G. Atkinson et T. Reilly, EFFECTS OF AGE AND TIME OF DAY ON PREFERRED WORK RATES DURING PROLONGED EXERCISE, Chronobiology international, 12(2), 1995, pp. 121-134
This study was designed to examine the effects of age and time of day
on work rates during prolonged, self-paced exercise. Eight young (19-2
5 years of age) and eight old (48-62 years of age) endurance athletes
volunteered for the study. At two times of day (07:00 and 17:00 h), su
bjects were asked to pedal on a Monark cycle ergometer (Varberg, Swede
n) at a self-chosen exercise intensity that they believed they could s
ustain for exactly 80 min. This self-chosen work rate rectal temperatu
re, skin temperature (chest, arm, and lower leg), oxygen consumption (
VO2), expired carbon dioxide (VCO2), minute ventilation (VE), heart ra
te, and perceived exertion (RPE) were recorded every 10 min during the
exercise. Preexercise resting measures of rectal temperature, VO2 and
VE were less affected by time of day in the older group than were tho
se in the young subjects (p < 0.05). In the morning, rectal temperatur
e was 0.3 degrees C higher in the older subjects than in the young adu
lts. Diurnal variation in mean work rate over the 80-min exercise peri
od was not evident in the old group (p > 0.10) but amounted to 10 W in
the young group (p < 0.05). Older subjects chose work rates 5.4 W low
er than did the young subjects in the morning test session (p > 0.10).
In the afternoon, age differences in work rate amounted to 14.3 W (p
< 0.05). For all subjects, work rates remained relatively constant thr
oughout the exercise period in the morning. In the afternoon, subjects
chose high work rates within the first 40 min of exercise, after whic
h work rate decreased sharply to values similar to those recorded in t
he morning (p < 0.01). These changes were mirrored closely by changes
in (VO2) and VCO2. Perceived exertion increased linearly throughout ex
ercise, irrespective of age or time of day. These results suggest that
, in young adults, the mean work rate over 80 min of exercise is highe
r in the afternoon than in the morning, although the work rate decreas
ed sharply toward the end of afternoon exercise. In agreement with stu
dies reporting age-related increases in ''morningness:'' age differenc
es in work rate appeared to be least when exercise was performed in th
e morning.