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Muscle tone:

This is the term for the typical resting length of muscles in a person's body. It is a comparitive term, so that one can have "low muscle tone," "low-average tone," "high tone," etc. Medically,  people sometimes use tonus in place of tone. What it looks like: high tone muscles are rigid and tight. They don't allow a full range of motion at the joints to which they are attached. For example, someone with high tone in their arm ("upper extremity"), will not be able to completely straighten their arm. Problems with muscle spasming or "locking" into different types of rigidity often accompany high tone. Low tone muscles are mooshy and soft and allow a great deal of range of motion at the limbs. They feel heavier than they look. People with medically low tone (see below) are at risk for impingement of muscles and nerves due to the extra space in the joints, and for dislocation of joints. Low tone can look like very low strength, since it takes more time and energy for these muscles to "get going" for a contraction.

The way that tone is tested is by looking at range of motion at joints, palpating the muscles, and sometimes tendon reflexes are looked at in conjunction with tone.

High tone refers to medically high tone, and likewise with low tone. Within the range of "normal tone," a person can have low-normal or high-normal. This is more likely what therapists will be referring to when working with children without a medical diagnosis of cerebral palsy, multiple sclerosis, or the like. Often, people don't clarify that they mean "high-normal," and will just say "high tone." This can be very confusing. Keep in mind the population that the professional discussing tone usually works with, and ask for clarification if needed.

Many children with sensory processing issues tend to have either low-normal or high-normal tone. In my experience, I see far more children with low-normal than high-normal tone. These are the kids who can't sit for more than a couple minutes without propping themselves up on their arms or hands. At circle time in Kindergarten, they are leaning on someone or something, lying on the floor, or seriously slouching. They are flexible and good at gymnastics or yoga when stretching is involved, but have a hard time holding involved positions against gravity. They are usually not very good at most athletics because they fatigue quickly and don't tend to move quickly. On the other hand, there are some high-energy, low-normal tone kids (usually with seeming problems with attention), who throw themselves around quickly and with force and manage to succeed pretty well at some athletics (dodge ball, soccer, football, frisbee). When it comes to holding a position or a contraction, though, they have a hard time and quickly go back to moving themselves quickly and recklessly about.

What to do about it:  muscle tone is an in-born trait and cannot be changed in the long run (I have heard from some physical therapists that it is possible with specific exercises; I think this is still a new idea). You can increase or decrease tone in the short-term using things like temperature, vestibular input (through brainstem reflexes), resistive exercises, and tendon tapping. Therapists will often do these things before a purposeful activity.  Neurological events (strokes, seizures) can change tone. However, by strengthening muscles, especially the muscles surrounding a specific joint, stability can be increased. Also, by increasing the actual number and bulk of fibers in a muscle (strengthening), the effect of overly-long (low tone) fibers can be reduced. High tone muscles should be stretched gently and regularly, especially just before skilled use.