Fitness training: Aerobic and Anaerobic exercise

Chronobiology – how timing could give you the edge

We may live in a high-technology 24/7 world, but the daily or circadian rhythm remains deeply ingrained in our physiological make-up. According to Andrew Hamilton, there’s plenty of recent research to suggest that athletes ignore this rhythm at their peril when conducting aerobic and anaerobic exercises.

Humans have evolved in and are surrounded by a world full of rhythms, and it would be incredible if these rhythms didn’t exert a significant effect on our physiological function and performance potential. In recent years, the field of chronobiology has confirmed that this is indeed the case.

Everybody is aware of the powerful circadian (daily) rhythm; it is after all what regulates your sleeping and waking patterns. However, other rhythms can also affect physiological function, although the magnitudes of their effects tend to be somewhat weaker, which can make some of them rather difficult to detect against the background of environmentally induced physiological variations.

Circadian rhythm

The circadian rhythm is the most powerful rhythm affecting humans; as well as the sleep/waking cycle, it affects hormone secretions, body temperature, mental alertness and physical performance capacity.

Due to these rhythmic fluctuations, many people experience maximum mental alertness, fastest reaction times and highest core temperature in the late afternoon/early evening period, while the peak in melatonin concentrations in the middle of the night period leads to maximum fatigue/sleepiness and lowest alertness.

It’s important to understand that while the circadian rhythm is modulated by environmental stimuli, it’s essentially a ‘free-running’ rhythm; put someone in an isolated, darkened room for a week, where they have no idea what time of day it is, and these rhythmic fluctuations will persist. However, in these free-running conditions, the circadian rhythm is not exactly 24 hours, but a little over(1, 2). Hence the need for external stimuli such as light to keep the rhythm synchronised with the 24-hour clock. It’s also true that within the basic circadian rhythm, different people may exhibit slightly differing physiological and behavioural responses.

Circadian rhythm and performance

A growing body of evidence suggests that manipulating the timing of training and/or your circadian rhythm can produce significant benefits. However, one of the problems that has beset researchers studying the effects of circadian rhythm on physical performance is that the magnitude of these effects tends to be small relative to the continual background ‘noise’ of other factors impacting on performance such as nutrition or psychological factors.

Moreover, studies of this type necessarily examine physical performance at different points in the rhythm and on different days; unless the sleep patterns remain constant between tests, significant errors can arise. There’s also evidence that the amplitude of these rhythms may be altered by varying exercise intensity, and that other rhythms can interfere with the circadian rhythm (especially the monthly menstrual rhythm in women(3)).

For example, a UK study published just over a year ago looked at blood lactate concentrations (a marker of physiological fatigue during endurance exercise) in 11 trained female endurance athletes at rest and during the final stage of an incremental multi-stage test on a Concept II rowing ergometer(4). Researchers were keen to discover what exercise intensity was required to produce a blood lactate concentration of 4mmol/l at different times of day (06.00h and 18.00 h), at two phases of the menstrual cycle (the midfollicular phase and the midluteal phase). The results showed that at the midluteal phase of the menstrual cycle, the 4mmol/l threshold occurred at a significantly higher exercise intensity, heart rate and oxygen consumption than it did in the midfollicular phase. But when researchers looked for a time of day interaction effect, none was evident.

Benefits of afternoon/evening training

However, the results above contrast sharply with a study by the same group of researchers on circadian rhythms and lactate threshold in male rowers the previous year(5). Eleven male athletes followed the same incremental protocol described above, but this time rowing at 02.00, 06.00, 10.00, 14.00, 18.00 and 22.00 hours (on separate days with full recovery between tests). The researchers gathered the data and, using statistical analysis, found that the oxygen consumption needed to produce a blood lactate concentration of 4mmol/l peaked at 21.39h while the highest heart rate needed to reach this threshold occurred at 20.32h. In plain English, the rowers were able to work most intensely for a given build-up of lactate around about 9pm in the evening, which also coincided with their peak core temperature. It also suggested that there was an interaction between menstrual and circadian rhythms in the female rowers previously mentioned.

More evidence for the link between circadian rhythm and aerobic/anaerobic performance came from a US study that evaluated the effect of the time of day on high intensity, constant-power cycle ergometry performance by both men and women(6). Fourteen subjects performed the tests both in the morning and the afternoon in randomised order. The load was set at 5 watts per kilo of body for the women and 6 watts per kilo for men (intense – a 75kg male would be working at 450 watts!). Compared to the morning, the total work performed was 9.6% greater in the afternoon and this afternoon work was associated with a 5.1% higher aerobic power and a 5.6% larger anaerobic contribution. Moreover, the trend was equally strong in both the men and the women.

A much more recent study looked at anaerobic power developed in 30-second cycling tests carried out at different times of the day(7). In this study, French researchers looked at the force and velocity of muscular contractions during cycle ergometry of 19 subjects tested at 02.00h, 06.00h, 10.00h, 14.00h, 18.00h and 22.00h on separate days, and how closely correlated to core temperature any performance changes were. The results were as follows:

  • Peak core temperatures occurred at just before 18.30h;
  • Maximum peak power tended to occur just before 17.30h (7.6% higher than average peak power);
  • Maximum mean power occurred at 18.00h (11.3% higher);
  • The changes in power output and core temperature were strongly associated (indicating that this was a circadian rhythm effect).

The researchers concluded that athletes could benefit by recording their temperature and timing their bouts of subsequent anaerobic training to coincide with peaks in their circadian rhythm.

Strength and circadian rhythm

Research into how circadian rhythms affect performance is not limited to anaerobic power/lactate studies. A 2002 UK study on the effects of circadian rhythm on strength found that the time of day affected maximal lifting strength in young female subjects with an 8% increase in maximal strength at 18.00h compared to 06.00h(9). However, this effect was only observed in the luteal phase of the cycle; in the follicular phase, there was no discernible effect.

