Original article written by Stefan Altmann, Ludwig Ruf, Rainer Neumann, Alexander Woll and Martin Bucheit.
Background
Submaximal fitness tests (SMFT) offer practitioners a viable means to assess how athletes are responding to a training programme without the physical and mental burdens that maximal assessments are renowned for.[1] In team sports such as soccer, monitoring average exercise heart rate (HRex) during a standardized bout of low-intensity running is a common SMFT used to evaluate fitness. This is based on the assumption that a chronically lower HRex is representative of positive training effects, in particular superior cardiovascular function and adaptations.[1] Practicality can often come at the cost of precision, unfortunately; which begs the question—can SMFT HRex be used to track real changes in fitness? Recent work hot off the press by Altmann & co.[2] sought to answer this question in an elite youth soccer population.
What the authors did
One hundred and seventy elite youth soccer players (U15 to U19 age groups) were recruited from three professional German soccer teams in the highest division of their respective age groups to participate in this study. Players were assessed on an incremental treadmill test twice during the summer pre-season, in two consecutive seasons. The treadmill test started at 6 km/h and increased by 2 km/h every 3 min until volitional exhaustion. HRex from the 12 km/h stage was retained for analysis. The running velocity corresponding to a lactate concentration of 4 mmol/L (V4; “anaerobic threshold”) was also estimated and used as an established measure of fitness.
The authors compared between-test changes in HRex with those of V4. First, the association between changes in HRex and changes in V4 were examined. Next, they looked at the agreement of “improvement” and “impairment” each outcome measure following the previous tests. Thresholds for improvement and impairment in HRex and V4 were set as ±4.5% and ±6%, respectively.[3] If a player registered an improvement in both HRex and V4, or an impairment in both measures, a classification of “full agreement” was made. If one measure was improved and the other was impaired, a classification of “full mismatch” was made. A “partial agreement” was classified when one measure was improved or impaired and the other showed no clear change in either direction. Lastly, the changes in HRex and V4 of a single player over a 3.5-year period were examined using both the approaches previously described (case study; n = 10 test comparisons from pre- and in-season phases).
What the authors found
The relationship between changes in HRex and V4 was moderate and negative (r = -0.40 [95% CI: -0.26 to -0.53]). In real-world money, this means that players who had a greater improvement in HRex typically showed greater improvement in V4. Similarly, subgroup correlations in U15–U19 players were also moderate and negative (range of r values = -0.48 – -0.31). In the case study, the correlation between change in HRex and change in V4 was large and negative (r = -0.68 [-0.38 to -0.98]).
In the classification analysis, there were 62% full agreements, 36% partial agreements and 2% full mismatches. For age grade subgroups, the range of full agreements, partial agreements and full mismatches were 56–68%, 28–44% and 0–5%, respectively. For the case study, these proportions were 80%, 20% and 0%, respectively.
Overall, these results back the theory that SMFT HRex can be used as an indicator of (cardiorespiratory) fitness in elite youth soccer players. The relationship between the two measures is probably not strong enough to accurately predict change in V4 from change in SMFT HRex, but it certainly supports the notion that they are both convergent the same construct. This is further affirmed by the classification analysis, where two out of every three instances of improvement or impairment were correctly classified, and there were ≤5% full mismatches. The findings from this study are similar to the author’s previous work conducted in elite senior players[3] and meta-analysis of 73 datasets from 29 studies examining the association between SMFT HRex and measures of maximal cardiorespiratory fitness in team-sport athletes.[4]
Limitations
Acknowledged in their paper[2] is the fact that influencing factors of HRex and V4, such as test time of day, training load over the previous days, hydration and or nutritional status, were not systematically controlled for. In addition, all data were obtained on a treadmill in a laboratory, and therefore the transfer of findings to the soccer pitch may not be like-for-like.
What this means for coaches
Fitness testing in elite soccer players can often be problematic due to the physical burden it imposes on players who are already carefully managed around high training loads and congested fixture schedules. SMFT offer a practical alternative to assess fitness changes without the unfavourable consequences of maximal tests. Monitoring HRex during the last minute of a standardized run (e.g., 4 min at 12 km/h) has shown strong associations with maximal fitness test performance in elite youth soccer players and could therefore be used as a practical alternative.[2,4]
The present work by Altman et al.[2] has demonstrated that when a threshold of ±4.5% is used to classify a change in SMFT HRex as a real impairment or improvement in fitness, roughly two thirds of decisions are accurate, with only around 1 in 20 decisions being inaccurate. From a practical perspective, this means practitioners are unlikely to misinterpret “true” changes in a player’s fitness using SMFT HRex. The trade-off between 100% clarity and accuracy for the time, effort and resource saved to perform a maximal fitness test is one that practitioners can take at their own discretion.
Reviewer’s comments
This was a brilliant paper both in methodological quality and practical application. The sample size from an elite population is impressive and the findings can be implemented into the applied setting almost immediately with little resource or complexity. This adds to the growing body of evidence supporting SMFT in team sports.[1,3,4]
I believe SMFT can be used in conjunction with established maximal fitness tests; with both modes of assessment performed at the usual infrequent intervals of the season (e.g., pre-season, mid-season and end-season, where SMFT acts as the warm-up for the maximal test), with SMFT additionally performed on a more frequent basis (maybe once every 2–4 weeks). The benefit of this is having a more continuous analysis of fitness changes that might be used to aid generally decisions on a players training programme, or more specific uses such as the adjustment of thresholds to programme high-intensity interval training.
Recommended resources
Article – The reliability and usefulness of an individualised submaximal shuttle run test in elite rugby league players – Tannath Scott and colleagues