Provided by Marion Main,
Superintendent Physiotherapist at The Hammersmith Hospital.
Below is a diagram and description of muscle groups and how they work.
Many of us either undertake or provide physiotherapy, and the following information is designed to aid understanding of this process and also to introduce us to the language used by physiotherapists and other professionals.
Spinal Muscular Atrophy - the weaker and stronger muscle groups
In the body the muscles usually work in pairs. One group to bend a joint, the opposite to straighten it. One group to move a limb sideways, the opposite brings it back to the middle.
Even in the "normal", one group is a little stronger than the other, because functionally, that is the way the body needs it. For example, the muscle group at the front of the thigh that straightens the knee (the quadriceps) is stronger than the group at the back (the hamstrings) that bend the knee. This is to help in walking, getting up stairs, getting up from a chair, etc.
In SMA Type II and III, and to a lesser degree in Type I, for many movements, there is a noticeable difference in strength between some of the opposite pairs of muscles. This leads to the problems of movement and function that we typically see and also can cause the contractures (fixed range of movement) and tightness from one muscle group pulling more strongly than another across a joint.
There are always exceptions and a few children and adults will not have this same pattern, but this is the general pattern for most people with SMA.
M. Main MCSP JTSMA conference 2002 | 
