Republication - the original can be found here
By Alan Couzens, MSc (Sports Science)
Those of you familiar with the training philosophies of Joe Friel (the guy decoupling big time in the shot above will have no doubt come across the concept of ‘decoupling’, i.e. a shift in the power: heart rate relationship as a workout goes on.
An example of this, from one of the athletes I work with, in the form of a rise in heart rate and a drop in power as the session progresses is shown below.
Clearly, as time went on the gap between the athlete’s power and heart rate widened, to the point that by the end of the session, the difference in power:HR compared to the start is 26%. Or in other words, it is taking this athlete an extra 30 beats/min to generate the same power!!
Detailed info on the calculation of decoupling can be found here, but the general gist is; we take the power/heart rate for the first half of the session and divide it by the power/heart rate for the second half. E.g. if that athlete did 105 watts at 100bpm in the first half (power/HR = 1.05) and 100 watts at 100bpm in the second, i.e. he lost 5 watts (power/HR = 1.00), then his decoupling would be 5watts/100watts = 5%.
When you think about it, this is a pretty perplexing phenomena. We assume physiologically that a given effort requires a given amount of energy, which requires a given amount of oxygen, which in turn requires a given amount of heart beats, at least for a particular individual! So what are the causes and implications of a need for more heart beats at the same workload?
To illustrate, let’s start with a typical exercise physiology scenario:
Say that I start pedaling a bike at 260W, a level of power that on average requires approximately 3.5 L/min of Oxygen. As I start the exercise & my muscles figure out “we’re gonna need more O2 captain”, my body goes to work transporting O2 to the working muscles.
Let’s assume that I have 12g of hemoglobin per deciliter of blood (an average amount). Assuming 100% saturation, this 12g/deciliter carries 16ml of O2, so 160ml of O2 per liter of blood. But I need 3.5 L of O2, so it’s going to take me about 22 liters of blood per minute to keep up with the demand (3500/160). Assuming I have a cardiac stroke volume of 150ml, it will take my body 150 beats per minute to pump these 22 liters (to the exercise physiology geeks, yes I’m ignoring the a-VO2 difference for the purpose of simplicity).
Pretty simple, eh? A given workload requires a given O2, which requires a given amount of heart beats. So, if the workload stays constant but the heart rate changes over time, what’s going on? At what step in the chain is the breakdown occurring?
The obvious one and the most commonly cited cause of increased heart rate for a given power is a change in stroke volume due to dehydration. If my cardiac stroke volume all of a sudden goes from 150ml down to 140ml my heart would need to beat 10 beats faster in order to get the same amount of blood per minute to the muscles. So, for my 260W, I would now be putting out 160bpm instead of 150bpm. The most common cause of this drop in stroke volume is a drop in blood volume via dehydration. For this reason, cardiovascular drift frequently occurs under hot conditions where some of the body’s fluids must be devoted to cooling rather than maintaining the integrity of the blood volume.
However, can an increase in heart rate for a given power reveal more?
Joe suggests that not only is decoupling of power and heart rate a sign of heat stress, he also uses it as an indicator of aerobic fitness. Is there a possible mechanism by which this metric could be used as a sign of not just heat tolerance, but also aerobic endurance?
Thomas and Chapman (2006) may be able to help answer the question of the validity of decoupling as a training metric. By observing VO2 during prolonged downhill walking on a steep grade, they saw a progressive rise in VO2 uptake with no change in body temperature or stroke volume. OK, you say, “the sweat thing made sense but what’s going on here?”
The break in the chain under these conditions occurs not in Oxygen transport, but Oxygen demand, i.e. at the top of the chain. During the eccentric exercise, as muscle damage occurs, the legs are forced to recruit larger, less economical muscle fibers. These fibers require a greater amount of O2 to exert a given level of power and the heart rate goes up for a given power output when the more economical fibers begin to fatigue.
In fact, type II fibers require ~twice the O2 for a given power output. Therefore, small fiber shifts result in relatively large differences in heart rate for a given power output (Coyle, 1992)
As we know, muscle damage isn’t the only cause of muscle fatigue. When a muscle fiber runs out of fuel (glycogen) it’s out of the game. Thus, decoupling can serve as an indicator of how our targeted muscle fibers are doing, both in terms of muscle damage and fuel stores.
As the targeted muscle fibers become stronger and more fatigue resistant, the time before the muscle fatigues to the point that it needs to call on it’s ‘big brother’ fibers increases. Therefore, as an athlete’s muscle fibers become more trained, decoupling over a training session decreases. In fact, the researchers above found that the effect disappeared when athletes were trained in downhill walking for a period of weeks. Or, in other words, as fitness for a given task increases, decoupling decreases.
Additionally, if we accept that HR:Power can indicate muscle damage and fuel depletion, we can also then use this metric to help determine if an athlete is adequately recovered for a key workout. If we know that typically an athlete takes 140bpm to run 7:00/mi (after warm up) we can use this number as a ‘check-in’ before key sessions. If the athlete takes 147bpm for the same pace (a difference of 5%) it may suggest that recovery is incomplete and the session should be postponed. Chuckie V wrote a great post on the practical implementation of this concept here.
Incidentally, a swing in the opposite direction can also indicate incomplete recovery via other mechanisms. In fact, over-reaching studies have typically found either decreased power/pace of ~5% for a given effort (e.g. Coutts et al. 2007, Jeukendrup et al., 1992), OR a decreased heart rate of 5% for a given power/pace (e.g. Hedelin et al, 2000). Therefore, ensuring athletes are within +/-5% of ‘normal’ power and HR is a good policy.
While it’s true that heart rate is subject to more confounding variables than other measures, it is not, as some coaches would suggest ‘useless’ as a training metric. The confounding variables can quite easily be accounted for by a good coach with effective communication and athlete knowledge. When used with a given athlete over a period of time, observing power:heart rate relationships offers the coach a fairly objective indicator of both the athlete’s base fitness and their readiness to work (two things that athletes notoriously over-estimate when left to their own devices). For this reason, in my opinion, decoupling is a key concept of science-based coaching.