The use of foot orthotics for Achilles tendonitis treatment has always bugged me. I used them and so did a lot of other people use them and all felt they helped. The problem is that I always struggled with understanding the rationale. People used to claim that orthotics stopped the pronatory ‘whip’ that the foot did to the Achilles tendon; others tried to say that the foot pronation strained the medial fibres of the Achilles tendon ... I just did not buy any of this as it made no sense to me.
The tissue stress model or theory to apply foot biomechanics to orthotic therapy did shed some light on this and Achilles tendonitis treatment is a really good example of the thought processes behind the tissue stress theory.
If someone has Achilles tendonitis, then think about the action of the muscles that are attached to the Achilles tendon on the foot. They really have two actions:
1. A plantarflexion moment (relative to ankle joint axis)
2. A supination moment (relative to STJ axis)
If a foot orthotic is to help treat Achilles tendonitis, then it must reduce both or one of these moments, so the question to ask is: Can a foot orthotic reduce one or both of these moments?
Heel raises are often used to treat Achilles tendonitis, but can they reduce the plantarflexion moment? This small study: Sharon J. Dixon; David G. Kerwin: The Influence of Heel Lift Manipulation on Achilles Tendon Loading in Running. JAB, 14(4), November 1998; showed that heel raises do not decrease the tension in the achilles tendon and actually may increase it, so a heel raise probably can’t reduce the plantarflexion moment. Think about it this way: if you are standing and you contract your calf muscles to raise on to your toes, a certain force will go through the Achilles tendon. If you added a heel raise and then raise up on to your toes, your calf muscles will have to contract with the exact same force to lift you up! So does this happen when running? – probably. So shortening the distance between the origin and insertion of the calf muscles with a heel raise does not appear to be effective in lower the force through the Achilles tendon.
So why do so many use heel raises for Achilles tendonitis treatment and find it helpful? I have data (we not published yet), that a 1cm heel raise reduced the static supination resistance force by 14%, which means the supination moment has been reduced with the heel raise, leading to the calf muscles not having to contract so much, reducing the load in the Achilles tendon. Inverted foot orthotics have been shown reduced the ankle inversion moment by up to almost a factor of 4 (Williams et al; McLean et al). So the foot orthotics also contribute by reducing the inversion(supination) moment, so the calf muscles do not need to contract so much, leading less load in the Achilles tendon as the calf muscles have a major reductionin the force they need to apply to the foot for the supination moment.
So I went from not understanding how foot orthotics worked in Achilles tendonitis treatment to applying the tissue stress theory and have a coherent and plausible mechanism by which they could work.
What does the evidence say?: We do have this study: Orthotic control of rear foot and lower limb motion during running in participants with chronic Achilles tendon injury. Donoghue OA, Harrison AJ, Laxton P, Jones RK. Sports Biomech. 2008 May;7(2):194-205; in which they reported:
- Statistical tests revealed an increase in maximum eversion with the foot orthoses
- Trends towards increased eversion range of motion and decreased ankle dorsiflexion maximum and range of motion angles were also observed.
- Increased eversion was unexpected as all devices were designed to provide pronation control as deemed necessary by the podiatrist.
- Despite this, participants reported between 50 and 100% (average 92%) relief from symptoms with the use of foot orthoses.
Why did the orthotics work in this study? Did they decrease the supination moment needed by the calf muscles? It was never about the motion and always about the forces.