To investigate the effect of increasing velocity within one gait on horse and rider movement and to describe the resulting changes in saddle forces, seven ridden dressage horses were examined on an instrumented treadmill. The speed ranged between 1.3-1.8 m/s at walk and 2.6-3.6 m/s at trot. Kinematics of the horse and rider, vertical ground reaction forces and saddle forces were measured simultaneously. Velocity dependency of each variable was assessed for the whole group with linear regression. With increasing velocity, the saddle forces at walk were mainly influenced by the accentuated rocking type of movement and at trot by the higher vertical dynamic and a more rigid horseback which resulted in increased counteracting force between horse and rider. Even small increases of velocity changed the dynamics of the movement pattern of the horse and consequently the forces emerging beneath the saddle: a 10% increase within the indicated speed range resulted in +5% (walk) and +14% (trot) higher total saddle force peaks. Accurate comparison of saddle forces requires speed-matched trials; velocity is therefore a factor which also has to be considered under clinical conditions.
IntroductionThe velocity at which a subject is moving within each gait has a fundamental influence on numerous biomechanical variables. In horses, several authors focused their studies on speed-dependent changes in kinetic and kinematic variables and found that increasing velocity reduces stride duration and extends stride length (Clayton, 1994(Clayton, , 1995Dusek et al., 1970;Leach and Drevemo, 1991) and although limb impulses decrease, peak vertical forces increase as a result of reduced relative stance durations (Khumsap et al., 2001a;McLaughlin et al., 1996;Weishaupt et al., 2010). Knowledge of the mathematical functions of these changes enables comparison of individual gait patterns studied at different velocities. On the treadmill, stride duration, stride length and limb impulses change in a linear fashion with increasing velocity, whereas relative stance and suspension duration, as well as peak vertical forces change exponentially . Khumsap et al. (2001bKhumsap et al. ( , 2002 utilised net moment and power of fore-and hindlimb joints to relate ground reaction forces to muscle function and found that with increasing velocity peak moments and power in the joints of the hindlimbs increased, providing more forward propulsion. In the forelimb joints, only minimal velocitydependent changes in net joint energies occurred, indicating that, compared to the hindlimbs, adjustments in muscle activity did not behave in the same way. Increasing velocity also influences back movement. In unridden horses at trot, a reduced flexion-extension movement of the back was observed caused by increased muscle activity of the trunk muscles (Robert et al. 2001a(Robert et al. ,b, 2002. However, there is no information as to how the movement of the horse's back adapt to increasing velocities at walk. Byström et al. (2009Byström et al. ( , 2010 investigate...
