If you have recovered well from your previous training sessions and your body is ready to take on the challenge of a huge new stimulus, then of course you should complete the workout. However, if you have not recovered very well from prior physical strains, the new session might cause a physiological setback which could make it difficult for you to train in your usual way for several days. Worse still, if your recovery has been particularly poor, the big workout might send you into a complete physical tailspin and leave you in an ‘overtrained’ state. You might be unable to perform decently for several weeks - or even longer. In other words, the big workout would have made you worse, not better.

 

So how can you tell if you are really ready for a critical workout? Many athletes use their prior histories to determine their readiness: if they have performed the workout before within the context of their usual training with no ill effects, they usually assume they are okay to repeat it. But history is an imprecise way to judge readiness for a major physical challenge, for several reasons:

First, if an athlete is training correctly, he is training progressively, meaning that the training load is becoming more difficult over time. Since the training load is increasing, an athlete can not simply assume that he has recovered to the same extent as before;

Secondly, if an athlete is training progressively, the big workout itself is ‘bigger’ and tougher, producing a greater-than-average physical challenge;

Finally, a prime workout may be something which has never been conducted before (in the case, for example, of an athlete who is moving into a new phase of training or trying a completely new type of session); in this case, prior history would provide no guidance at all.

 

Some athletes rely on their perceptions of overall vigour and fatigue to determine their training readiness. If they feel fit and energetic, they forge ahead and tackle significant sessions without major concerns, but if they are feeling a bit below par, they may choose to wait awhile before embarking on a grandiose effort. However, a fair number of athletes plunge into major new exertions even if they are generally feeling below-par, and an athlete’s overall perceptions of energy and fitness can be misleading. Individuals who are feeling great can nevertheless go into a physical tailspin following a significant workout, and athletes who are feeling sluggish may be enlivened by what might appear to be a too-great physical challenge.

 

As you can see, determining readiness for a significant physical challenge is an imprecise science. In fact, until now very little science at all has been involved in the process. However, some exercise scientists are engaged in trying to fill this particular void, and one such effort centers on the fickleness of the heart.

 

Although we tend to think of our tickers as rock-steady, dependable fellows which hammer away in our chests without need for rest on a ‘24-7’ basis, the truth is that the performance of our hearts can waver considerably over the course of a single day. In addition, the amount of wavering seems to depend on the degree of physiological stress being experienced by the individual concerned.

 

Research in this area began in the 1960s, when scientists noticed that subtle changes in the intervals between heart beats preceded the appearance of fetal distress in unborn human babies. From this finding, it was logical to conclude that inter-beat-interval variation could be used to predict troubles in fetuses. An extremely interesting aspect of this research was that heart rate might remain rock solid (if decreases in intervals were matched by increases) while the fetus was on the brink of severe distress; thus, a better initial predictor of problems was the variation in between-beat intervals (the extent to which these intervals varied around a mean).

 

In the early 1980s, scientists confirmed these findings by showing conclusively that inter-beat intervals were far from random; in fact, it was clear that specific physiological rhythms were ‘embedded’ in the beat-to-beat heart-rate ‘signal’. In addition, it soon became apparent that heart-rate variability was a strong and independent predictor of mortality following an acute heart attack.

 

The heart, of course, becomes more-than-mildly involved when a rigorous exercise programme is carried out, and exercise scientists soon began to realise that the inter-beat interval itself, as well as the variation in such intervals, can respond rather strongly to sustained periods of strenuous exercise; (a well-known example of the exercise-induced alteration in inter-beat interval length is the slowing in resting heart rate – and consequent expansion of interval length – which occurs as fitness improves). Scientists reasoned that if changes and variation in interval length were truly predictable, athletes might be able to determine objectively how well their training was going and lose their reliance on subjective indicators. A certain level of inter-beat variation, for example, might indicate that an athlete was on the verge of overtraining, just as fetal inter-beat variation had predicted the onset of fetal distress. In other words, athletes and coaches might have some real science to guide their training efforts.

 

Researchers in France and Switzerland have helped to lead such explorations. In one of their recent investigations, six previously sedentary men (average age 33), who were free of cardiac abnormalities, trained intensely for eight weeks, recovered for one week, trained very strenuously for four weeks, and then recovered for two additional weeks. During the eight-week period of intense training, the subjects completed three training sessions per week (on Monday, Wednesday, and Friday), each consisting of the following:

- ten-minute warm-up;

- five-minute maximal-effort test on cycle ergometers;

- 15 minutes of easy cycling;

- four five-minute work intervals at 85% of the power sustained during the all-out test, with three-minute recovery intervals of easy cycling.

