The gains from training effort vary greatly both within and between individuals. Genes and life-time training history play a large part in determining differences between individuals in response to training. However, genes cannot feasibly be altered and life-time history takes a life-time to create. But there are also factors that we can control in the present that have the potential to influence how much benefit we get from training.
Perhaps the most important controllable factor is the balance between stress and recovery. Training creates stress – there is microscopic damage to muscle fibres and an increase in catabolic hormones that break down the tissues of the body. During recovery, local repair processes such as the engagement of satellite cells in muscle aided by systemic anabolic hormones such a growth hormone and vascular endothelial growth factor which promote the development of new capillaries, and the synthesis of mitochondrial enzymes, make the body stronger and fitter. However, if the training stress was too large or too sustained, the body fails to adapt and might even suffer irreversible degradation – the over-training syndrome.
Autonomic indices of stress and recovery
Because the ways in which the body adapts to stress are complex there is no single physiological measurement that allows us to monitor stress and recovery. However one of the most reliable measures is the balance between the two divisions of the autonomic nervous system: the sympathetic system that employs adrenaline as the signalling molecule to mobilise the body in response to stress; and the parasympathetic system that employs the signalling molecule acetylcholine to mediate recovery. Because increase in sympathetic activity increases heart rate while increase in parasympathetic activity increases beat-to-beat variability in heart rate (high frequency HRV) we can monitor the balance between stress and recovery by measuring heart rate and/or high frequency HRV.
However, there are three major complications that make this tricky.
- First of all, the balance between the sympathetic and parasympathetic systems is affected many transient circumstances, such as the occurrence of loud noise or time of day, that have only minor relevance to the balance between stress and recovery on the time scale relevant to repairing the body.
- Secondly, the way in which the autonomic system is regulated is not simple. Although initially stress causes an increase in sympathetic activity, it the stress is too great or too sustained, the body reacts by a compensatory increase in parasympathetic activity – the rabbit in the headlights situation.
- Thirdly, as we get fitter, the amount of parasympathetic activity in any standardised restful situation increases, while the amount of sympathetic activation required to generate a particular power output decreases. This third factor provides additional useful information: increased high frequency HRV at rest or decreased HR at a given level of power output provides a useful measure of increased fitness, but there is a catch. Changing fitness level produces a shifting baseline if we wish to use sympathetic/parasympathetic balance to assess the current balance between rest and recovery, while transient stress such as sleep disturbance, can interfere with the use of either resting HRV or HR in response to load as a measure of fitness.
One way of dealing with these problems is to perform regular measurements, perhaps daily, under very carefully standardised conditions – such as immediately after getting up in the morning in a quiet setting while breathing at a predetermined rate – as recommended for HRV measurements using the Ithlete, developed by Simon Wegerif. I have followed this procedure over a period of many months, using my Polar RS800CX to provide the required information about beat to beat variation in heart rate, rather than the Ithlete. However, at least with my varying daily schedule and sleep patterns, apparently random fluctuations tend to overwhelm the systematic changes of interest. Furthermore, I found it difficult to distinguish the increases in HRV that denote healthy recovery from those heralding the onset of over-training. While I cannot rule out the possibility that Simon’s Ithlete algorithm is more successful than my own interpretation of the data, the inconsistency of some of the variation in HRV led me to conclude that additional data acquire under a range of circumstances might be more informative.
