A Guide To Lower Extremity Muscle Testing
- Volume 26 - Issue 10 - October 2013
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Testing the muscles of the lower extremity can provide valuable diagnostic insight for physicians. Accordingly, this author reviews the basic physiology of muscles as well as pertinent biomechanical principles to provide a practical guide to muscle testing in the office.
In order to understand muscle testing in the lower extremity, it is important to understand just how muscles work. The activation of a muscle is the result of many steps.
Once the person chooses to activate the muscle, the motor cortex sends a signal down the upper motor neuron where it synapses in the spinal cord with the lower motor neuron. That neuron has an action potential that will release neurotransmitters. This creates an action potential in the muscle and the muscle contracts.
For muscles that have a tendon, the tendon must slide when the muscle contracts in order to produce a moment or torque at a joint that will alter motion. Of course, the joint at which you are testing motion/moment must be able to move to be able to assess function of the muscle. The force of the contraction will be in proportion to the strength of the lower motor neuron signal to the muscle.
Remember that if the evaluated muscle is weak, then the deficit can be at any point along the above pathway. Sometimes “weakness” is the patient not choosing to give maximum effort. Pain may be a reason that someone chooses not to activate the muscle.
Understanding Basic Muscle Physiology
Muscles are a conglomeration of actin and myosin units arranged in series and parallel to each other. A single actin and myosin unit will be able to produce force with motion by having the cross bridges slide. The more units there are side by side, the more force a muscle can produce. The more units there are end to end, the greater the distance the muscle is able to shorten.
This arrangement explains the length tension curve visible in muscle. When the muscle lengthens, there are fewer cross bridges that can produce force. So the extremes of lengthening the muscle will not produce as much force as when it is at its optimal length. When the muscle has shortened maximally, the actin and myosin units will be all bunched up and not be able to produce as much force in comparison to the force produced at the resting length.
An example of this is making a fist and then attempting to flex your wrist while maintaining the fist. The long flexors of the fingers do not have the ability to shorten enough to flex both the wrist and the fingers. In the foot, most of the time, the range of motion of the joint corresponds well with the range of the optimal length of the muscle. I will discuss the one major exception below.
A Closer Look At The Mechanics Of Muscles
When a muscle contracts, it will produce a force at both its origin and its insertion point. In order to figure out what the force will do, you have to look at the direction, magnitude and point of application of that force.