For hamstrings, do "10 and 2" stationary lunges. You can put a barbell on your back for these, but start with bodyweight at first. If you're weak moving through this range of motion then you could end up straining any one of the muscle groups involved.

Pretend you're standing on a clock. Step with your left leg to the 10 o'clock position and perform a controlled eccentric (negative) and powerful concentric on each rep. Then with the right leg, step to 2 o'clock and do the same. Do 4 sets of 10 reps for each leg.

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