If sore knees are keeping you from enjoying your ride use the Cashel English Stirrup Iron Cushion. These Iron Cushions have foam sewn into heavy duty durable nylon and fit on the base of your stirrups and secure with a hook and loop closure. The exclusive Grip Strip helps keep your foot securely in the stirrup. The sizes are measured from side to side Medium: 4"-4.5" Large: 4.5"-4.75"