The laws of physics tell us that a body will continue in its state of rest or motion (constant velocity) in a straight line unless compelled to change that state by external forces exerted upon it. This is Newton's first law of motion, sometimes called the law of inertia. We inherently know that starting that heavy barbell moving is hard but that once we've got it moving it wants to keep going. So heavy lifts have what is called a sticking point where the amount of muscle force that can be exerted on the load (which is moving slowly and resisting changing that state) is only just able to get the weight moving. Once past that point, the weight starts to accelerate and, toward the end of the lift (even if less muscle force can be used), the lift is easier. Of course, that pesky thing called gravity is trying to slow the weight down but unless it is a really heavy weight we can get the load moving upward at a reasonable pace, and as stated, once we get it going it will be easier to keep it going. This is why you can lift more in a push press than a strict press: your much larger leg musculature gets the weight going up and you only have to use the smaller upper-body muscles to keep it moving to the finish. This pattern of resistance is one of nature's very common forms of variable resistance.
However, not all resistance motions we perform result in reduced torques as the load is accelerated. If the load moves farther from the joint center, the torque can increase even if the load is being accelerated. For example, in the middle part of kettlebell swing to overhead, the torque will be very high even though you already have the kettlebell moving fast (this because the load's is a full arm's length from your shoulder). So in nature if loads have to be moved from close to farther away from the body you will get, yes, variable resistance.
Working with free weights also teaches us when we really need to keep the torques as low as possible for a given weight. The gravitational line of action on a weight is always vertical, and the distance between this vertical line of action and a joint center is crucial. If, for example, you do not get a heavy press directly overhead (elbows into your ears, weight above feet) the resultant torque is going to demand much more effort from your shoulder muscles and may in fact cause you to lose balance and drop the weight. Similarly, if want to make that max deadlift, you better keep that bar on your shins, reducing the torque on your lower back. Training a multitude of movements in your workouts prepares you for a variety of real-life situations where loads can sometimes be kept close and sometimes not. The bottom line is that you don't need a machine to provide artificial variable resistance.
If you go back to the first diagram you'll see that the torque drops to zero toward the end of the curl. Well, if you take a light weight (say something around your 15-rep max) and do an explosive push press, you can get the barbell moving upward so fast that although gravity is pulling it back it'll carry on upward for a bit without much, if any, more effort from you. In fact, if you get enough momentum on it, you can let go of the barbell and the bar will continue up past your full extension height (as with wall ball, for example). In this scenario you will actually have to grip tight and stop the barbell from continuing upward. However, with much heavier weight (your one- to three-rep max, for example) you will have to push hard during the entire motion. That is why it is also beneficial to work with a wide range of loads. Varying the weight is essentially another way, along with every other natural movement, of creating variable resistance.
I will admit research has shown that variable resistance does improve strength throughout the full range of motion better than "non-variable resistance." This benefit, however, is only compared to strength training on non-variable resistance machines, and these studies tend to look at force production at the specific intensities trained (like 10-rep maximums, for example). But what about the effective application of strength? By this I mean the skill component—the development of the motor control required to effectively lift heavy weights. If we can keep the line of action of a heavy weight close to the joint center of rotation, lifting it requires less torque than lifting a lighter weight whose line of action is farther from the center of rotation. This is why, as discussed above, lifting heavy weights is not just about pure strength. Studies that suggest that variable resistance machines are a great way to train are looking at one narrow parameter—one very specific definition and isolated definition of strength—but not at the functional expression of athleticism or power in the real world.
In this article I have criticized machines in general and specifically the claim that variable resistance machines are better as they mirror the torque-angle relationship of a muscle/joint system. But the question remains: "Are they bad?" As an educator and trainer, I have to accept that someone who is doing any kind of resistance work is better off than the majority of the population. However, real-life movement and strength requirements will challenge all ten physical skills, and so should your training. If you are in the military, a first responder, or a martial artist, you don't need me to suggest machines are not an effective training tool. The bottom line on this question? If you put a CrossFitter on a variable resistance machine, they will do well. If you give a reasonably heavy barbell to someone who only trains on a machine and tell them to get it overhead, I suggest you stand clear!
Tony Leyland is Senior Lecturer in the School of Kinesiology at Simon Fraser University, Vancouver, British Columbia. He has taught at the university level for 25 years and has been heavily involved in competitive sports such as soccer, tennis, squash, and rugby as both an athlete and a coach for over 40 years. He is a professional member of the National Strength and Conditioning Association, a Canadian National B-licensed soccer coach, and a level-1 CrossFit trainer. He can be reached at [email protected].
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