The Effect of Stirrup Iron Style on Normal Forces and Rider Position.

The stirrup iron has the potential to modify the forces experienced by a horse and rider during ridden exercise. A range of stirrup designs are available, but no previous studies have investigated if these modifications influence riders' position and interaction with the horse. Novel flexible (F) or flexible and rotatable (FR) irons versus traditional (T) stirrups may positively impact the welfare and performance of the horse and rider. Four riders rode using the three stirrup types (T, F, and FR). Hip, knee, and ankle angles and toe position from film, and the normal force exerted bilaterally on force sensors on the stirrups tread were evaluated at the highest (HP) and lowest point (LP) of the posting trot (n = 4) and canter (n = 2). Statistics included Shapiro-Wilk's test, Friedman's test, and Wilcoxon signed rank test (significant at P < .05). No significant difference was seen between joint angles, toe position, or forces between the types of stirrups. At the HP, mean hip, knee, and ankle angles were 169.4° ± 10°, 150.7° ± 9.7°, and 94.5° ± 9.6°, and 139.1° ± 9.6°, 123.9° ± 10.9°, and 92.7° ± 9.5° at the LP. Riders had an 8.74° ± 6.66° difference of right versus left joints. Right toes rotated more laterally (P = .02) regardless of stirrup type. The mean trot and canter forces applied (N)/body weight (N) were 0.72 ± 0.15 (HP), 0.19 ± 0.15 (LP), and 0.18 ± 0.05 (canter). Riders shortened the stirrup leathers with F or FR. Stirrup style minimally impacted rider position or the forces experienced; however, forces differed by gait. Future studies regarding how a rider's experience and painful joints may contribute to asymmetries are warranted.