Contact Sports After Thread Lift?

I would ask your physician, but it takes about 6 weeks to build scar tissue that begins to take over for the thread lifts.
Generally, we would advise you to refrain from contact sports for a 6 week minimum, but other physicians may advise you to hold off for 3-4 months.

Check with your doctor for sure, but 6 weeks should do the trick.  By the way, taking blows to the head as in from boxing and jiu-jitsu has been proven to cause brain damage.

A thread lift does not last very long even in the best of situations. If you are active in contact sports or other activities that produce a lot of garring then your results will decline quickly. Consider other forms of rejuvination such as fillers.
If you do proceed with a thread lift, wait at least 6 weeks before having any contact or vigerous activity. The longer the better because you need internal healing to provide some strength.