Sensitivity of Minimally Invasive Protocols to Monitor Changes in Endurance Performance in Elite Junior Football (Soccer) Players

Purpose: Adequate physical fitness is a key requirement for elite footballers. Maximal fitness tests are the standard for evaluating fitness but are often fatiguing and time consuming. This study investigated the ability of various minimally invasive protocols to reflect changes in standard parameters of football players' physical fitness. Methods: Twenty-one elite junior footballers performed 2 standard fitness tests (stepwise incremental treadmill test with blood-lactate and heart-rate [HR] measurements and Yo-Yo Intermittent Recovery 1 test) and various proxy fitness evaluations (submaximal runs with measurement of exercise HR and HR recovery and external-to-internal load ratios with data collected during match play) 3 times over the course of 19 weeks. Changes in the standard parameters were compared with changes in proxy parameters. Results: Changes in maximal endurance performance showed small to moderate associations with HR-based proxy parameters (r = -.1 to -.46; P = .004-.56), whereas changes in the running velocity at anaerobic threshold had trivial to moderate associations (r = -.07 to -.39; P = .01-.67). Alterations in HR recovery following 2 short, submaximal runs were very largely correlated with changes in the running velocity corresponding to an HR of 170 beats<middle dot>min(-1) (r = -.74 and r = -.75; P < .001). Changes in external-to-internal load ratios derived from match play were not significantly correlated with changes in any of the standard parameters (r = -.03 to .31; P > .05). Conclusions: Parameters of maximal running performance and parameters describing physiological thresholds do not seem to be adequately captured by different minimally invasive protocols. In contrast, HR recovery following short, submaximal runs may be a valid tool to monitor changes in aerobic endurance adaptations.