Stride parameters and hindlimb length in horses fatigued on a treadmill and at an endurance ride

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Abstract
The relationship between fatigue and stride and/or muscle stiffness requires further study. To measure stride parameters in horses undergoing fatigue associated with running at submaximal speeds both on a treadmill and in an endurance ride. Stride frequencies and estimates of hindlimb stiffness would be decreased in fatigued horses. Horses were fatigued using 2 paradigms: run to exhaustion at a treadmill (4.5 m/sec, 6% incline) and finishing an 80 km endurance ride. Videos were digitised before and after fatigue and analysed for stride parameters: hind limb length, stride frequency, time of contact, step length, duty factor and stride length. In fatigued horses, stride durations were 5% longer (P = 0.007) resulting in lower stride frequencies (P = 0.016) and longer stride lengths (P = 0.006). The time of contacts (tc) for stance phase were not different (P = 0.108) nor was duty factor (tc/stride period, P = 0.457). Step length (speed x tc) and hindlimb lengths were also not different (P = 0.104, P = 0.8). For endurance horses, stride data for nonfatigued horses were consistent with data extrapolated to 4.5 m/sec from nonfatigued horses on the treadmill. Endurance horses slowed (P = 0.002) during the race from 4.55 to 4.03 m/sec and stride lengths were shorter. Despite a slower speed, other stride parameters were unchanged. Hindlimb length was shorter in fatigued horses. Horses fatigued on a treadmill and during the natural course of an endurance ride responded differently, biomechanically. On the treadmill, where speed is constrained, stride frequencies decreased and stride lengths increased. During one endurance ride, stride frequencies were the same, although speeds were substantially reduced. Limb length was shorter in fatigued endurance horses. It remains to be determined if these changes in mechanics are advantageous or disadvantageous in terms of energetics or injury. Further examination of endurance rides is also warranted.