Rotating plank

Muscles worked: Erector spinae and abs, pectorals
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Start: Start on your hands and knees and prop yourself up in a push-up position on your hands and toes, your body in a straight line from head to toe. Look down and keep your neck aligned with your spine. Try not to sink into your shoulders. Tighten all your core muscles and breathe normally. 
Technique tip: Drop your hips until your body sags and you feel increased weight in your lower back. Then raise your hips back up to the point where the weight is lifted from your lower back and more equally distributed from head to toe. Maintain this position.
1 of 3 Photo by Lynda Churilla for MSN Health & Fitness

Finish: From the basic plank position, rotate your torso outwards to the left into a side-bridge, resting your body weight on your right arm and opening your left arm straight up reaching to the ceiling. Allow your legs to cross as your body turns, top leg in front of the lower leg. Then rotate inwards to return to the plank, and rotate out to the other side. Repeat, alternating sides.

Safety tip: Make sure that you are strong enough to maintain a perfect plank position for at least 30 seconds, and that you are able to maintain good form doing a basic plank and moving your arms and legs in different variations of this exercise, before you progress to this move. If you feel any strain in your lower back or shoulders, choose an easier variation to help build up more strength. 

2 of 3 Photo by Lynda Churilla for MSN Health & Fitness