 

Cardiac variability was assessed by means of Holter recordings twice a week, once on the day following a workout (Thursday) and once on Sunday evening after two days of complete rest.

 

During the one-week recovery period which followed the eight weeks of intense training, the subjects completed a maximal five-minute test on two separate days and a VO2max test on a third day, with no other training carried out during the week. A 24-hour Holter recording was carried out on a day when there was no exertion.

 

During the four-week ‘overload’ training period which followed the seven days of recovery, the quantity carried out in the initial eight-week period was multiplied by a factor of 1.67. The three original training sessions were retained on Monday, Wednesday and Friday, but two additional workouts were added for Tuesday and Thursday. These latter sessions consisted of ten minutes of warm-up followed by five five-minute work intervals at 85% of the intensity achieved during the five-minute max test, with three-minute low-intensity cycling recoveries. Saturday and Sunday were again treated as rest days. During the two recovery weeks that followed, the six subjects once again carried out only exercise tests – a five-minute test on three separate days during the first week and a five-minute test on two days plus a VO2-peak test on a third day during the second week.

 

While monitoring this interesting programme, one of the variables the researchers were interested in was what they called ‘RR’ – the interval (in seconds) between the beginnings of consecutive heart beats; in an individual with a heart rate of 60 beats per second, for example, RR would be one second. Of most interest, of course, was the variation in RR (which, if you remember, had been linked with fetal distress and heart-attack recovery), and the researchers used a number of different statistical techniques (including ‘time-domain’ indices and ‘Fourier-transform’ indices) to assess RR variation. Discussion of these techniques is beyond the scope of this article, but it is believed that these methods can discriminate between the effects on the heart of two key elements of the human nervous system.

 

Those elements are the ‘sympathetic’ and ‘parasympathetic’ nervous systems, and together they make up what is known as the ‘autonomic nervous system’ (ANS). Overall, the ANS controls what is often called the internal environment of the human body, including the heart itself, as well as the glands and all the ‘smooth’ muscles in the body, including those which line the digestive organs, blood vessels, respiratory passageways, urinary structures and reproductive organs. The control centres of the ANS, which operates beneath the level of consciousness, are found in the midbrain, hindbrain, and spinal cord.

 

Sympathetic vs parasympathetic

 

As mentioned, the ANS is divided into sympathetic and parasympathetic components, and these tend to act in opposition to each other in an effort to keep the body in relative homeostasis, or balance. In general, the sympathetic system tends to ‘rev’ the internal organs up; its name originated from the observation that it appeared to help the human body prepare for activity in response to emergencies. For example, the sympathetic system elevates blood glucose levels, shunts blood toward skeletal muscles, dilates the pupils of the eyes, stimulates the sweat glands, boosts adrenaline production, dilates airways, shuts down the gastrointestinal tract, spurs adipose cells to begin breaking down fat and pushes reserve blood out of the spleen. By contrast, the parasympathetic system tends to exert opposing effects which amount to calming the body down.

 

With regard to the heart, the sympathetic effects are to increase the strength of contraction of the heart muscle, to amplify the rate of conduction of electrical messages across the heart (so that one complete beat takes less time) and to magnify the rate of beating (by reducing inter-beat interval length). Meanwhile, the parasympathetic system has the power to decrease the rate of conduction and slow the heart’s rate of beating.

 

It is important to note that the systems work simultaneously at all times, and it is the balance achieved between them which determines your basic heart rate, inter-beat interval length and inter-beat interval variation. A second key point is that the two subsystems respond and adapt differently to strenuous physical training, with knock-on effects on heart rate, RR and RR variation. If the sympathetic system becomes less dominant as a result of training, for example, and the parasympathetic system becomes more dominant, RR will broaden, and, during the dynamic broadening process, the variation in RR will rise. As mentioned, the hope is that these changes in RR variation will have a systematic and predictable relationship to fitness and overall physiological status, so that they may be used to assess the risk of overtraining and the readiness for additional hard work.