The role of the central governor in setting heart rate
As you get fitter HR at a given power output falls. There is a fall in HR relative to maximum heart rate, due to increased efficiency of the delivery of oxygen to muscles, but also an appreciable fall in maximum heart rate. Maximum heart rate is usually identified as the rate beyond which there is no further increase despite increasing work load when power output is increased in a graded manner. Interestingly, it is sometimes possible to achieving a rate greater than this maximum by an initial brief, intense burst of activity at a power output exceeding that required for peak HR (as in a Wingate test), and then allowing power output to decrease gradually. This suggests that maximum heart rate is not fixed but is set by a ‘central governor’. I do not think that the ability to trick the governor into allowing a higher HR is a useful procedure, but the realisation that HR at high power output might be regulated by a governor led me to reconsider an observation that I reported in my post of 12th September 2009 and have repeated on several occasions since: namely that following a period of excessive training, a major subjective effort required to push HR even to the level of the second ventilatory threshold (beyond which acidity accumulates very rapidly creating a very powerful respiratory drive). In that post of 12th September 2009 I had described the experience of overwhelming subjective difficulty in maintaining a high power output on the elliptical cross trainer despite a lower heart rate than usual at that power output, at a time when I was suffering from over-training, following over-enthusiastic recommencement of training after illness. It felt as if a powerful restraining influence was preventing my heart from beating too hard.
This suggested that the most sensitive way to assess impending over-training might include not only resting measurements of HR and HRV but also measurements across a range of work-loads spanning the full aerobic range. Although in principle, such measurement might be performed either running or on the elliptical cross trainer, on account of the fact that I am limited mainly to training on the elliptical when my arthritis is troublesome, I initially developed a protocol for the elliptical cross trainer. I have subsequently developed a version of the test for use to measure HR when running at various paces. The running version works similarly, but because I have acquired data on the elliptical at the beginning and end of my recent period of base-building, I present the elliptical version in this post and will describe the running version in my next post.
The criteria that I applied in developing the protocol were:
1) It should proved a comprehensive set of measures of autonomic function across a range of work-loads from rest to near the second ventilatory threshold (identifiable in practice by the need to increase respiratory rate to around 80/minute, and corresponding approximately to the anaerobic threshold).
2) It should be feasible to include it within a normal warm up, requiring less than a total of 20 minutes and demanding only a short period of high effort.
3) The key measurements should be based on heart rate, though during development of the test I used Polar RS800CX to record the R-R interval between consecutive heart beats, thereby allowing comparison of the predictions based on HR with those based on HRV.
I modelled the major features on the protocol on the sub-maximal cycling test developed by Lamberts and Lambert, in which heart rate is measured at three different work-loads.
The test protocol
The key component of the test is an assessment of heart rate at three work-loads spanning the aerobic range: a load in the lower part of the aerobic zone, with heart rate around 60% of maximum heart rate; a medium load with heart rate around 75% of maximum, and a high load with heart rate around 85-90% of maximum. The low and medium loads are maintained for 4 minutes each, during which time heart rate reaches a stable plateau, while the high load is maintained for only three minutes. At the high load, the HR does not reach a plateau. Achieving a plateau at this work-load takes about 10-15 minutes and would make the test unnecessarily stressful.
The three work-loads were achieved at resistance settings 4, 8 and 12 on the elliptical. I maintained a constant cadence of 80 cycles/min (160 steps per minute) giving a power output of 62, 135 and 199 watts at resistance settings of 4, 8 and 12 respectively. I would anticipate that a younger and stronger person would require higher resistance settings and greater power output to achieve the target heart rates for the three levels. In addition to recording heart rate, I also record the subjective effort level at each of the three levels on a scale from 1 to 20 where 1 is very easy (typical of relaxed walking, 3 is slight effort (eg faster walking or very easy jogging), 8 is comfortable running, while 16 requires determined effort to sustain and 20 is the hardest effort I could imagine.
To provide a more comprehensive picture of the autonomic responses the test also includes measurement of resting HR and HRV while standing in a relaxed state for one minute before the start of the test, and measurement of heart rate recovery (HRR) while standing in a relaxed state for one minute at the end of the high load phase. Between the initial minute of relaxed standing I do a very gentle warm-up for 2 minutes, at around 55% HRmax, so as to exclude that the initial period of erratic HR (apparently due to somewhat erratic opening of capillaries) from the low-load test period. After the second period of standing, I do 4 minutes of gentle cool down at the same work load as the initial warm-up to assess the stress accumulated during the test, and to make a transition to subsequent activity.
