Exercising under fatigue changes ones mechanics. Form gets sloppy. In the world of sport, performing under less than ideal conditions (and ever more so in the tactical world) is mandatory. You cannot ask for an extra few minutes to stretch, warm up, shift the angle of the sun or change the arena temperature. One of the mechanisms humans have over other animals for running is our spring mass complex in the Achilles tendon. An understanding of how it behaves when fatigued will help explain overall mechanical adaptations.
Girard, et al; 2013 had 12 competitive triathletes run on a 200m track with a 5m platform used to determine ground-reaction forces, velocity and determine vertical forces, horizontal forces and the spring-mass characteristics used by the runners.
Average completion time was 17.5 minutes. During the middle part of the race they decreased velocity by 11.6% but returned to almost first lap speeds during the last lap. Stride length and frequency both dropped by 7.4% and 4.1% respectively and consequently contact time increased 8.9% and stride duration by 4.1%. This lead to peak vertical forces dropping 2.0% and leg compression dropping 4.3%. Centre of mass vertical displacement increased by 3.2% indicating a general loss of form. Vertical stiffness decreased by 6.0% but leg stiffness only decreased by 1.3%.
This demonstrates that the spring-mass system of runners does decrease over time with fatigue. Given that many football, American football and rugby players all play under some level of fatigue, training sprints pre-fatigued for one cycle of training may be needed so players learn how it feels and at what rate they will perform. Also, training the aerobic and lactic systems to maintain high output rates as long as possible may be an approach to minimize the fatigue.