# Pacing of Human Locomotion on Land and in Water: 1500 m Swimming vs. 5000 m Running
## 4 Discussion
This effect is thought to result from increased motor unit recruitment and the use of anaerobic energy reserves [35]. The speed variations of each split, with respect to the average speed, shows that runners of all ranking and non-medallist swimmers maintain a speed close to the average for the first part of the competition, and then climb above the average speed in the second half, until the acceleration of the end spurt. This is supposedly due to the athlete's inability to tolerate constant loads during extended maximal performances, being more inclined to drop-offs in work [26]. Due to excessively fast starts among these athletes, major disturbances of muscle oxidative capacity, tissue oxygen saturation index, and the recruitment of additional type I and II motor units with muscle fatigue can impair performance in endurance events by earlier disruption of muscle contractile processes, causing poorer technique [36; 37]. Optimal pacing strategy during long races, both on land and in water, may be attained by improving the athlete's physiological parameters, such as the ability to delay the accumulation of blood lactate or a higher maximal oxygen uptake, but also by a lowered energy cost of locomotion through improved biomechanical abilities [38]. Medallist swimmers maintained a nearly even pace throughout the competition, while non-medallists completed a significantly faster first half and a significantly slower second half of the race. Energy expenditure can vary substantially from individual to individual during swimming because of different strokes and skill levels. For any given speed in the aerobic range, the more proficient swimmers expend 50% less energy than less proficient ones [16]. Thus, energetic and propulsive efficiencies, although not collected in the present study, could explain the differences between the groups of swimmers in relation to pacing. Consistently, a review study on pacing behaviour in swimming demonstrated that medallists adopt a more conservative pacing behaviour compared to swimmers ranked fourth to eighth place [39]. A more conservative initial pace that allows for later increases of speed seems to be associated with success, but athletes need mental confidence and physical talent to put these strategies into practice [11]. On the contrary, no significant differences were found among medallist and non-medallist runners for the whole race. In the case of endurance running races, individuals of lower absolute ability were reported to be able to maintain contact with superior athletes through taking advantage of drafting benefits, which cannot be utilized in lane swimming [34]. This could respond to the so called \"herd behaviour\" of competitions where drafting is allowed, and athletes follow the behaviour of surrounding opponents regardless of their rational decision making [5]. Typically, the winners of a 5000 m race tend to remain in the top five runners throughout the race in order to not use unnecessary energy taking the lead or setting the pace. The goal of this tactic is to minimise physiological interference, by keeping a steady pace at the beginning and acquiring a good position for the sprint at the end [40]. Decisions about energy expenditure during the race are based on relationships between internal factors, such as the athlete's physiological/biomechanical capacity, and external factors, such as the opponents. In competition, the opponents present a variety of affordances that affect motivation, attentional focus, ability to endure fatigue and pain, positioning, drafting, fall risk, and collective behaviour [9; 41]. This could explain the homogeneity of pacing strategy among runners, independent of ranking. Meanwhile, in swimming, competitions are characterized by lanes separating participants, which impedes tactical behaviour such as trailing behind another competitor. Therefore, swimmers do not have to compete for the ideal line, and their speed profile resembles that of time trial athletes, allowing them to be more independent of each other.