 

As it turned out, the initial eight weeks of intense training worked well for the subjects, who boosted their average VO2max values by 20% (from 42.9 to 50.5 ml/kg-min) and their five-minute peak performances by 26% (from 275 to 348 Watts). As these changes were occurring, RR and the variation in RR were also steadily transforming themselves. Essentially, RR – measured at night during sleep – increased significantly from 0.92 to 1.08 seconds during the eight-week period, reflecting a drop in night-time heart rate from 66.2 to 56.5 beats per minute. Inter-beat variability also swelled dramatically. Overall, both the sympathetic and parasympathetic nervous systems became more active during the eight-week training period, but there was a ‘power shift’ in favour of the latter. Thus it appeared that increased inter-beat variability was an indicator of enhanced overall fitness rather than a predictor of physiological distress.

 

During the subsequent four-week overload period, nocturnal RR no longer increased, inter-beat variability actually dropped, and the parasympathetic system had a smaller impact on the heart (as assessed by the decrease in parasympathetic indices of interbeat-interval variability). During the two-week recovery period, however, the parasympathetic system rose up again, and interbeat variability also began to march upward, again in line with enhanced physiological status.

 

Overall, the balance in the autonomic nervous system shifted towards the sympathetic branch during the overload training and the parasympathetic branch during recovery. Seven weeks after training had completely ended, the parasympathetic system was still in control, although its dominance was not as great as during the initial two weeks of recovery.

 

It was clear that inter-beat variation was responsive to training, and the results paralleled those achieved by the investigators in prior research with experienced runners. In that earlier study, seven male middle-distance runners (average age 25), who had been training for a minimum of three years and were ranked at the national level in France, were compared with a control group of eight healthy, sedentary male university students. The training programme for the athletes comprised four-week training cycles, with three weeks of very exhaustive training followed by a fourth (recovery) week of very easy exertions. During the three-week periods of intense training, the runners carried out six to ten workouts per week; Halter recordings were again taken, with caffeine and alcohol avoided during the preceding 24 hours.

 

Training load in the runners was monitored via a simple system which scored four different types of activities, as follows:

Rest day – 0

Endurance training – 1

Sprint, strength or extended workouts – 2

Maximal, exhaustive training sessions – 4

 

Weekly training loads were determined by adding up the daily scores. Feelings of fatigue were plotted on a scale from 0 to 10, with 0 corresponding to a total absence of fatigue and 10 associated with maximal levels of tiredness.

 

As it turned out, nocturnal heart rate (measured by the Holter devices between midnight and 4am while the subjects slept) increased progressively during the three-week period of exhaustive training; by the end of the third week, night-time heart rate was 3.74 beats per minute higher than at the beginning of the first week – a statistically significant shift. It took just one week to bring this down, however; during the fourth week, nocturnal beating decreased by 5.85 beats per minute – also a statistically significant shift.

 

As in the cycling study, measures of inter-beat interval variability generally increased as overall physiological status improved. In this study with middle-distance runners, the improvement (and increase in variability) occurred during the fourth, recovery week of the cycle. During this week, average levels of fatigue plummeted by a remarkable 32%, corresponding with the 40% reduction in total training load. Statistical analyses suggested that the sympathetic system was upgrading its dominance over the parasympathetic system during the first three weeks of the four-week cycle, while the parasympathetic system was fighting back during the recovery week. Overall, this study confirmed that the autonomic nervous system makes quick, impressive changes in functioning in response to vigorous training, and that these responses tend to occur in predictable ways. By contrast, healthy individuals who are not involved in training display remarkable stability in RR variability; indeed, the control group in the running study exhibited no significant change whatsoever in heart-rate variability over the 28-day period.

 

What does all of this mean to you as an athlete? First, bear in mind that, while these studies are extremely interesting, they are really only starting points. They reveal that inter-beat variability tends to decrease systematically in response to very strenuous training and increase during periods of recovery, regardless of the initial RR of the athlete. Overall, it may be possible to use decreased RR variability as a valid indicator of impending fatigue and overtraining and enhanced RR variability as a sign that training and recovery are going well, so that additional extremely challenging workouts may be attempted. It remains for exercise scientists to further validate these ideas – and then ‘put some numbers’ on the relevant upturns or downturns in RR variability. If this can happen, athletes will at last have a scientifically valid way to monitor training progress, judge their readiness for extreme efforts and avoid the dreaded overtraining syndrome.