A couple of weeks ago, we re-visited the Manchester United fitness test with one of our soccer teams. A follow up to last spring, and a follow up to our heart rate data. The group performed much differently however, and I don’t just mean individual performance. The entire group LITERALLY performed opposite to how they did last time. After the last run of the test, I tried to break down the numbers to find patterns or some reliably consistent indicators that linked HR values to success on the test. Admittedly I don’t know everything about heart rates, or even heart rate monitors for that matter. I did however, find a semblance of what I had expected (funny how you manage to find what you’re looking for sometimes.) What I had arrived at last time, was the following:
- Our athletes who tested best (passed most levels) recorded higher “MAX” HR’s.
- Our athletes who tested best (passed most levels) recorded more levels completed while operating below 90% of their recorded “MAX” HR.
- Our athletes who tested best (passed most levels) averaged more levels completed while operating above 90% of their recorded “MAX” HR.
The information in the small charts below is sorted by quartile and “median performance” in terms of levels passed. Generally speaking, as levels passed increased, max heart rate also increased. I interpreted this to mean that athletes with higher max heart rates have greater capacities. Secondly, these groups also recorded more levels both below 90% (aerobic/early anaerobic capacity) and above 90% (anaerobic capacity.)
In the most recent trial of this test we saw some inverse relationships, albeit in a larger population. Increased levels meant lower heart rates, and more often we saw an increase in levels completed ABOVE 90%. My intuition told me that this is likely because our aerobic capacity was down, and that we are achieving threshold sooner. Therefore we were trying to complete “levels over 90%” EARLIER in the test. Overall decreased scores on the test suggested that this is reasonable, because once threshold is achieved, there are limited glycogen stores available, and co2 concentrations outweigh o2 concentrations (lactate cannot be converted,) and exercise is not sustainable above threshold as long as it is below threshold.
I should note that I chose 90% MHR as our cutoff for levels passed above and below because it is generally recognized that this is where the anaerobic threshold occurs. Actually, it is suggested that it will be hit somewhere between the 80-85% zone – intensity, not of MHR – (Benson, Connolly 2011.) However, when I divided up the data on the original charts, that zone only lasted about 18 seconds due to the incremental nature of the test. Additionally, we should expect to see HR increase fairly linearly up until about 90 or 95% HRmax before levelling off. (Anecdotally, I have also seen a final SPIKE up to “max” before a steep decrease.)
Of course, if we are going to seriously investigate the occurrence of the anaerobic threshold, more information is required. Oxygen consumption, and lactate measures would also bolster our case for identifying the ventilatory and lactate thresholds. It appears that most of these measures will occur AROUND the same point, because as the demands of the incremental test exceed the capacity of the aerobic system there is an increase in recruitment of “fast twitch” fibers to keep up with the workload of the test. At this point, and due to the nature of preferential substrate usage of the fast twitch fiber, carbohydrate is the primary source of fuel for these intensities. When carbohydrate is used as the primary substrate in the absence of adequate oxygen and carbon dioxide concentrations increase (ventilatory threshold) the required resources to convert lactate (a byproduct of carb metabolism) are not available and it therefore accumulates (lactate threshold and onset of accumulation shortly after) in the system. It is then the presence of hydrogen ions (another byproduct of metabolism) and not “lactic acid” (which doesn’t actually exist) that makes the legs feel heavy and decrease muscle function. At this point you will see the athlete appear to have run into a stiff wind during the test, and they will fail out. In fact, LT can occur anywhere between 65-95% of heart rate max depending on trainedness. This should be enough evidence to suggest the difficulty in pinpointing the associated HR measure.
No heart rate measure that I am aware of has been “confirmed” as a consistently reliable measure of threshold, but some evidence suggests that these phenomena occur in the same neighbourhood of intensity. With respect to oxygen consumption, the VO2max occurs in a much different way than MHR because it is not physiologically possible for these two events to occur at the same point. At the onset of exercise, HR and VO2 will both increase linearly, with HR increasing more quickly. Up until about 75 or 80% intensity (different than HRmax,) HR will begin to plateau and level off, while VO2 continues to climb.
So with all this in mind, what would improved fitness look like as measured by an HR monitor?
- Greater speeds can be achieved at the same comfort level.
- Higher HR’s can be recorded at the same comfort level.
- Lower HR’s at the same workload/intensity.
- Possibly lower HRmax over the same workload.
- Lower resting heart rate.
- Upward shift in anaerobic threshold.
- HRmax (absolute value) may remain relatively UNCHANGED.
In this case, a lower MHR and more levels completed below 90% suggests to me an increased capacity of Aerobic Fitness. A lower MHR and more levels completed above 90% suggests to me increased anaerobic power, and possibly increased anaerobic fitness. No change and increased levels completed in either case suggests an overall improvement in fitness (MHR can vary as much as 3-7% in the same person depending on test format and environmental conditions such as heat, humidity, RHR on that day etc.) Decreased MHR and decreased levels completed suggests overall a deconditioned effect.
The greatest limitation in this method is that this test toes a line in terms of intensity. HR monitoring may falter slightly in this case as the sole monitoring tool because of delayed HR Response. In bouts of higher intensity exercise, it is possible/likely that the heart rate will increase at a later time point than the actual cessation of effort. So there is a chance at some of the later levels that the heart rate actually hit a higher point during the early part of the “recovery” run on the way back, than they did at the end of the sprint portion.
R Benson, D Connolly. Heart Rate Training. Human Kinetics, 2011.