This study aimed to determine whether training aids (side reins and a Pessoa) increased the use of the longissimus dorsi when horses are being lunged. Horses were lunged on a circle under four different conditions on the left and right reins in walk and trot, and electromyographic (EMG) and speed measurements were taken using surface EMG at T16 and GPS, respectively. The EMG intensity was substantially greater for the longissimus dorsi on the inside of the circle. Differences occurred in both the timing and the intensity of the EMG between the conditions. At walk, the EMG intensity was the greatest for the control condition and at trot the EMG intensity was the greatest for the control and Pessoa control conditions. It is concluded that the training aids of side reins and a Pessoa do not increase the use of the longissimus dorsi to stabilise the back.