There is no real danger in kids performing machine based training under the proper supervision and appropriate guidelines. Many studies done the world over have concluded that strength based training programs done on this kind of fitness equipment is very safe for young children (again under appropriate guidelines). My issue is not whether or not kids CAN perform this kind of training, my question is WHY they need to.
Back tracking for a second, I have watched (as I'm sure we all have) a very young baby struggle to get to their feet. In terms of strength output, this equates to a near maximal load. No one seems to be concerned about it until that child becomes eight or so years old and wants to lift weights … Then people want to call the police on you because you had the ignorance to let a child perform strength training! The bottom line is that kids CAN handle strength training based loads … heck they do everyday – hopscotch, tag, bowling, ANY sport – all these things require varying degrees of strength.
Now the question of why. I just have never been convinced by any article, book or study champion child strength training that kids SHOULD use machine based fitness equipment. The reality is that sport AND life is based on the functionality of movement. Juan Carlos refers to it as the Four Pillars of human movement; Paul Check has a similar model which incorporates six stages. My point being is that weather you are dealing with a young athlete or just a young fitness participant, your goals should involve obtaining health and / or sporting proficiency on a useable level. Machines provide support (I have trained so many young athletes who simply can not produce stability in all three planes), and the force application is both pre-set (which just begs for biomechanical dysfunction) as well as pre-guided (unfortunately sport and life are not).
My other concern with machines is that they inhibit two very important concerns when dealing with young athletes (actually when dealing with anyone):
1. It is very hard to train unilaterally when using machines (one side of the body at a time). Unilateral training, in my mind, is one of the most crucial components of developing young athletes.
2. You simply can not train (either produce force through or learn to stabilize) the transverse plane. As referred in many Kinesiology based books, over 85% of our core musculature is orientated horizontally or diagonally – we are designed for rotation, yet machines do not allow for it.
My suggestion for working with young athletes (and this is based on several factors including age, emotional maturity, current physical proficiency) is as follows:
Do not engage kids in exercises that promote external stability or useless force production. The key to working with young athletes in any sport is to promote mobility, stability and balance in conjunction with force. Young athletes need to have a virtual warehouse of athletic based skills in order to reach optimum levels. This is achieved by moving and stabilizing the body through various planes and producing force through various vectors. A common sequence as put forth by esteemed professionals such as Paul Check is as follows: flexibility before stability – stability before strength – strength before power.
Incorporate unilateral strengthening activities (one leg at a time, for example). Most force production in sport is unilaterally based. This strategy also assists in training balance and avoiding unilateral overuse injuries.
Add dynamic flexibility activities into workouts on a regular basis. Range of motion style flexibility is proven to be far more important to athletics than traditional static flexibility.
While bodybuilding type training training is entirely useless to young athletes, a well educated professional should start teaching the movements and techniques associated with the Olympic and Power lifts at an early age (11 – 12 years old). These lifts include cleans, squats, push-press, snatch etc. Keep the exercises unweighted (ie use a broom handle rather than a weight bar) and concentrate on developing perfect and explosive power through technique.
Do not underestimate the importance and value of basic 'games' such as tag, tug of war and single leg tug of war.