the cue of a beep every 2 seconds, and then hold the exertion for 1 second (Ex3)". In the start phase of measurement during which larger exertion (above 80% maximal voluntary grip contraction) of grip strength is possible, the exertion values and muscle oxygenation kinetics were not significantly different among the 3 instruction conditions. However, a significant difference was found in muscle oxygenation kinetics after the middle phase (3 minutes) of measurement and also in the peak forces. Particularly, oxygen supply to the skeletal muscle as compared with oxygen consumption was greater in Ex2 than in Ex3. This study found that the evaluated muscle endurance differs considerably under various instruction conditions. The following instruction may be useful in the assessment of muscular endurance: "Squeeze as hard as you can quickly at the cue of a beep and then immediately relax". The present findings suggest that proper instructions are essential in various physical function tests and in the dynamometer handgrip test for occupational therapists and strength and conditioning professionals to obtain accurate measurements.