A Guide To Lower Extremity Muscle Testing
There may be many points of application of a force. There will always be a point of application at the origin and insertion of the muscle. Many of the muscle-tendon units will curve around a bone and when this occurs, the bone may act as a pulley. A pulley will change the direction of force of the tendon, or rope, wrapped around it. The tendon will apply a force to the pulley/bone and the bone will apply an equal and opposite force to the tendon. This is how a tendon can have additional points of application of force. One uses the point of application of force to calculate a moment created by the force in the tendon. The moment is equal to the perpendicular distance between the line of action of the force and the joint axis.
A good example of this is looking at the Achilles tendon and ankle joint in the sagittal view. The point of application of the force is the insertion of the Achilles tendon on the posterior surface of the calcaneus. The line of action of the force goes from this point to the average point of origin of the gastrocnemius and soleus muscles. One can consider the ankle joint axis to be a point at the center of the ankle and the leverage from the force in the tendon will be the distance from the tendon to the joint axis.
The use of the joint axis to calculate the lever arm of the tendon is a slight simplification. In fact, the force in the tendon will create a force couple. A force couple occurs when a body applies two forces to an object and those forces are not directly opposite each other. Another way of saying that is that the forces do not have the same line of action. The magnitude of the moment created by the force couple is equal to the distance between the two lines of action times the magnitude of one of the forces. (The forces will be equal in magnitude in this situation.) The Achilles tendon will pull the foot upward. The foot cannot go upward because the tibia is in the way so the tibia will apply a downward force to the top of the talus.
The force couple of upward force on the calcaneus and downward force on the talar dome is what creates a plantarflexion moment on the foot. Newton’s third law can verify the concept of origin insertion inversion. Sometimes the distal part moves and sometimes the proximal part moves. With an ankle joint plantarflexion moment, the top of the tibia will rotate posteriorly. When the tendon pulls the calcaneus upward, the calcaneus pulls the tendon downward. The downward force from the calcaneus acting on the tendon and the upward force from the talus acting on the inferior surface of the tibia will create a moment that will tend to rotate the top of the tibia posteriorly.
Examining the force couples also demonstrates that muscle contraction increases the compressive forces at the joints. It has been calculated that during a slow jog, the compressive force at the ankle joint can be higher than 11 times body weight because of the upward acceleration of the body in addition to the muscle compression of the ankle joint.1 However, for simplicity from this point on, I will use the joint axis method for determining the action of a muscle.
The examination of the force couples also helps to understand how multi-joint muscles work. When a muscle exerts force, it is simultaneously pulling on both the proximal and distal segments. So a muscle cannot act on the proximal segment before it acts on a distal segment or vice versa.
Current Insights Into The Function Of The Gastrocnemius And Soleus Muscles And Achilles Tendon
Looking at the ankle joint again, the Achilles tendon crosses both the ankle joint and the subtalar joint. The force in the tendon will simultaneously create a moment at both joints. The tendon runs posterior to the ankle joint axis and therefore will plantarflex the ankle joint.