The influence of level of spinal cord injury on the physiological and metabolic response to exercise and performance
Compared with our knowledge concerning the responses to exercise of able-bodied athletes there is relatively little known about the exercise responses of athletes who have a spinal cord injury (SCI). The purpose of this thesis was to examine the physiological characteristics of endurance trained wheelchair athletes; to assess the importance of various physiological factors to the endurance performance of wheelchair athletes; to describe and examine the physiological and metabolic responses and performances of wheelchair athletes to endurance and brief high intensity exercise; and to assess the influence of level of SCI or wheelchair racing class on these responses to exercise. The mean peak oxygen uptake (V02 pk) achieved by the group of 27 wheelchair athletes during wheelchair treadmill exercise was 2.11 ±0.53 I. min-1. When the athletes were grouped according to the paralympic racing classification system (TK2; TK3; TK4) the V02 pk values were 1.28 ±0.16 l. min-1,2.10 ±0.43 I. min-1 and 2.32 ±0.52 I. min-1 respectively. The mean peak heart rates of these groups were 112 ±4 b. min-1,190 ±9 b. min-i, and 200 ±9 b. min-1 respectively. The relationship between V02 pk and endurance performance (r=-0.69; p<0.01) was not as high as found between % V02 pk utilised at a given submaximal propulsion speed (r=0.89; p<0.01) or a propulsion speed equivalent to a reference blood lactate concentration (BLA) of 4 mmol. l-1 (r=-0.87; p<0.01). During a 10 km treadmill trial the group of athletes maintained a speed equivalent to 78.4 ±13.6% V02 pk. The TK2 racing class appeared to exhibit lower heart rates and respiratory exchange ratio (R) values throughout the test than the other racing classes. During a one hour endurance test at 80% of top speed (TS), an exercise intensity meaningful to the wheelchair athlete, the group were working at 64.6 ±13.5% V02 pk. The responses observed were indicative of steady state exercise. Oxygen uptake and ventilation rate remained stable, there was no cardiovascular drift, there was a decrease in R value, BLA peaked after 15 minutes and then decreased throughout the remainder of the test. Blood glucose (BGL) concentrations remained similar to the value observed at rest throughout the test. In general, it was found that wheelchair athletes were able to maintain a propulsion speed equivalent 75% V02 pk for prolonged periods of time, irrespective of their level of SCI. There were no differences between the physiological and metabolic responses of the two paraplegic racing classes. The tetraplegic group appeared to exhibit lower heart rates, BLA, BGL and R values throughout the duration of the test. The concentrations of plasma free fatty acid, glycerol, ammonia and urea postexercise indicated a tendency towards higher values in the wheelchair racing class with the lowest lesion level athletes (TK4). During a 20 s high intensity exercise fixed work test there was greater metabolic activity in the TK2 racing class than the TK4 racing class. The metabolic responses of the TK3 racing class to this test were, in general, between these two groups. A 30 s arm sprint resulted in a decrease in power output of greater than 50% for each racing class. The mean power outputs (MPO) generated by the TK2, TK3 and TK4 racing classes were 100.2 ±21.7 W, 188.3 ±48.9 W and 247.2 ±40.3 W respectively. The physiological and metabolic responses showed a similar tendency. A 30 s test at 80% MPO showed that the metabolic responses of each racing class were similar. The results of the thesis suggest that the amount of muscle mass available for recruitment during exercise and the degree of disruption to the sympathetic nervous system play an important role in determining the responses to exercise and the racing performance of wheelchair athletes with a SCI.