In addition to lifting too heavy, the type of weight machine you choose to train with can also put you at risk. For instance, a leg extension machine, which involves pushing weight upwards with your feet from a seated, reclined position, may put more stress on your knee joints than necessary. Certain machines meant to work your abs and back can be risky, as well, because they put pressure on or twist your spine. If you feel pain or strain while using a workout machine, you should probably ditch it or ask a trainer for assistance. There are other exercises that use the same muscle groups you could try instead. In the case of the leg extension machine, for instance, you may opt for squats or lunges. Instead of ab machines, try planks or another core exercise you can do on the floor.