Muscle fibers are recruited and activated on an as-needed basis. The more muscles you work at once, the more fibers you recruit. The heavier you train, the more fibers you activate. The harder you train, the more you will recruit additional fibers to complete your set. Your ability to recruit more fibers will increase based on your use of heavy, compound lifts such as the squat and deadlift. Consult a health care practitioner before beginning any strength training program.
Perform complex lifts such as the squat. Squatting recruits more fibers and activates more of your muscles than the leg extension. To squat, hold the bar securely on your upper back and descend as deeply as you can, then stand back up. Avoid leaning forward or rounding your back.
Lift heavy. Heavy lifting recruits more fibers. Gradually increase your training weights until you are training with at least 80 percent of what you can lift for a single repetition.
Train to positive failure. Continue lifting until you cannot perform another repetition with good form. Rest three minutes and repeat.
Train with light weights, using no more than 70 percent of your one-repetition maximum. Lower the bar under control, but on the way up, accelerate as quickly as possible.
Employ all three methods for maximum fiber recruitment -- heavy training, speed training and repetitive effort. If you only employ one method, you are only doing one-third of what you need to achieve your goals.