Date Published: February 1, 2017
Publisher: Public Library of Science
Author(s): Olivier Girard, Franck Brocherie, Jean-Benoit Morin, Sébastien Racinais, Grégoire P. Millet, Julien D. Périard, Øyvind Sandbakk.
Examine the mechanical alterations associated with repeated treadmill sprinting performed in HOT (38°C) and CON (25°C) conditions.
Eleven recreationally active males performed a 30-min warm-up followed by three sets of five 5-s sprints with 25-s recovery and 3-min between sets in each environment. Constant-velocity running for 1-min at 10 and 20 km.h-1 was also performed prior to and following sprinting.
Mean skin (37.2±0.7 vs. 32.7±0.8°C; P<0.001) and core (38.9±0.2 vs. 38.8±0.3°C; P<0.05) temperatures, together with thermal comfort (P<0.001) were higher following repeated sprinting in HOT vs. CON. Step frequency and vertical stiffness were lower (-2.6±1.6% and -5.5±5.5%; both P<0.001) and contact time (+3.2±2.4%; P<0.01) higher in HOT for the mean of sets 1–3 compared to CON. Running distance per sprint decreased from set 1 to 3 (-7.0±6.4%; P<0.001), with a tendency for shorter distance covered in HOT vs. CON (-2.7±3.4%; P = 0.06). Mean vertical (-2.6±5.5%; P<0.01), horizontal (-9.1±4.4%; P<0.001) and resultant ground reaction forces (-3.0±2.8%; P<0.01) along with vertical stiffness (-12.9±2.3%; P<0.001) and leg stiffness (-8.4±2.7%; P<0.01) decreased from set 1 to 3, independently of conditions. Propulsive power decreased from set 1 to 3 (-16.9±2.4%; P<0.001), with lower propulsive power values in set 2 (-6.6%; P<0.05) in HOT vs. CON. No changes in constant-velocity running patterns occurred between conditions, or from pre-to-post repeated-sprint exercise. Thermal strain alters step frequency and vertical stiffness during repeated sprinting; however without exacerbating mechanical alterations. The absence of changes in constant-velocity running patterns suggests a strong link between fatigue-induced velocity decrements during sprinting and mechanical alterations.
Accelerating over short distances is crucial in many team sports, where short-duration efforts (e.g., accelerated runs over 10–30 m or 3–5 s) are commonplace . This is in turn associated with important moments in a game, such as gaining an advantage over an opponent or creating scoring opportunities. During football match play for instance, a straight sprint is most often observed prior to a goal being scored . The ability to maintain sprint performance as maximal or ‘all out’ efforts are repeated (i.e., repeated-sprint ability) during a game is thereby crucial for athletes engaged in these disciplines.
Although there is compelling evidence to suggest that compared to temperate conditions, larger performance decrements occur when repeated sprints are undertaken in hot environments due to the development of hyperthermia , the mechanical causes underlying these decrements are not completely understood. In the current study, the nature and the extent of fatigue-induced alterations in running mechanics during multiple-set RSE performed with and without environmental heat stress were examined for the first time. The major findings are that greater thermal (i.e., core and skin temperature) and perceptual strain during RSE under heat stress is associated with decrements in propulsive power, step frequency and vertical stiffness, along with longer contact time; however, running mechanical alterations induced by sprint repetitions were in general not more pronounced in hot compared with cooler ambient conditions. Furthermore, low and high constant-velocity running patterns displayed no pre-to-post RSE changes in either condition.
In summary, higher thermal and perceptual strain during multiple-set RSE under heat stress is associated with decrements in propulsive power, step frequency and vertical stiffness, along with longer contact time. However, running mechanical alterations induced by sprint repetitions were in general not more pronounced in hot compared with cooler ambient conditions. Moreover, completion of multiple-set RSE did not influence 1-min low and high constant-velocity (i.e., 10 and 20 km.h-1) running patterns. The absence of changes in constant-velocity running patterns therefore suggests a strong link between fatigue-induced velocity decrements during sprinting and mechanical alterations. While these results shed light on the biomechanical manifestation of fatigue when athletes repeatedly sprint at their maximum under heat stress, additional studies are needed to demonstrate how to improve RSE tolerance and optimize performance when competing in a hot environment. Given the preponderant role of cardiovascular and perceptual strain on performance and mechanical alterations, future studies should consider RSE inducing heavier thermal strain and the impact of potential countermeasures (e.g., pre-cooling, acclimation) aimed at mitigating its influence.