Day 4 21-Day Jumpstart Fitness Challenge.

Grab your heavier dumbbells and prepare to work your glutes, hamstrings and quadriceps. This circuit-style strength workout hits all the big muscles of the lower body (and a few smaller ones, as well). Rear-foot elevated lunges require a step, BOSU or chair to place your back foot on. No equipment? No lunges? No worries. We've got you covered.

Tools: booty band, moderate & heavy dumbbells, chair/step/bench/BOSU