I have a client doing matwork who has continuous cramps in her Harmstrings. Whenever we do Bridging Exercises she has to stop because it is so bad. She is a very disciplined person, big Horsebackrider, very tight and tensed in her whole body, already two disc herniations in the past.
Has someone a good tip how to help her? I cannot change her personality..

May be a silly question- but I had a client who had a similar issue. When I had her consider engaging the glutes prior to using the hamstrings- well it has worked like a charm. Good luck!

I have also had good luck with having them down on their heels initially

If the spring load is heavier, it will actually be easier on the hamstrings. Also make sure she is engaging her glutes and low back-if those areas are weak the hamstrings will take on more than they should. Also make sure she is hydrated and eating lots of potassium rich foods!

I ask my clients to lift their toes of the floor and emphasize the lower abs on the take off of the movement to bypass the hamstrings as much as they can. On the reformer I also keep their heels on the foot barre.

Try asking her to bring her feet closer to her glutes to help switch off her hamstrings.

Sounds like she's got delayed glutes, which can cause the hamstrings to over fire and cramp.

A few ideas:

1) Have her try bridging with the knees straight (but unlocked of course) elevated on a box. Since the hamstrings also flex the knee, this can sometimes partially take them out of the game.

2) Try putting a ball or roller between the knees or a band just above the knees, which can activate the lateral slings and sometimes increase glute activation.

3) Try foam rolling the inner thighs/quads/hamstrings before bridging to decrease some of the noise/tension and see if helps improve muscle timing.

4) There's a good chance you've tried this (and a lot of what I already said), but try cuing her to relax as much as possible and then to push into the ground through the whole foot to lift the hips. Sometimes the initial push off can help people feel their glutes (especially if you cue it) to make for a more successful bridge.

Hope this helps! :)