Glengarvan,
The jumping or twitching is just random electrical impulses, causing the muscle to contract. This actually occurs all the time, through out the day. But, we are usually using our muscles, so the occasional twitch goes unnoticed or is overridden by the stronger contraction the muscle is already undergoing.
Twitches are most noticable when a person is resting. So, after surgery we tend to rest the muscles involved (since it hurts to contract them). So, the twitches become apparent.
In some special cases, when the tension within a musculotendinous structure has changed (like when they debrided the tendon or in cases of a tendon repair/transfer), it takes a little while for the muscle to get used to its new resting length, so the twitching can increase.
There are some systemic causes, like imbalances in electrolytes, vitamins, or minerals. But, these usually cause twitching in all parts of the body.
In any case, they are usually benign and go away with an increase in activity and time. You can try gentle stretching (within the limits set by your surgeon) or massage.
If, however, they become cramps or they just bother you, contact your surgeon.
Good luck.