This is an excerpt from Strength Training for Young Athletes-2nd Edition by William Kraemer & Steven Fleck.
We've established that most machines allow movement only in a predetermined plane or movement path so that balancing the resistance in all directions is not necessary. However, free-weight exercises require balancing the weight in all directions, which makes learning exercise technique for them more difficult than for machine exercises. Balancing the resistance requires the use of muscles other than the prime movers. For example, the prime movers in the military press are the deltoid, located on the outside of the shoulder, and the triceps, located on the back of the upper arm. However, the muscles of the upper and lower back, smaller muscles around the shoulder area, and even the abdominal muscles are all used in balancing the resistance. The involvement of these secondary, stabilizing muscle groups needed for balancing the weight is greater with free weights than with machines. Advocates of free weights point out that sporting events and daily life activities require balancing any resistance moved, so having to balance the weight during resistance training is an important aspect of preparing for those activities. Conversely, machine advocates believe that the lack of a need to balance the resistance allows greater isolation of the prime movers and so provides a greater training stimulus to those muscles. In addition, machine advocates believe that proper exercise technique is easier to achieve because movement is allowed only in one plane and direction. The truth is that both machines and free weights are beneficial at different points in a weight-training program. Also, some exercises, such as knee extensions and knee curls, cannot be safely performed with free weights.
Both machines and free weights offer training advantages. Machines that allow movement in only one plane and direction, thus isolating a muscle group, are useful when the training goal is to increase strength/power or local muscular endurance. Additionally, this ability to isolate a muscle group is ideal for some rehabilitation programs after an injury or when the goal is to increase strength/power or local muscular endurance of a muscle group or joint that is prone to injury or that is the weak link in the performance of a certain sport. Free-weight exercises are a good choice if the training goal is to strengthen total-body movements and improve coordination between various muscle groups.
Additionally, although the research is not conclusive, it appears that for adults free-weight exercises, such as squats, result in greater increases in vertical jumping ability than do machine leg press exercises. This is probably due to the greater mechanical similarity of squat exercises to the jumping motion. However, both squat and leg press exercises can cause an increase in vertical jumping ability.
Types of Machines
There are several types of weight-training machines. Today variable-resistance machines are the most common. These machines have a kidney-bean-shaped cam that varies the resistance throughout the exercise's range of motion. When the belt or cable leaves the cam a short distance from the cam's center, or point of rotation, the resistance seems light, and when it leaves the cam further from the center, the resistance seems heavy.
If a machine does not have a cam and uses only round pulleys for the belt or cable, the resistance does not vary. Other machines emphasize the lowering, or eccentric, portion of each repetition. These machines are utilized in what is commonly known as negative training, in which a heavier weight than can be lifted during the positive (concentric), or lifting, phase of a repetition is used for the lowering portion of each repetition. Eccentric strength is important for many athletic events and daily life activities; simply walking down steps involves eccentric actions of the quadriceps. However, the use of heavier weights than are possible during the lifting phase of a repetition to bring about optimal strength, muscle size, and fitness gains is controversial, especially for young children. Although negative training may have a place for advanced adult lifters, it is not generally recommended for children or beginning lifters.
Children and Machine Fit
When training prepubescent or pubescent children, the most important equipment consideration is that it fits them. With free weights, body-weight exercises, or exercises in which a partner supplies the resistance, fit is not the critical concern it is when using machines. Although some manufacturers do make machines designed to fit children, most resistance-training machines are made to fit adults and will not properly fit many children despite allowing many adjustments. Most prepubescent children's limbs are too short for many machines, which makes correct technique and full range of motion of the exercise virtually impossible. Most critical, a body part could slip off of its point of contact with a machine, such as a foot pad or an arm pad, resulting in injury to the child.
With some machines you can make simple alterations that allow a child to safely use the machine; for example, you can use additional back or seat pads on a knee extension machine. However, simply adjusting the seat often is not enough to make a machine fit the child; you may also need to adjust for proper positioning of the arms and legs on the contact points of the machine. In addition, changing the seat position might make it impossible for the child's feet to reach the floor. In many exercises the feet need to touch the floor to aid in balance, so you may also need to place blocks under the child's feet.
Altering a piece of equipment to fit one child does not guarantee that the equipment will fit another child. Check each child for proper fit before the equipment is used. Insure that padding and blocks do not slide during exercise performance, which could result in injury. Sometimes you can avoid this by attaching rubber matting to the top or bottom of the pads or blocks. Remember that safety is the major issue. If you cannot safely adapt a piece of equipment to properly fit a child, he should not use it.
This is an excerpt from Strength Training for Young Athletes.