At one extreme, a muscle may be fully relaxed, when much electromyographic evidence shows no activity; the muscle is electrically silent. This contrasts with earlier accounts by electrophysiologists, who held that “full relaxation” was still accompanied by “background” activity rotating among a few units. Misconceptions concerning such “muscle tonus” were based on these views.
MUSCLE TONUS: Some writers discard the terms “muscle tone”, or “tonus”, or define it as the response of skeletal muscle to stretch. The electromyograph registers no activity in a resting intact muscle, but such a muscle is certainly different from one with its nerve supply cut, and certainly has no slack to take up before contracting. It may be that muscle tissue itself has an intrinsic elasticity.
“Relaxed” muscles in the body are comparatively firm. This tone is apparently a result of low levels of contractile activity in some motor units driven by reflex arcs from receptors in the muscles; tone is abolished by dorsal root section.
The stand-at-ease position has been considered as movement upon a stationary base in view of the slight swaying to and fro and side to side that eventually occurs. When a person is steadily balanced in this position, little if any activity is necessary or can be detected in the trunk, thigh and anterior leg muscles, but the calf muscles are active. The line of the center of gravity passes in front of the ankle joint where the erect position is maintained either by an accurate, if precarious, balance or by activity of the calf muscles. whereas at the hip and knee joints, with the gravity line respectively behind and in front, ligaments may take the strain before muscles are necessary.
However, no continuous activity can be detected in the trunk and thigh muscles in the standing-at-ease position, where the effect of gravity to rotate the trunk backwards on the hip joint is counteracted by tension in the iliofemoral ligament. At the knee, the weight of the body may produce hyperextension, passively checked by the anterior cruciate ligament, but usually a balance is reached with the knee not fully extended. Any swaying from a position of balance may be seen or felt counter-acted by a brief contraction of the appropriate muscle group.
In the leg, however, the muscles one might expect to feel relaxed in the to-and-fro movement are very firm compared to their softness felt in a leg hanging free.
ADVANCED HYPERTONICITY: Ligamentous action of muscles occurs when they are unable to lengthen sufficiently to allow full movement of a joint. The hamstrings in the majority of people do not allow full flexion of the hip with the knee extended, an explanation of their inability to execute the high kick. Because of tight pectoral muscles, certain persons are unable to raise both arms to the vertical.
The act of touching the toes with the knees straight requires relaxation of the hamstrings and adductors; inability to relax the same muscles also prevents the execution of the high kick.
It is a principle in physical training that exercise should be balanced to secure both the full contraction and the full relaxation of a muscle.
