The Achilles tendon connects the calf muscles to the heel of the foot. The calf muscles pull on the heal through the Achilles tendon allowing propulsion of the human body over the foot for walking and jump activities. The length of the calf muscles and Achilles tendon combined should be short enough to contract strongly and generate enough power for daily activities and should stretch long enough to allow 10-20 degrees of ankle dorsiflexion. If the calf muscles and Achilles tendon cannot stretch and allow ankle dorsiflexion to this extent, the midfoot and the forefoot see abnormal stresses leading to pain and conditions like plantar fascitis, flat feet, posterior tibial tendon dysfunction, diabetic forefoot ulcers, stress fractures and arthritis.
The normal human foot is a dynamic structure that changes from a flexible unit to adapt to uneven ground during weight bearing and becomes rigid to push off by the calf muscles. The loosening and stiffening automatically happen by locking and unlocking of the midfoot during walking and running. The same mechanism can be used in a reverse fashion by stiffening the foot by locking the midfoot by maximally lifting the big toe and its metatarsal bone away from the ground or the level of the fifth toe and its metatarsal (supination of the forefoot).
The Achilles tendon can be stretched by daily activities and specific exercises that force the foot up against the leg. In adults, weight bearing exercises are more useful. Children benefit more from manual stretching by adults. Both adults and children can use dynamic splints that constantly stretch the tendon. Current devices do not stiffen the foot during Achilles stretching.
Any stretching of the calf muscle-Achilles tendon unit will be more effective if the foot acts as a rigid level and transmits all the stretch to the Achilles. If the foot is not rigid, some of the stretching forces will stretch the arch of the foot and create or worsen an existing flatfoot and will be less effective in stretching the Achilles tendon. Supination of the forefoot by an external devices makes the foot a rigid lever and allows better stretching o the calf muscle-Achilles unit.