Stand with your feet together, holding a dumbbell in each hand, either by your sides, or with bent elbows as shown in the photo.
Put weight into your left foot and step the right foot back into a lunge position, so both knees are at 90-degree angles, with the back knee just barely touching the floor.
Step the right foot forward.
Lunge the left foot back, then step it forward.
This counts as one rep.
Complete as many alternating reverse lunges as you can for 20 seconds. Put the weights down and take a 10-second rest.