If this helps I wrote this about spasticity:
The nervous system is incredibly complex and takes years to learn about after regular medical training. Every neurologist worth their salt admits that we know very little about the brain and nervous system at all. I will try to explain spasticity in layman’s terms using illustrative examples.
Our most obvious movements could be thought of as basically two muscles at a joint, for the sake of simplicity. Taking two muscles most people know, such as the biceps and triceps, we know they are both in the arm but are used for opposite functions. To bend the arm, the biceps is pulled tight so it gets shorter, while the triceps gets longer and stretches, allowing the joint to move. To extend the limb, the opposite happens – the triceps shortens and the biceps lengthens. In reality there are more muscles at play but, if you hold your arm out and bend your elbow, so that your hand comes towards your face, you can feel the biceps contracting and the triceps stretching. Now straighten your arm out again and you feel the opposite happening.
When spasticity is involved, as the brain sends the command to the biceps to contract, there is background noise like static. The muscle hears static over the real message, and so hears ‘Contract – no, get longer – no, contract.’ However, the static is also getting in the way of what the triceps is hearing and so it is hearing much the same. Both muscles try to contract and the result is some crazy, jerky movements.
This is why spasticity causes a lot of stiffness and deep muscle aching.
Medications can help reduce and control spasticity. They usually work by reducing the amount of static, which was confusing the muscles. As a result, people often complain they are weaker when on these medications, because the static was masking a lot of their true weakness with all the extra contractions. Now, without the static in the background,
the muscles are more relaxed and so the weakness is more apparent.