The time course of the HR during the test is shown in figure 1. R4, R8 and R12 denote the three resistance settings required to produce the target heart rate for the three different levels of the test. The red trace is a recording in the week immediately preceding a 10 week block of base-building with an average weekly training volume equivalent to 50 miles of running. The blue trace is a recording at the end of this training block. Comparison of the red and blue traces reveals a similar proportional decrease in HR at each load. This is also shown the bar chart in figure 2. The height of the bars represents the heart rate at each of the stages of the test. There is a reduction in heart rate of approximately 5% at each of the three test loads. This fairly uniform reduction in heart rate across the aerobic range should be compared with the fairly uniform reduction of about 10% in heart beats/Km across the entire aerobic range achieved by Hadd’s protégé Joe following 16 weeks of base-building at a weekly training volume that was mostly above 80 miles per week. Bearing in mind that I am an elderly runner with limited capacity to withstand hard training, I am pleased that I achieved a roughly similar amount of improvement relative to training volume to that achieved by Joe. I will describe this base-building in more detail in a later post.
The traces in figure 1 show beat by beat variation, and appear more erratic that the 5 second averages produced by most heart rate monitors. The high variability during relaxed standing at the start indicates large high frequency variability in heart rate, indicative of a high level of parasympathetic activity. Similarly the rapid increase in variability after about 50 seconds of standing during the recovery period activity demonstrates the return of parasympathetic activity. The sporadic sharp spikes at around 4 minutes in the record for 13th March (red trace) are probably premature atrial contractions, which are fairly common in an elderly runner.
The detection of over-training
At the beginning of the year I had suffered an episode of mild arthritis. As the joint pain began to resolve in late January I was eager to build up the volume of training on the elliptical cross trainer. However, in my eagerness I built up the training volume too rapidly, and at the end of the month, I was becoming exhausted. Figure 3 depicts the heart rate at each test phase during tests performed on 27 and 29th of January and again on 1st Feb.
The crucial informative feature was my rating of subjective effort at the high load. During all 5 tests in the preceding two weeks in mid-January, I had rated the effort at the high load as 14 on a scale from 1 to 20. On 27th Jan I also assigned a rating of 14 but two days later, on 29th, there was a clearly perceptible change in the effort required. It required a determined effort to complete the three minutes and I rated the effort at 16. This increase in effort was accompanied by a sudden drop on HR of almost 5% at each of the three loads. I recognise that this was the onset of an over-trained state and rested for two days. By 1st February, the heart rate reading were again back to the levels recorded on 26th Jan, and I found it quite easy to complete the three minutes at the high load. I assigned an effort rating of 13.
Thus, the comparison of 27th and 29th January reveals a drop in HR very similar in magnitude to that achieved by 10 weeks of systematic base-building a few months later. The crucial factors are that this reduction of almost 5% occurred over a 2 day period and was accompanied by a very easily perceived increase in subjective effort. However the problem resolved rapidly with two days of rest indicating that it was only the early phase of over-training.
The effect of moderate stress
During the systematic base-building phase starting in March, I built up training load in a well controlled manner and did not experience any dramatic sudden reductions in heart rate at a given load. However other aspects of my life were less easy to control. In mid-May experienced a demanding period at work. On 13th and 15th May I had only a very short night’s sleep. I did continue training according to my plan, but in a sub-maximal test on 16th May, I noted that the HR at the various levels of the test was 3-5% higher than it had been two weeks earlier. It is probably significant that this effect was most noticeable at the low load and also in the standing HR. By the beginning of June, the stressful time at work was behind me and the heart rate at each level was back to the value at the beginning of May (though slightly lower at the R4 and R12 levels, perhaps indicating a small gain in fitness due to training). Thus, the immediate effect of stress was an increase in HR that reached its peak following a night of seriously curtailed sleep, but it is also noteworthy that the gains in fitness over the entire month were only marginal.
