contraction is biceps activity as the weight is slowly low- ered during elbow extension. Eccentric contractions are extremely common. In fact, every movement in the di- rection of gravity is controlled by an eccentric contrac- tion. Other examples include sitting, squatting, lying down, bending forward or sideways, and going down stairs. The amount of energy expended during an ec- centric contraction is always less than that observed during a concentric contraction for the same muscle.
The amount of metabolic work associated with eccen- tric contractions is one-third to one-thirteenth the work of concentric contractions. In SEMG recordings, the mi- crovolt amplitude of a concentric contraction is always larger than it is for the eccentric contraction, given the same amount of weight. For example, the erector spinae of the low back work harder and show higher levels of recruitment when returning from a flexed posi- tion than when going down into forward flexion from the neutral position. Researchers believe that the eccen- tric contractions require less SEMG activity because much of the work has to do with “breaking” existing cross-bridges rather than building new cross-bridges.
Another form of contraction is isotonic contraction, a subset (special class) of the concentric and eccentric contractions. Isotonic contractions occur when constant muscle force is employed as the muscle either shortens or lengthens. This type of force is studied most com- monly on instruments in which the force is controlled over the range of motion.
Surface EMG recruitment patterns may differ when they are observed in an open versus closed kinetic chain. In an open kinetic chain, the distal segment is free to move—as in the case where a person is not bearing weight in the lower extremities. The open kinetic chain entails a movement that is not resisted manually or through weight bearing. Movement of one joint does not necessarily cause movement in other joints. In a closed kinetic chain, the distal segment is fixed, as in weight bearing for the lower extremities. Movement at one joint induces movement at another joint. For exam- ple, in sitting with the leg hanging over the edge of the table (open kinetic chain), a person can move the ankle without moving the knee or hip. But when the person stands (closed kinetic chain), he or she cannot move the ankle without affecting other joints. Surface EMG re- cruitment patterns are typically greater under the con- ditions of a closed kinetic chain compared to those observed under the conditions of an open kinetic chain.
For example, the level of SEMG activity from rectus femoris during a squat (closed kinetic chain) is greater
than when the muscle is contracted from the seated po- sition and the knee extends without resistance (open ki- netic chain).
locity. An artificial example of this functionality is the knee jerk. When the patella tendon receives a swift blow, the quadriceps muscle is unpredictably and sud- denly elongated, and the muscle spindle is stretched.
This leads to excitatory outflow to the motor neuron, which results in the contraction of the quadriceps muscle. This, in turn, quiets the muscle spindle. The briskness of this response is commonly graded in neu- rological examinations. On a functional level, the brisk- ness of the response is determined by the output of the gamma motor system. Edmund Jacobson11conducted studies on progressive relaxation procedures, in which he was able to demonstrate a substantial diminution (if with the gamma motor system. These gamma motor
nerves are smaller and have slower conduction veloci- ties than the alpha motor nerves. The gamma motor system primarily emerges from the lower centers of the brain, while the alpha motor system originates prima- rily from the cortex. There is commonly a CNS linkage or coactivation of the alpha and gamma motor systems—a relationship that was first observed by Hunt and Kuffler9in the 1950s, and elaborated upon by Vallbo10in the 1980s.
The primary purpose of the muscle spindle, then, is to regulate the muscle length, make postural adjust- ments, and maintain a predicted muscle length and ve-
Figure 2–6 Relationship of muscle spindle and Golgi tendon organ to extrafusal muscle fibers. (A) Spindle (located within the intrafusal muscle fiber) is parallel to the muscle fibers so that passive muscle stretch causes a secondary contraction or stretch reflex. (B) Muscle contraction slackens tension on the spindle. Golgi tendon organ is in series with the muscle fibers (resides between the muscle fibers and their bony attachments) so that both passive and active contractions of the muscle cause the receptor to become active.
Source:Netter Anatomy Illustration Collection, ©Elsevier, Inc. All Rights Reserved.
not frank absence) in the knee jerk in patients who were deeply relaxed. Thus, one can learn to alter and control the sensitivity of the stretch receptor through al- terations in the level of gamma motor activity through deep relaxation.
In real-life situations, the muscle spindle monitors the effects of the gravitational field upon a person’s posture and helps keep the person erect. For example, when a person walks along an uneven surface, the muscle spin- dle constantly adjusts the tone of the muscle (excitatory input to the alpha motor neuron) to compensate for the unpredicted stretches upon the muscle. The resting tone of the gamma motor system is especially important for an athlete. If the muscle tone is too high or too low, the timing and effort of the athletic performance can be thrown off. Researchers have recently found that the muscle spindle responds to emotional arousal.4Perhaps this response explains why professional and Olympic athletes work on their emotions and attitudes toward their performances just as much as on their motor coor- dination skills.
Recently, the muscle spindle has been suggested as a potential site for the trigger point.12Trigger points are anatomical locations that, when activated by pressure or movement, refer pain to the immediate area or a dis- tant site.13Dynamic SEMG studies14have reported that during a symmetrical movement (i.e., forward flexion of the head), muscles that have an active trigger point tend to contract at a higher level of activation than do the contralateral muscles without trigger points. This find-
ing suggests that an active trigger point in a muscle tends to alter the sensitivity of muscle spindles for the in- volved muscle, making it more sensitive to changes in its length.
Another important sensory organ within the muscle is the Golgi tendon organ (see Figures 2–6 and 2–7), which is found at the muscle tendon junction and runs in series with the muscle. It is exquisitely sensitive to the tension placed on the tendon, and it perceives the effort given out by the muscle. On its most basic level, the out- put from the Golgi tendon organ loops through the cord and inhibits the alpha motor neuron for the 5 to 10 motor units that are pulling on it. In essence, the in- hibitory influence of the Golgi tendon organ protects the muscle from tearing itself loose from its attachments.
On a more subtle level, the Golgi tendon organ informs the CNS of the effort that is under way and facilitates the necessary inhibition needed for learning. For exam- ple, when a piano student is learning to distinguish be- tween playing the piano softly and playing it loudly, the student must learn to press on the keys in soft and hard fashion. While the ear can readily hear the difference, the Golgi tendon organ can feel the difference; it pro- vides the proprioceptive basis for knowing when the key has been pressed just hard enough. The output of the Golgi tendon organ terminates in the lower centers of the brain (i.e., basal ganglia). This output does not reach the cortex, so we are unconscious of its presence. Yet it is through the interaction of the information supplied by the Golgi tendon organ with the information provided Figure 2–7 Relationship of alpha and gamma motor neurons to muscle spindles and Golgi tendon organs. The alpha and gamma motor systems must coordinate to maintain the correct tension on the muscle spindle.
Source: Netter Anatomy Illustration Collection, ©Elsevier, Inc. All Rights Reserved.
sensitive to one degree of arc, and never habituates (turns off). It continuously provides the nervous system with information concerning joint angle and position.
One of the important aspects of this sensory system is that we are conscious of it. Although we are not con- scious of the level of tension in our muscles, we are readily aware of our limb and joint position. For example, it may make more sense to talk to a patient about how high the shoulders are than to talk about how tense the upper trapezius muscles might be. Similarly, it is easier for patients to respond to the request to lower the shoul- ders than to a request to relax them. This terminology simply makes more sense to patients, because they can be consciously aware of joint movements yet con- sciously blind to their tension levels.
The sensory network of the muscles provides a wealth of information to the central nervous system concerning muscle function. Some of these influences are excitatory, some are inhibitory, some reach the cor- tex and are conscious, and some terminate in the lower brain and are not consciously perceived. All pass through the cord and influence the segmental intelli- gence of the total system.