In previous papers, we focused on the importance of an analysis based on the athlete’s individual characteristics. We described a whole series of parameters that play a key role in the performance analysis of soccer players and split them into two categories: those which are independent from each athlete’s characteristics (total energy, equivalent distance index and average power) and those which are closely connected with his/her aerobic fitness (anaerobic energy, anaerobic index and high-intensity events).
Heart rate is another helpful parameter that falls into the second category and that should be monitored during training in order to obtain some feedback on the aerobic intensity levels produced during exercise. Within the same heart rate value, for instance, it is much more appropriate to focus on ball drills, rather than on sessions of dry-land training.
Heart rate and oxygen consumption
The linear relationship between HR and VO₂ has been widely investigated in the scientific literature. Heart rate is a reliable and effective parameter in the assessment of VO₂ values during steady-state exercises.
figure 1: relationship between heart rate and VO₂ (from WL Kenney, JH Wilmore, DL Costill. Physiology of Sport and Exercise).Check the difference between relative values expressed both in terms of HRmax and VO₂max.
figure 2: relationship between heart rate reserve and VO₂ reserve (from Swain DP, Leutholtz BC. Heart rate reserve is equivalent to %VO₂ reserve, not to %VO₂max. Med Sci Sports Exerc 1997; 29: 410-4).
The discrepancy detected between relative intensities expressed both in terms of VO₂max and HRmax (figure 1) lead us to evaluate them with reference to their reserve rate, i.e. the difference appreciated between the maximum and the resting value (see figure 2). By following this approach, heart rate measurement might be taken into account as a reliable and effective parameter to assess aerobic intensity in steady-state exercises.
Nevertheless, in certain sports, soccer included, the training session is characterized by technical and tactical exercise drills during which high-intensity phases alternate randomly with the recovery process.
For simplicity’s sake, let’s break down the VO₂ formula:
VO₂ = SV ∙ HR ∙ (A-V diff.)
where SV = stroke volume and A-V diff. = arteriovenous oxygen difference
In aerobic exercises performed at steady state, the stroke volume and the arteriovenous oxygen difference factors remain quite constant, thus showing how heart rate variations go hand in hand with VO₂ values. As it is clear from the above-mentioned consideration, this is the kind of situation in which heart rate might be taken into account as a reliable and adequate parameter to estimate VO₂ values.
On the contrary, in exercises characterized by a frequent alternation of low and high-intensity phases (soccer), the three factors which determine VO₂ values vary simultaneously. Thus this is the case in which heart rate cannot be set as the single parameter to estimate VO₂ variations.
The example illustrated below clarifies what we just stated: let’s assume that during the recovery phase following a high-intensity action the three factors decrease by 20%. The heart rate, the stroke volume and the A-V diff decrease to 80%.
VO₂ = SV ∙ HR ∙ (A-V diff.)
VO₂ = 0.80 ∙ 0.80 ∙ 0.80 = 0.512
VO₂ rates will decrease to 51.2% of the value previously detected. In this scenario, should we have taken into account heart rate as the only parameter used to estimate VO₂ values, we would have incurred in a gross miscalculation of that value. (80% vs 51%)!
The table below shows data concerning the analysis of specific exercise drills performed by a soccer player in a training session.
Table 1: AS = average speed of exercise; EC = average energy cost of exercise; EDI= equivalent distance index; AP = average power; VO₂ = estimate of average oxygen consumption; HR = average heart rate measured during exercise
continuous running: the EDI is very close to zero, thus indicating a lack of accelerating and decelerating phases, the EC value matches the one we measured when describing running at constant speed on a flat terrain in our 2010 study and intensity evaluated both as VO₂max and HR ratio reaches similar levels (exercise at steady state).
interval running: the EDI and the EC slightly increase due to standing starts (high-intensity activity); power modifies according to running intensity, it is directly correlated to the ratio between exercise and recovery and might vary according to the basic aim pursued by exercise. Notable differences may be observed between intensity intended as VO₂max ratio (67%) and intensity expressed as heart rate ratio (81%). Should we have considered heart rate as a reliable feedback parameter, we would have surely overestimated the aerobic intensity of the exercise.
match simulation: (a typical official match scenario), the EDI and the EC values are higher than those observed previously thus implying an increase in terms of acceleration phases. On the contrary, the decrease of average power is caused by the fact that activity performed in acceleration is followed by low-intensity events that allow the athlete to pay off the oxygen debt he/she owes to high-intensity phases. As data reported in the table clearly show, a significant difference might be noted also between average intensity expressed as VO₂max ratio (54%) and intensity considered as heart rate reserve ratio(86%). Indeed, as we have already stated, aerobic intensity developed in interval exercise phases cannot be estimated relying exclusively on heart rate ratio value.
interval training: a training session during which short ball drills performed in acceleration alternate with recovery phases (exercise/recovery ratio: 1:4 – 1:6). The average speed figures are very low, as a matter of fact, if we had taken into account the ratio between exercise and recovery, data wouldn’t have been different. A substantial increase of acceleration phases results in a considerable variation of EDI and EC values, the latter reaching significant peaks which were not observed in exercise drills previously outlined. The average power does not increase dramatically and dissimilarities between aerobic intensity expressed as VO₂max ratio (41%) and aerobic intensity expressed as heart rate ratio (85%) are much more consistent.
Detailed data on the exercise drill are a key factor to enhance performance. Once the coach has met his goals in terms of technical and tactical priorities, he/she needs to be well informed about the kind of exercises he/she proposes and the tangible results they provide. Indeed, it is of paramount importance to know how to handle exercise timing, exercise and recovery phases and the whole training session as well.
The strength and conditioning coach plays a fundamental role in this sense. As a matter of fact, he/she is able to evaluate and, if necessary, modify the training drill, thus improving the athlete’s performance. An appropriate feedback approach should be developed in order to obtain an exhaustive analysis. As the article has pointed out, there are occasions in which heart rate may be taken into great account in terms of performance analysis, whereas, in other approaches, it has to be weighed up much more carefully and associated to other parameters to help outline a more reliable evaluation and a more accurate scenario.