Fatigue in selected lower limb muscle groups while walking in a full pressure suit
Dyck, John Waldo, Jr.
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The purpose of this research was twofold; first the purpose was to use the technique of frequency analysis to interpret EMG signals generated in an operational situation which involved isotonic exercise to develop baseline indexes of local muscle fatigue and, second, to fill descriptive gaps about changes in the EMG signal as selected muscle groups were exercised to develop fatigue. A unique combination of exercise, electrodes, muscle groups, and analysis were used to capture EMG signals from the thigh and calf muscle groups with surface electrodes while each of four subjects walked with and without a full pressure suit at 1 mph, 2 mph, and 3 mph. The purpose of the full pressure suit was to load the selected muscle groups and to provide muscle fatigue data from loaded muscles for comparisons with muscle fatigue data from unloaded muscles. The captured EMG signals from the selected muscle groups were recorded and analyzed with the frequency analysis technique to provide the percentage of EMG power in a low, broad frequency band (3-30 Hz). Results of the non-parametric statistical analyses and graphic comparisons showed the individual activity of the muscle groups and the interdependent relationships between the effect of walking speed and suited or unsuited conditions on each muscle group. These relationships were interpreted from the patterns of EMG percentages in the time-series samples taken at five-minute intervals during each 15-minute exercise trial. The individualized activity of each muscle group is dependent on the walking speed and whether the walking speed was accomplished in the suited or unsuited condition. The muscle activity, as measured by EMG percentages, can be easily differentiated into suited and unsuited conditions at 1 mph, less easily at 3 mph, and cannot be differentiated at 2 mph. The EMG percentages differentiated the activity of the muscle groups at each walking speed for the suited calf and thigh muscle groups and the unsuited thigh, but the results for the unsuited calf muscle group were not as obvious. The results for the unsuited calf at each walking speed were mixed, i.e., the different walking speeds were not distinguished by different patterns of EMG percentages. There was not a definite pattern of the highest and lowest EMG percentages at the different walking speeds, i.e., neither the calf nor thigh muscle groups consistently generated the highest EMG percentages across the three walking speeds and/or the suited and unsuited conditions. The original operational definition of muscle fatigue, which focused on an EMG power shift at the onset of muscle fatigue, was altered in favor of an operational definition that used the largest EMG percentage to indicate muscle fatigue. By using the largest EMG percentage as a criterion for muscle fatigue, several relationships evolved between the independent variables to give an indication of muscle fatigue which, for the EMG data collected in this research, was not available with the original operational definition. The largest EMG percentage showed that the thigh and calf muscle groups fatigued at different rates with the calf muscle group fatiguing before the thigh muscle group. Also, both muscle groups fatigued in the suited condition before they did in the unsuited condition. The largest EMG percentage showed different rates, or times, at which muscle fatigue occurred for each walking speed, e.g., at 1 mph fatigue occurred after approximately 12 minutes of exercise for both the unsuited calf and thigh, but after seven minutes and five minutes for the suited thigh and calf, respectively. At 2 mph, the suited and unsuited thigh fatigued after 15 minutes of exercise, the suited thigh after ten minutes, and the suited calf after approximately seven minutes of exercise. Then, at 3 mph, the unsuited calf and thigh fatigued after ten minutes of exercise, the suited thigh after seven minutes, and the suited calf after five minutes of exercise. From a different perspective, fatigue occurred after about seven minutes of exercise for the suited thigh at 1 mph, the suited calf at 2 mph, and the suited thigh at 3 mph. There was no difference in the time of fatigue for the unsuited calf at 2 mph, or the unsuited calf and unsuited thigh at 3 mph. The unsuited calf and thigh fatigued after approximately 12 minutes at 1 mph and the suited and unsuited thigh at 2 mph. These rates of fatigue indicate that, generally, the onset of fatigue occurred earlier at 1 mph and 3 mph than at 2 mph. At first this result may appear contradictory, but all the subjects agreed that ivalking at 2 mph was the most comfortable, natural pace, while walking at 1 mph was unnatural and awkward, and walking at 3 mph was truly "fatiguing". It was concluded that the frequency analysis technique is an operationally feasible, and valuable, tool for interpreting the complex interference pattern of the electromyogram from muscles involved in isotonic exercise for the purpose of describing the onset of local muscle fatigue, and detailing rates of local muscle fatigue. Future research using the frequency analysis technique should investigate two parallel paths. The first would be to further develop the exploratory/baseline muscle fatigue data from this research with more elaborate research designs which include frequent time-series sampling to further detail the onset of muscle fatigue and delineate levels of muscle fatigue. The second, and parallel, research effort should attempt to find the behavioral, correlates that accompany various levels of muscle fatigue in the hope of providing useful information to industrial designers that will help them predict the onset of muscle fatigue.