This exercise: works the lower back part of your legs (calves), helps your calves not get tired as quickly. 

Steps: 

1. Sit tall in your chair with your feet flat on the floor. 
2. Lift your heels off the ground, while keeping your toes planted on the ground. 
3. Hold for 5-10 seconds before lowering your heels back down slowly. 
4. Do 3 rounds of 10-12 counts.