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Two things could be going on here: Either your weights are too light, which is often the case for women, or they're too heavy, and you're letting momentum or gravity take over. Either way, your muscles aren't being sufficiently challenged, which is why they're not getting more toned. "To see an improvement in definition, you need to have an increase in the protein content of muscle fibers, and that happens when the muscles are stressed and being called on to exert more force," Bushman explains.
The Solution: If you don't believe you've done just about all you can do by the end of a set, pick a heavier weight. "You want there to be a bit of strain on the second-to-last and last reps," Bushman says. Reach for a lighter dumbbell when you aren't moving the weight with steady control as you lift and lower.