Improve leg strenght

This exercise is great for people who have weak legs. What you have to do is go to hack squat machine and do not load any weights.
Use only body weight.

Flex hips and knees to descend until knees are just short of complete flexion. Raise sled by extending knees and hips. Repeat. Well that is normal hack squat exercise.
But try this one my way if you want to improve your leg strenght: Descend down extremely slowly and count for 60 seconds (so you will go down slowly for 60 seconds). Come down until hamstrings are parallel with the floor. Than go up but like this: Count for 60 seconds and come extremely slowly up.

To explain it again: You will come down (descend) in extremely slowly motion. Take your time to come down because you have 60 seconds. You legs will start shaking. You will see. Than when your hamstrings are parallel with the floor, start coming up. Go slowly here and take your time because you have 60 seconds to come up.

Do 3 sets of this exercise. And you will see results.