Depending on your fitness levels, you could start your leg presses at the same weight as your mate. There's no set weight for women or men starting out in any exercise. It's really just trial and error, but be careful. If you try a weight that's too heavy to start, the error could leave you sidelined indefinitely.
A rule of thumb used by many personal trainers is to start with a weight at which you can do eight leg presses fairly easily with 10 to 12 repetitions leading to muscle failure. Meaning, your legs quiver if you try doing even one more. Of course, you have to use common sense or else you could be there all day starting at an insignificant weight and moving up until your muscles fail. You should feel some resistance from the start. On the other hand, it's better to go too light in the beginning than too heavy. If you find the weight you choose is a little too easy, up it a little for a second set and then start at that weight next time.
Another way to tell if the weight is too heavy is if you can't complete at least eight reps in proper form. If you find yourself throwing your back into it, literally, that's not a good thing. Your lower back should stay plastered to the seat throughout the entire movement and all your reps. You shouldn't have to thrust your legs out either. As the name suggests, you should press them out and then be able to control the return so it's slow and controlled.
Targeting Different Muscles
With your feet centered on the plate and about hip-width apart, the leg press machine works your quads, hamstring and glutes. Moving your feet down will emphasize the quads more, while moving them up emphasizes the glutes. Placing just your toes on the bottom of the plate will work the calves. If you choose to target different muscles, you may find that the proper weight will change for different moves, so you'll need to experiment.
For your muscles to continue to grow, they must be constantly challenged. Once you can do eight reps fairly easily, continue to add reps up to 12. Once 12 reps become easy, increase the weight. Doing the same exercise indefinitely will also lead to plateaus. As a beginner you can continue doing leg presses for about eight to 10 weeks, then it will be time for a change. Try other machines, like combining the leg extension and hamstring curl, or try free-standing or ball squats or standing or walking lunges. After your initial period, you should change exercises about every four to eight weeks, and you can circle back to the leg press as well as other exercises you've done once your muscles are no longer accustomed to them.
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