The target muscles of the seated leg curl machine are the hamstrings. But the answer to whether using the seated leg curl machine can also lead to bigger calves is that it depends. Machines vary, and certain ones work your calf muscles more than others. Body types vary too, so whether your calves get "bigger" or actually look slimmer, more shapely or hardly change at all can depend on your genes. And then there's weight versus repetitions.
If your gym has a seated leg curl machine, chances are it's one of two designs. In one design the lap pad rests just above your knees with both that pad and the lower pad being fixed. This design does not work your calves. In fact, hardly any muscles are worked outside of the hamstrings. In the other design, the upper pad rests just below your knee. Often, with this design, both the knee pad and the lower pad roll. This causes your feet to flex back on the curl -- called dorsiflexion if you want to impress your friends -- working your calves as well as your hamstrings.
You may have heard the terms endomorph, ectomorph and mesomorph. While they may sound more like something out of a video game, they are really body types that affect how you respond to resistance training. Endomorphs tend to have bodies that are softer and rounder, while ectomorphs are more slim and linear. Mesomorphs are more likely to bulk up with resistance training. So, if you're a mesomorph, your calves -- and your thighs -- are more likely to get bigger. As an ectomorph, they'd probably stay slim and maybe gain some shape, and if you're an endomorph, you'll need more work, not less, to harden those calf muscles.
So how can you work your hamstrings if the only seated leg curl machine at your gym works your calves, and you can no longer zip those boots you bought last year? One option is to do more repetitions and sets with less weight. However, if you're a full-blooded mesomorph, even that may not work. Another solution is to eliminate extra calf work you may not realize you're doing. For example, move from the stair stepper to an elliptical, which uses less calf muscle, and definitely skip the squats, lunges or leg presses, targeting your quads with the leg extension machine instead. Good mornings and straight-legged deadlifts done with dumbbells, cables or a barbell are among the few non-machine exercises that target the hamstrings without involving your calves.
In your quest to slim your calves, don't neglect your back. Most hamstring exercises can strain your back if done improperly. For the seated leg curl, make sure your back is plastered against the seat back. Also, set the machine to limit the range of motion if you have knee problems. You need to be especially careful with deadlifts. Always err on the side of caution with your beginning weight and build gradually. Follow your hamstring workout with stretches. Extend your leg in front of you with your heal on the ground or on an elevated surface. Flex your foot back and bend forward at your hips while bending the knee of your supporting leg until you feel a stretch in the back of your thigh.
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