A Guide To Lower Extremity Muscle Testing

Author(s): 
Eric Fuller, DPM

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.

   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.

   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.

   In testing the posterior tibial muscle, it is best to eliminate the effects of other muscles that might contribute to the moment at the joints of concern. The anterior tibial muscle can contribute some supination moment so the foot should be plantarflexed when one is attempting to test the posterior tibial muscle.

   The examiner asks the patient to relax the foot and then places the foot in a plantarflexed and adducted position. Place a hand on the medial side of the first metatarsal and place the other hand on the lateral leg. Then push the hands toward each other.

   Use this principle for testing all other muscles of the foot. Place the foot in the position where the maximum pull of the tendon would place it. Then ask the patient to prevent you from moving the tendon. I have found these instructions to be the easiest for patients to understand. Also, in this position, when the forces exerted by the examiner are strong enough to overcome the muscle strength, then you will see motion. If the joints were at the opposite end of the range of motion, the resistance to the examiner’s forces may just be the physical end of range of motion of the joint and not the strength of the muscle.

   Flexor digitorum longus tendon. The flexor digitorum longus tendon runs in a very similar course to the posterior tibial tendon. Instead of inserting into the navicular tuberosity, the tendon travels laterally and plantarly deep in the foot to insert, after splitting into four slips on to the base of the distal phalanx of the toes. Since its path is similar to the posterior tibial tendon, the flexor digitorum longus will have a similar effect at the subtalar joint. It will also have a similar lack of effect at the ankle joint. Distally, it runs inferior to the midtarsal joint and will create a plantarflexion moment there.

   To test the flexor digitorum longus muscle, you want to eliminate the effects of other muscles and this is fortunately easy to do as it is the only muscle that plantarflexes the distal phalanges of the toes. Just ask the patient to curl the toes. All the lesser toes should curl at the same time because the one muscle belly pulls on the tendon that splits to all of the lesser digits.

   The similarity between the path of flexor digitorum longus tendon and the posterior tibial tendon means that the flexor digitorum longus tendon is a good supinator of the subtalar joint. This fact explains the phenomenon of flexor stabilization. When a patient subconsciously chooses to get an extra subtalar joint supination moment from the flexor digitorum longus muscle, there will be a flexion contracture of the toes that can be visible in gait.

   Flexor hallucis longus tendon. The flexor hallucis longus tendon differs from the other medial ankle tendons in that it passes posterior to the talus before traveling distally inferior to the midtarsal joint before finally inserting into the base of the proximal phalanx of the first toe. A certain percentage of the time, the flexor hallucis longus tendon sends slips to the distal slips of the flexor digitorum longus tendon. This arrangement is called the master knot of Henry. When the tendon of the flexor hallucis longus slides proximally, it will also plantarflex the second toe and, in some feet, the third toe as well.

   The flexor hallucis longus tendon runs posterior to the ankle joint so it can plantarflex the ankle joint. The muscle belly is much smaller than the gastroc and soleus, and the flexor hallucis longus has a shorter lever arm so it is a much weaker plantarflexor of the ankle joint than the muscles attached to the Achilles tendon. The flexor hallucis longus tendon runs more medial than the flexor digitorum longus tendon and posterior tibial tendon, and therefore it has less leverage to affect the subtalar joint. The flexor hallucis longus tendon runs plantar to the midtarsal joint and can plantarflex that joint. Finally, the tendon inserts into the first distal phalanx and is the only muscle that can plantarflex that joint.

   When testing the strength of the flexor hallucis longus tendon, one must be aware of its effect at the ankle joint. This muscle is similar to the long flexors of the hand in that the muscle cannot shorten enough to plantarflex both the ankle and hallux interphalangeal joint. So one needs to test the muscle with the ankle joint dorsiflexed.

What To Look For In Testing The Peroneus Brevis Muscle And The Peroneus Longus Tendon

Peroneus brevis. The course of the tendon of the peroneus brevis muscle runs very close to the ankle joint axis and therefore has little effect on ankle joint motion. The tendon runs the farthest lateral to the subtalar joint and therefore is the best pronator of the subtalar joint. The tendon also runs lateral to the midtarsal joint and is an abductor of that joint. The peroneus brevis is a direct antagonist of the posterior tibial tendon at both the subtalar joint and metatarsal joints.

   To test the strength of the peroneus brevis, one should pronate the subtalar joint and abduct the forefoot on the rearfoot. Ask the patient to hold the foot in this position while you place one hand on the lateral side of the forefoot and the other hand on the medial side of the leg, and push both hands together.

   Peroneus longus tendon. The peroneus longus tendon has a very similar course to the peroneus brevis tendon so it will have a similar effect to the peroneus brevis tendon in the joints they both cross. It will have minimal effect at the ankle joint, a strong pronation effect at the subtalar joint and an abduction effect at the midtarsal joint. Additionally, the tendon courses through the notch in the cuboid and inserts into the base of the proximal phalanx. The pulley effect at the cuboid will tend to create a dorsiflexion moment at the midtarsal joint. The attachment at the base of the proximal phalanx will cause a plantarflexion moment of the first ray.

   The peroneus longus muscle is difficult to isolate as it has a similar action to the peroneus brevis and extensor hallucis longus muscles. The extensor hallucis longus muscle can plantarflex the first ray through the windlass mechanism. Being aware of the extensor hallucis longus effect can avoid this problem. To test the muscle, have the patient sitting relaxed and dorsiflex the first ray with the palm of the hand. While dorsiflexing the ray, note the range of motion of the first ray. Activation of the peroneus longus will plantarflex the ray and create the appearance of a higher arch.

Other Muscle Testing Tips

Anterior tibial muscle. The anterior tibial muscle has the most leverage to dorsiflex the ankle because of an interesting arrangement of the extensor retinaculum. The retinaculum holds the extensor digitorum longus and extensor hallucis longus much closer to the ankle joint. The subtalar joint axis exits out of the foot close to the talar head, which is very near the insertion of the anterior tibial muscle.

   However, with abduction or adduction of the forefoot, the insertion can move relative to the subtalar joint axis. Accordingly, when the forefoot is abducted, the insertion of the tendon can end up lateral to the subtalar joint axis and tension in the tendon will tend to pronate the subtalar joint. When the forefoot adducts, the insertion will be medial to the subtalar joint axis and the anterior tibial muscle can be a supinator of the subtalar joint in this position.

   Testing of the anterior tibial muscle occurs by resisting dorsiflexion of the ankle joint.

   Extensor hallucis longus and extensor digitorum longus. Both of the long toe extensors pass close by but anterior to the ankle joint so they will weakly dorsiflex the ankle. These tendons are lateral to the anterior tibial tendon so they will be more pronators and less supinators than the anterior tibial tendon. The exact action depends on the location of the forefoot in the same manner that the position of the forefoot affects the anterior tibial tendons’ effect at the subtalar joint. The easiest way to test the strength of these muscles is to have the patient dorsiflex the toes while one attempts to plantarflex the metatarsophalangeal joints.

   Dr. Fuller is in private practice at Berkeley Foot Specialists in Berkeley, Calif. He is board-certified by the American Board of Podiatric Medicine.

Reference
1. Scott SH, Winter DA. Internal forces of chronic running injury sites. Medicine Sci Sports Exer. 1990; 22(3):357‑69.

   For further reading, see “Conducting A Quick And Easy Functional Lower Extremity Exam Of An Athlete” in the June 2013 issue of Podiatry Today.

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