Muscles as Team Players
By Erik Dalton, PhD
Synergistic dominance occurs as “helper” muscles are recruited to take over function when a “prime mover” muscle fails, much like when a football coach calls in the substitute players when a key player is injured.
These synergistic stabilizing muscles are designed to help, but not be primary contributors, to a particular movement. Synergistic may be defined as “acting together to enhance the effect of another force.” Therefore, if muscles perform the same task at a particular joint, they are termed synergistic.
Altered reciprocal inhibition occurs when a muscle is activated (the agonist), when it should not be. Excessive stress on the agonist decreases the signal strength to the opposing muscle (the antagonist). In altered reciprocal inhibition, the agonist muscle is being activated even though it is not actively contracting. Altered reciprocal inhibition is often the culprit causing synergistic dominance. For example, in forward-head postures, the client’s suboccipitals are often maintained in a hypercontracted state as they battle gravity to keep the eyes level with the horizon. As the head cocks back and moves forward on the neck, the antagonist longus capitis muscles – which bind the anterior surface of the upper cervical vertebrae to the occipital base, become overstretched and weak (Figure 1).
Fig. 1 (© www.erikdalton.com) Sensing the longus capitis muscles can no longer carry out their duty as primary head-on-neck flexors, the brain calls on the powerful sternocleidomastoids (SCMs) as pinch-hitters. The SCMs are reliable neck flexors when allowed to fire in proper order. However, they serve as poor subs for longus capitis due to their insertion at the mastoid process. When reciprocally weakened from suboccipital hypertonicity, longus capitis muscles give way to the powerful SCMs causing them to fire first in an effort to hold the head upright on the neck. But, instead of holding the head upright, the SCMs “extend” the head on the neck, causing a forward head posture. Neural and vascular structures embedded under the posterior O-A joint aren’t happy with this excessive compression.
Fig. 2 (© www.erikdalton.com) When the neck’s normal firing-order sequence is disrupted, synergistic muscles begin pulling the head in different directions, sending torsional and compressive forces through the facet joints and intervertebral discs. This often results in chronic degenerative conditions such osteoarthritis (spurring), degenerative disc disease and ligamentous laxity. The client may come in complaining of migraines, radicular pain in the arms or thorax, or even an unsightly dowager’s hump (Figure 2).
At some point, the brain may get “fed-up” with the flood of noxious mechanoreceptor and possibly chemoreceptor input, and decide to lock the area down with protective spasm. Of course, this may further alter the firing order pattern causing a pain-spasm-pain cycle that’s often hard to break. The client’s gait may reveal certain body parts that appear frozen in time, as chronically embedded compensations have caused the brain to sacrifice complexity of movement for stability. Fortunately, simple tests help determine if synergistic dominance exists at a particular joint.
Fig. 3 (© www.erikdalton.com) Neck Flexion Test
Forward bending of the head and neck with the client in a supine position should initiate the following firing-order sequence: longus capitis, longus colli, SCMs and anterior scalenes. The deepest intrinsic muscles must fire first starting with longus capitis (flexing the head on the neck) followed closely by longus colli, which initiates the beginning of neck flexion. Anterior scalenes and SCMs can then join forces to produce smooth head-and-neck flexion.
Fig. 4 (© www.erikdalton.com) The most commonly seen substitution pattern (SCMs, anterior scalenes, longus colli and longus capitis) causes the chin to reach toward the ceiling rather than tucking into the chest during the first two inches of flexion efforts (Figure 3).
The neck flexion test alerts the therapist as to which musculofascial tissues need lengthening and which must be strengthened. By performing the head-raise test before and after each neck session, aberrant substitution patterns can be easily identified and corrected. Tension-length imbalances are usually easy to fix once proper assessment is made. The technique demonstrated in Figure 4 is one of my favorites for treating adhesions and contractures in the SCM muscles and accompanying fascia.