Electromyographic activity (EMG) was used to determine how hindlimb muscle activation patterns vary with speed and incline in the horse. EMG was recorded using surface electrodes over the gluteus medius and tensor fasciae latae muscles during treadmill locomotion at trot for different combinations of speed (3.5 to 6 m/s) and inclination (0, 3 and 6%). Raw EMG signals were processed to determine stride duration, activity onset and end, and integrated EMG (IEMG). Stride and stance phase duration decreased linearly with increasing speed. Stride duration was not influenced by the slope. Onset and end of muscle activity came significantly earlier in the stride cycle when speed increased and later when inclination changed from 0 to 6%. The relative duration of the burst (percentage of stride duration) increased as running speed increased, but tended to decrease with increasing slope. The IEMG of the muscles increased with increasing speed and slope, the largest increase being observed in the tensor fasciae latae. It is concluded that both increases in speed and inclination lead to an increase in the integrated electromyographic activity and hence to a higher workload of the 2 hindlimb muscles investigated.