Clonus is an involuntary rapid and rhythmic contraction of a muscle that occurs when the affected limb is put in a specific position and usually continues for the duration that position is held. In Multiple Sclerosis (MS), clonus is usually the result of a lesion in the spinal cord between C1 and T12 (basically the neck to the middle of the back). How low or how high up the lesion is in the spinal cord will determine what muscle group is affected by clonus. People with MS usually see clonus in muscles affected by spasticity such as the legs. Treatment is rather limited and usually consists of physical therapy exercises but in some cases, Botox injections may be used.
Personally, I experienced clonus in my foot/ankle when my ankle was rapidly bent so that my toes would point up towards my body. This is pretty typical in MS and most neurologists will check for signs of clonus by jerking your foot into this position. My clonus was also sensitive enough that if I put weight on my toes while sitting down the shaking would begin. Below is a video of myself triggering clonus but this side effect has slowly (over the years) faded away. I never really did anything to treat it as it was very easy to avoid positioning myself in a way that would trigger it so it never really bothered me.