Muscle tone: “The slight constant tension of healthy muscles which contributes a slight resistance to passive displacement of a limb” - J. Field, 1960
Muscle tone may be defined as "the slight constant tension of healthy muscles which contribute a slight resistance to passive displacement of a limb." (ref 7)
The newborn's neuromuscular examination includes an assessment of both active and passive muscle tone. If all newborns were normal and healthy, both active and passive tone could be used routinely to assess neuromuscular maturation. Active muscle tone, (motility, activity, or efforts at righting oneself) is markedly affected by states of illness, recent maternal medications, acute perinatal compromise and level of alertness. Hence active muscle tone is not consistently useful in evaluating baseline neuromuscular maturity. Passive tone essentially is unscathed by those same factors that profoundly affect active tone. Hence passive tone is useful for evaluating maturational development of the neonatal brain, regardless of the infant's state of alertness or level of wellness.
Passive tone may be further subdivided into extensor and flexor tone. The human fetus, lying primarily with limbs extended in the very early phases of development, gradually assumes a progressively flexed attitude. This is true whether development occurs in utero or in the nursery, (ref 8) and thus reflects maturation of the central nervous system rather than extraneous compressive forces of the uterus. Passive flexor tone gradually overcomes passive extensor tone as maturation progresses.
Progression of neuromuscular tone development proceeds in a caudo-cephalad and centripetal direction; i.e., lower extremity passive flexor tone develops slightly ahead of upper extremity tone, and distal passive flexion precedes proximal passive flexion. (ref 8)