Mix balance with weights.
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When we work out, we think about pushing ourselves muscularly or cardiovascularly, but why not also challenge yourself neurologically? Balancing on one leg or incorporating exercise balls into the workout – whatever weird variations you can think of – forces you to focus on the position because you've got, like, six things happening simultaneously. You just get fried from it. Use the exercise ball to challenge your stability, and not just for core work and push-ups. Instead of doing the same bench press you always do, lie back on a ball, and press dumbbells. Rather than doing your curls in front of the mirror, sit down and do them on a ball. Do a one-arm shoulder press while sitting on it. Implement the ball into everything. Believe me, if you're strong, that'll be like learning it all over: All of a sudden, you've got yourself some work.