The influence of exercise/rest schedule on the physiological and psychophysical response to isometric shoulder-neck exercise

Six female subjects, aged 24-34 years, performed shoulder-neck exercise for 1 h or until they were exhausted by holding out their arms horizontally at 60° to the sagittal plane. One continuous and six intermittent protocols were applied, all with a mean load corresponding to the torque of the arms, i.e. about 15% maximal voluntary contraction (MVC). The intermittent protocols varied according to cycle time (10 s, 60 s, 360 s) and duty cycle (0.33, 0.50, 0.67, 0.83). Electromyogram (EMG), mean arterial blood pressure (_a), heart rate (f_c) and perceived fatigue were monitored at regular intervals during exercise. Blood concentrations of potassium, lactate and ammonia were determined in pre- and postexercise samples of venous blood. Before and up to 4 h after exercise, measurements were made of MVC, pressure pain threshold, proprioceptive performance, and of EMG, _a and f_c during 1-min arm-holding at 25% MVC. Endurance times ranged from about 10 min to more than 1 h, significantly relating to both cycle time and duty cycle. The _a, f_c EMG amplitude and perceived fatigue increased early during all protocols and continued to increase throughout the exercise period. Duty cycle influenced all of these variables, while only _a and fatigue perception were related to cycle time. Cardiovascular and neuromuscular recovery was incomplete for hours after several of the protocols, as indicated for example by a sensitizised response to the 1-min armholding. The protocols differed substantially as regards the relationship between different responses. Thus, ranking of the protocols in terms of physiological strain was different, depending on the criterion variable. The result stresses the relevance of applying a comprehensive selection of variables when evaluating the responses to intermittent shoulder-neck exercise.