Stand holding a pair of medium-weight dumbbells in each hand, arms at your sides, with your knees slightly bent.
Keeping your arms straight and knees slightly bent, slowly bend at your hip joint, not your waist, and lower the weights as far as possible without rounding your back, which should remain straight. Looking forward, not at the ground, will help you avoid rounding your back. Keep the weights close to, almost touching, your legs.
Squeeze your glutes to slowly pull yourself up. This counts as one rep. Be sure not to use your back or round your spine!
Do as many reps as you can with correct form for 40 seconds. Take a 20-second rest.