Circadian (24 hour) rhythms in respon- ses to exercise have been the ... THE JOURNAL OF SPORTS MEDICINE AND PHYSICAL FITNESS. September 1988 ...
EFFECT OF THE CIRCADIAN RHYTHM IN BODY TEMPERATURE ON OXYGEN UPTAKE DAVID W. HILL, Ph.D., K I R K J. CURETON, PI?.D., MITCHELL A. COLLINS, Ed.D., S. CHERYL GRISHAM From t h e Physical Educntiorz Departn7eizt, Urziver-sity o f Georgia, Atlzens, GA, U.S.A.
Circadian (24 hour) rhythms in responses to exercise have been the focus of recent study and review.' I t is not clear if there is a causal relationship between the rhythms in body temperature and in responses to exercise. While a change in body temperature due, for example, to a pyrogen alters metabolic rate, this Qlo effect generally is not noted when temperature change is due to circadian variation. However, it has been reported that part of the daily variation in resting VOz is explained by the rhythm in body temperature? The purpose of this note is to describe the relationships between the rhythm (i.e., the morning-to-afternoon difference) in resting body temperature and the rhythms in metabolic resporlses to graded cycling exercise.
Method Subjects and methods. Subjects and methods are described in our related paper. Briefly, once in the morning (AM) and once in the afternoon (PM), subjects rested for 30 minutes in a darkened room. Oral body temperature was measured using a Yellow Springs telethermometer placed sublingually for 5 minutes. Then, subjects performed cycling exercise tests with work at 60 watts for 4 minutes, and work rate incremented 20 watts-min-' thereafter. 0 0 2 was measured at each work rate. Ventilatory threshold was de310
termined from the break-point in ventilatory equivalent for oxygcn plotted against VO2.l
Data analysis. In addition to measuring v O ~ ,we calculated the rate of increase in VO2 from one work rate to the next (i.e., the slope of the VOz-work rate relationship). This slope is similar to the delta efficiency described by Gaesser and Brooks;' however, our slope value should not be compared to theirs, nor interpreted as a measure of efficiency, as our subjects were not a t steady state. Because diurnal variations (i.e., AM-PM differences) in VOz were more pronounced a t work rates above the ventilatory threshold, the OOz and 002-work rate slope were calculated separately for the work rates above and below the ventilatory threshold. Correlations were calculated between body temperature and both 0 0 2 and VO2-work rate slope, above and below the ventilatory threshold, for both AM and PM tests. In addition, relationships between the rhythms were quantified by the correlations between the AM-PM differences in temperature and the AM-PM differences in ii02 and VOz-work rate slope, above and below the ventilatory threshold. Results Data were analyzed for 27 subjects who completed the minute at 160 watts. Venti-
THE JOURNAL OF SPORTS MEDICINE AND PHYSICAL FITNESS
September 1988
T A B L EI.-Mean
w i t h ( S E ) IiOz in the AM and PM. WORK RATE (\r,at!s)
Tcsr rimc 60
80
100
I20
140
160
AM
1.07 ** (0.02)
1.22 * (0.02)
1.40 ** (0.02)
1.66 ** (0.02)
1.86 ** (0.03)
2.12 ** (0.03)
PM
1.14 (0.02)
1.27 (0.02)
1.SO (0.02)
1.74 (0.03)
1.99 (0.02)
2.26 (0.03)
*Different from PM value, p
