Phasic muscles have a tendency to become weak and inhibited as a result of the action of postural muscles. This is the reason why red fibers are often referred to as “tonic”, referring to tone or posture. The red fibers are “slow twitch” fibers and are important in endurance activities or as a base in the activation of a strength effort. The neurons are smaller and easier to activate, constantly sending impulses with correction messages to the postural muscles to maintain body position. This is why postural muscles have abundant red fibers. Hennemans Size Principle states that the smaller the fiber the easier it activates, the larger the fiber the faster it conducts. They are composed of red muscle fibers, because they contain significant myoglobin and are innervated by small Alpha a2 motor neurons. Postural muscles have a tendency to become overactive, hypertonic, shortened and weakened because of the physical demands placed upon them. The muscles involved in hip extension and abduction can be divided into two groups, postural and phasic (Table 1). The hypertonicity of the psoas muscle has a tendency to flex the lumbar spine, causing the lumbar spine to diminish in lordosis especially when a weakness of the erector spinae is present. An example of change in posture is one in which the person stands slightly flexed at the waist, unable to stand erect because of hypertonicity of the psoas muscle and weakness of the erector spinae muscle. The changes in posture may be observed while the person is sitting, standing or walking. They can develop because of repetitive motion, injury, pain, illness, muscle deconditioning or sedentary lifestyle.Ĭhanges in posture are often noted with an altered movement pattern in the body. Altered movement patterns do not occur randomly. The altered movement pattern is a sign of muscle imbalance in the body because of muscular dysfunction. As a result, a different sequence of muscular contractions occur called “an altered movement pattern”. The primary muscle responsible for the specific joint movement may become weak and inhibited, causing a synergistic muscle/muscles to become the primary muscle/muscles responsible for that joint movement. The physician must be able to recognize the muscular imbalance and the altered movement pattern in order to prescribe appropriate relaxation and strengthening exercises to restore the normal movement pattern in the body.Īn ” altered movement pattern” is a movement pattern in which a change occurs in the coordination of the muscle firing sequences for a specific group of muscles, facilitating a specific joint movement. The tendency for a specific muscle to become overactive and another muscle to become inhibited occurs over time creating a muscle imbalance and an altered movement pattern. An example of a phasic muscle is the gluteus maximus muscle, a hip extensor. On the other hand, phasic muscles have a tendency to become weak and inhibited. An example of a postural muscle is the psoas muscle, a hip flexor. Postural muscles have a tendency to become overactive, hypertonic, weak and shortened in length. Janda classified muscles into two groups “postural and phasic”. According to Vladimir Janda(1), muscle imbalances develop between muscles that have a tendency to develop tightness and other muscles which are prone to inhibition. Muscle strength can change due to different reasons, which include sitting or standing, repetitive motion, injury, lack of exercise and deconditioning causing a muscle imbalance in the body. Muscles facilitate joint movement in the body and help maintain posture.
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