A more recent Iranian study, published 16 months ago, looked at isometric and isokinetic leg strength in eight women during the follicular phase only of the menstrual cycle (to prevent any masking effect), under conditions of both adequate sleep and partial sleep loss(10). The researchers also assessed the strength of involuntary contractions in the quadriceps produced by electrical stimulation (this technique is used to help screen out any effects of varying levels of motivation at different testing times). The results showed that the peak torque generated by the leg muscles was 4.5-5.9% higher at 18.00h compared with 06.00h and that the performance rhythms were synchronised with rectal temperature (ie circadian rhythm). Furthermore, partial sleep loss did not alter the magnitude or variations in muscle strength with changing time of day.

These results were supported by a French study on circadian variations in strength in men and women published at the same time and in the same journal(11). Twelve men and eight women were tested for maximal isometric voluntary contraction force of quadriceps and hamstrings at six different times of the day (02.00h, 06.00h, 10.00h, 14.00h, 18.00h and 22.00h), the order of which was assigned randomly. At each of these times, three trials were performed separated by three minutes’ recovery, and the highest value recorded. Rectal temperatures to track the circadian rhythm were recorded and involuntary force values were also recorded when an electric stimulus was applied (to control for motivation effects). The results were as follows:

  • Circadian peaks (highest core temperatures) occurred at 17.29h and 16.40h for males and females respectively;
  • Maximum voluntary leg strength occurred at 17.06h in males (increase of just over 2.5%) and 15.35h in females (increase of just under 3%);
  • The increase in voluntary leg strength in the men was not large enough to be considered statistically significant; however, when the involuntary contractions (via electrical stimulation) were considered, there was a very significant increase in strength. This suggests that in the men, there was a strong circadian effect, but they were somehow able to compensate for it by increasing the strength of voluntary contractions when they were not near their circadian peak.

There is also evidence from earlier studies that circadian rhythm affects strength. For example, a study conducted 10 years ago by researchers from the University of Dijon on the variation of maximum isometric elbow torque in PE students at different times of day found that peak torque tended to occur at 17.58h, and was nearly 7% higher than the averaged peak torque figure over the whole day. Moreover, when the experiment was repeated and spread out over a period of six days, the peak torque figure was calculated to occur at 17.55h – just three minutes earlier. This led the researchers to conclude that the circadian rhythms affecting muscular activity are remarkably constant(12).

Why does circadian rhythm affect performance?

There are a number of possible explanations as to why performance may be enhanced during the hours around the peak of the circadian rhythm, but increased core temperature almost certainly plays a major role. Higher body temperatures result in less viscous blood flow and muscles that are more supple, with less energy loss from internal friction. However, there is evidence that increased core temperatures in the afternoon/evening as a result of the circadian rhythm may also help because the body is in more of a ‘heat loss’ mode than compared with early morning ‘heat gain’ mode when core temperatures are low.

British researchers looked at heart rate, core temperature, sternum skin temperature and forearm skin blood flow during exercise, and throughout a subsequent 30-minute recovery period in 12 males exercising at 70% VO2max at both 08.00h and 18.00h(13). Comparisons were made of the changes of heart rate, temperature, and skin blood flow produced by the exercise at the two times of day. What the researchers found was that the increases in core and sternum temperatures during the afternoon exercise were significantly less than the morning, even though the workloads were not significantly different. Also, resting forearm skin blood flow (a measure of the ability of the body to lose excess heat) was higher in the afternoon exercise bout and the rate of change of blood flow as exercise was commenced was also higher.

Summary – manipulating circadian rhythm for performance gains

Although some early studies have reported little effect of circadian rhythm on athletic performance(14), the weight of more recent research suggests that for high intensity aerobic/anaerobic and strength training, circadian rhythm significantly affects performance potential. The obvious question for athletes and coaches therefore is how they can they can train in harmony with this rhythm to maximise performance. Here are some suggestions:

  • Measure your own circadian rhythm; this is best done by taking your temperature every two hours during a rest day following several days of a normal, regular sleep pattern. Plot the figures and observe when the peak occurs (normally late afternoon/early evening);
  • Try where possible to schedule important and/or strenuous workouts within an hour or so of circadian peak; you will almost certainly gain quality over attempting the same workout earlier in the day;
  • Early morning workouts should be performed at a gentle pace and a more thorough warm-up performed to reduce the risk of injury;
  • Getting up much earlier than usual (ie when your circadian rhythm is in a trough) to ‘squeeze’ in a workout may be counterproductive; the quality of the workout is likely to be reduced, the risk of injury is increased and you will of course be losing sleep into the bargain!
  • Adaptation to hot conditions during a workout seems to be more efficient during circadian peak; ensure plenty of fluid/hydration during hot morning workouts;
  • Athletes trying to build strength should time workouts to coincide with their circadian peak; research suggests that late afternoon weight training produces a more favourable post-exercise anabolic hormone profile, with higher levels of testosterone and lower levels of cortisol (a hormone associated with physiological stress and muscle tissue breakdown)(15);
  • For competition (where the time of the event is usually fixed), you may wish to experiment with manipulating your circadian rhythm so that you’re nearer your peak at the time of the event (see box above). The same applies when competing abroad in different time zones;
  • Unless you’re trying to manipulate your circadian rhythm, try to maintain regular bedtime and waking hours; irregular hours can disrupt circadian rhythm, leading to a generalised drop in performance.

Andrew Hamilton BSc, MRSC, trained as a chemist and is now a consultant to the fitness industry and an experienced science writer

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