The first one is Isometric Lunge. 

Why it's great: This single-leg-focused hold helps even out your lower-body strength and improves running mechanics.

How to: Stand up tall, then take a large step forward. From here, bend knees and lower body until knees are bent to 90 degrees. Hold for up to 30 seconds. That's one rep. Perform five on each side, then immediately continue on to your next move. When you've completed all of your exercises, rest for 45 seconds. Then, repeat twice more for a total of three rounds.

The other method is that you can try this product. It can exercise your leg muscles and enhance leg strength.