Don't use a chair with rollers for the assisted pull-up.
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Because the pull-up is an advanced exercise that not all lifters can do, most gyms have assisted pull-up machines -- the machine provides a counter weight that pushes your body up, providing assistance as you lift yourself toward the bar. But if you're working out at home, you may not have access to such a machine. However, you can still perform assisted pull-ups if you have a sturdy chair available.
Have a Seat
Use a chair that is sturdy and can safely hold your weight. Place it under the pull-up bar and make sure it is level and doesn't wobble. Select a chair with a high back, which will be useful as you grow stronger and need to increase the intensity of the movement. The chair doesn't have to be a specific height, but it should be tall enough that you can easily place your feet on the seat when you are hanging from the pull-up bar, and it should be short enough that you don't have to excessively bend your knee to place your foot on the seat of the chair.
Pull Up a Chair
Set the chair so it faces you when you are hanging from the pull-up bar. Position it so the front edge of the seat is almost directly under the bar. Grab the pull-up bar with an overhand, shoulder-width grip and place one foot on the seat of the chair. Allow the other leg to hang down toward the floor. Press through your foot as you pull yourself up. Only provide as much assistance as you need to complete the pull-up. Focus on pulling with your upper body, specifically your back muscles.
On the Edge of Your Chair
Once you become stronger, you won't need quite as much assistance, but you still might not feel comfortable doing a set of pull-ups with no assistance. To increase the intensity of the movement and decrease the amount of assistance, turn the chair around so it faces away from you and push it a couple feet away from the pull-up bar. When you perform pull-ups, place one foot on the back of the chair. This leg will support some of your weight, but because you have to extend your leg to reach the chair, you won't have as much leverage, decreasing the amount of assistance you can provide.
Fall Off Your Chair
Always have one leg hanging directly below you as you perform pull-ups. This way you can catch yourself if you lose your grip or slip. When using the high back of the chair, you have a greater chance of causing the chair to tip. To counterbalance your pressure on the back of the chair, have a workout partner place one foot on the chair or set something heavy -- like weight plates -- on the seat of the chair.