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Most people do not perform standing, weighted side-to-side crunches with proper posture, so it creates too much strain on the spine and can lead to lower-back injuries, says Kim Truman, certified personal trainer and owner of Kim Truman Fitness. Since the 90-degree burn only uses your body as resistance, you place less stress on your spine while still working your obliques.
To do the 90-degree burn (pictured), lie on right side with legs and feet together, upper-body propped up on right elbow. Bend left arm across chest with fingertips lightly touching the floor. Extend legs and feet slightly in front of body, and lift to about a 45-degree angle, keeping hips stacked. Hold at the top of the movement for about 3 seconds, and then slowly lower legs back to the floor. Perform equal reps on both sides.
Don't waste your time (or risk injury) with these moves the pros won't touch
female doing crunch on medicine ball