A Guide To Lower Extremity Muscle Testing
To examine the effect of the pull of the Achilles tendon at the subtalar joint, it is easiest to examine the transverse plane where force in the tendon points at the viewer. The average subtalar joint axis passes lateral to the insertion of the tendon and therefore an upward force in the tendon will tend to cause supination of the subtalar joint. A tendon will produce a moment at a joint with only the component of the force that is perpendicular to the axis of the joint. Looking in the sagittal plane, the average axis of the subtalar joint is approximately 45 degrees to the weightbearing surface and the tendon is perpendicular to the weightbearing surface. With this arrangement, roughly half the force in the tendon will create a supination moment at the subtalar joint.
However, since the gastrocsoleus complex is one of the strongest muscles in the body, this will still create a significant moment. Since the ankle joint axis is closer to perpendicular to the line of action of the force and the lever arm of the tendon at the ankle joint is greater than the lever arm of the tendon at the subtalar joint, the Achilles’ most important function is to plantarflex the ankle joint. A secondary function is to supinate the subtalar joint directly.
There are also indirect effects of muscle contraction. With plantarflexion of the ankle, the center of pressure of ground reaction force will shift to the forefoot. Force on the forefoot will, in the average foot, create a pronation moment. The pronation moment from the ground reaction force may be greater than the direct supination moment from the tendon. This is how equinus can cause a pronation even though the Achilles is a direct supinator.
Since the function of the Achilles tendon is to plantarflex and supinate, gauging the resistance of those motions is the best way to test the strength of those muscles. One can best do this when the patient is seated with the bottom of the foot pointing toward the examiner. Place your hand against the lateral forefoot and ask the patient to push toward you.
Keys To Testing The Posterior Tibial Muscle, The Flexor Digitorum Longus And The Flexor Hallucis Longus Tendon
The function of muscles is to create a moment at joints. In order to test this function, the examiner has to create a moment that will oppose the moment created by the muscle. Therefore, the examiner needs to be aware of how best to create a moment at the joint(s) that opposes moments created by the muscle that one is testing.
Posterior tibial muscle. The posterior tibial muscle crosses the ankle, subtalar and midtarsal joints. At the ankle joint, the line of action of the tendon passes very closely to the ankle joint axis. Therefore, the tendon has very little leverage to create a moment at the ankle joint. To best understand the relationship of the posterior tibial tendon to the subtalar joint, you should be looking at the foot from a position where the subtalar joint axis points toward you.
The subtalar joint axis usually runs in line from the posterior lateral heel to the central part of the talar head. The tendon is roughly perpendicular to the subtalar joint axis and has a longer lever arm than other extrinsic muscles of the foot. At the midtarsal joint, the tendon runs along the medial side of the joint and attaches to the navicular tuberosity. The tendon’s insertion does spread out over several bones. However, in terms of figuring out its mechanical action, use the average point of insertion. This average point is more medial than plantar so the effect of the tendon is to adduct the forefoot on the rearfoot at the midtarsal joint with a small amount of plantarflexion of the forefoot on the rearfoot at the midtarsal joint.