One-handed load carrying — Cardiovascular, muscular and subjective indices of endurance and fatigue

Summary

A group of 5 women and 11 men walked on a treadmill, each carrying a weight in the right hand. In separate experiments, the mass was varied to give total exhaustion within 3 min, 5 min, 9 min, and 13 min. In additional experiments 50% and 25% of the masses leading to exhaustion after 5 min were used, and these were stopped after 16 min. Heart rate (f _c) and systolic blood pressure (BP_s) were measured noninvasively. There was a consistent increase in (f _c) x BP_s during the experiments leading to exhaustion, while steady-states were obtained in the nonexhausting trials. An electromyogram (EMG) was recorded with cutaneous electrodes over the flexor carpi ulnaris and flexor digitorum superficialis muscles and the number of zero crossings (ZC) of the EMG signal per time unit were analysed. As the subjects approached exhaustion, the number of ZC declined exponentially, reaching approximately 50% of their initial values. In the nonexhausting experiments, however, the decline was slower and less marked, and during the second half of the experiment the number of ZC increased again. Subjectively, endurance was underestimated by all the subjects. It was concluded that cardiovascular and muscle criteria of fatigue in carrying coincided. Prolonged carrying in one hand of more than 6 kg or 10 kg for young healthy women and men respectively should not be recommended, since it could lead to cardiovascular non steady-states and EMG signs of fatigue.