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Abstract

The physiological responses of negative work involving predominantly eccentric muscle contraction were compared to positive work involving predominantly concentric muscle contraction in twelve older healthy subjects between 39 and G5 years of age. A motorized cycle ergometer was used for steady state exercise testing. To study the physiological response to positive and negative work, pedalling frequencies of 35, 55, and 75 rpm and a constant power output of 60 Watts were chosen. Steady state values of oxygen consumption (VO2), heart rate (HR), minute ventilation (VE), tidal volume <VT) and breathing frequency (fb) were obtained during six test conditions, namely, positive and negative work at each of the three pedalling frequencies. All physiological measures were greater during positive work than negative work (p<0.001) except for fb (p>0.05). The greater VO2 (1.14±.0.13 and 0.62±0.15 1/min (mean+standard deviation) during positive and negative work respectively), HR (95. 8±.10. 7 and 81.8 ± 13.6 bpm) and VE (26.7±5.5 and 16.5±5.2 1/min) during positive work were consistent with the greater energy efficiency of negative work. The greater VE during positive work reflected a greater VT (1.46±.32 1/br) than negative work (0.99±.31 1/br) while fb was the same (18.7±.4.0 and 17.5±5.6 br/min) for both positive and negative work. During positive work, all physiolgical variables were greatest at 75 compared to 35 and 55 rpm (p<0.05) except for fb which showed no significant difference across the three pedalling frequencies

0.05). During negative work, VO2 and HR were greatest at 75 and 35 rpm compared to 55 rpm <p<0.05), and VE and VT were greater at 75 than at 55 rpm (p<0.05), whereas fb was not different among pedalling frequencies (p>0.05). The slopes and intercepts of the regression lines relating HR and VO2, VE and VO2, VT and VE, and fb and VE were identical between positive and negative work except for a higher intercept for the VE and VO2 relationship during negative work. Thus, it was concluded that at a power output of 60 Watts, physiological responses such as VO2, HR and VE during positive and negative work were qualitatively similar. When changes in VT and fb were compared from baseline to steady-state for positive work, however, VT and fb both increased. In contrast, for negative work, VT increased minimally while relatively greater increases in fb were observed for pedalling frequencies of 35 and 55 rpm. The relatively greater effect of negative work on fb compared with positive work is not predicted from the known ventilatory responses to low intensity exercise. Further study is needed to elucidate the precise mechanism for this predominant increase in fb during negative work.