Standing heart rate and HRV
Standing HR shows similar pattern of variation with training (fig 2), over-training (fig 3) and stress (fig 4) as the HR at each of the three test levels. However, in general standing HR is less reliable than during exercise because it is more sensitive to current stress levels. In fact the standing and R4 scores alone can provide a very quick and easy test of current stress level, but detection of the early over-training is more reliable when the full range of scores is obtained.
On days when standing HR was low, high frequency HRV was usually high, confirming strong parasympathetic activity and generally denoting good recovery. When standing HR was less than 48 beats/min, the root mean square of deviations (RMSSD) was typically greater than 70 milliseconds, whereas at times of stress, standing HR exceeded 50 beats/min, and RMSSD was usually much lower, sometimes as low as 41-43 milliseconds. The lowest recorded standing HR during base-building was 45 and this was accompanied by an extremely high RMSSD of 163 milliseconds. Thus RMSSD provides a rough guide to the level of recovery, though the extremity of the swings in RMSSD made it an unreliable measure. On 29th Jan, when I was showing signs of over-training RMSSD was 94, a moderately high value indicating substantial parasympathetic activity, but the definitive evidence of over-training was provided by the low HR and high effort at the high load (R12 level).
Heart rate recovery (HRR)
Heart rate recovery (HRR) tended to increase steadily as I became fitter. The drop in HR during 1 minute when standing in a relaxed state immediately after cessation of the high load epoch was typically 50 beats/min in January and February as I began the build up training volume. It had increased to 55 beats/min by the time I commenced regular base-building in mid-March and was in the low 60’s by the completion of the block of base-building in early June. It was atypically low (45 beat/min decrease over 1 min) when I showed sign of over-training on 29th Jan and atypically high (71) when I exhibited signs of stress on 16th May. Thus HRR augments the picture provided by the changes in HR at the high work load (R12) but shows less consistency.
While the HR at the three work-loads provides the most reliable information, the picture is consolidated by the overall pattern, including standing HR, HRR and HRV. The use of this sub-maximal test gave me the confidence to build up my training load at a fairly rapid rate, while nonetheless avoiding serious over-training or injury. However it was also noteworthy that the data indicated that I achieved only a small increase in aerobic fitness during the month in which overall stress level was quite high, suggesting that I might have benefitted more if I had taken more rest during that month.
Addendum added 28 June 2013.
In his comment below, Ewen suggests that large amount of data generated by this test appears a bit over-whelming. He reports that he himself simply measures resting HR. As I noted in my response to him, I consider that resting HR is certainly a worthwhile measure, but the reason I have spent time designing this more complicated test is that I found that a single measures such as resting HR or resting high frequency HRV are too sensitive to various different transient circumstances, making it difficult to interpret the results reliably. To some extent one can deal with this by performing a single measurement such as resting HR or HRV under constant circumstances every day, but with my variable life routine this doesn’t work well enough. So in the test described in this post, I have adopted the approach of performing multiple different measurements within a more complex test. It is still necessary to standardise the circumstances, but the overall picture provided by multiple measurements makes it possible to obtain a more reliable estimate of changes in both fitness and recovery. However, it does produce a bewildering amount of information. While I find this large amount of information quite interesting I accept that it might appear a bit over-whelming. So maybe for routine use, one should focus on one key measurement and only examine the other variables when the key measurement is out of line with expectation.
I think that the single most informative measurement is the heart rate at a medium work load. Provided the load is consistent, this measurement is the one that is most consistently related to overall recovery and fitness. If this HR is decreasing slowly over a period of many weeks during a block of training, all is well. If there is a rapid jump upwards from recent values, it probably indicates transient stress. This can be confirmed by noting an increase in standing HR (and perhaps a decrease in HF HRV if you have a monitor that records beat by beat variation). It is no cause for alarm, but it is worth identifying and dealing with the stress. A sudden drop in HR at medium load is the important marker. If this is accompanied by additional effort that is most noticeable at the higher workload, it is probable that you are in the early phase of over-training and need to adjust training accordingly.