A common dilemma that every thriving lifter faces is “Should I use free weights, cables or machines?” Bro-science on this topic is of little help, as most of it is superficial knowledge. In this three-part article we will inspect the similarities and differences between exercising with free weights vs machines, and give you some useful hints to keep in mind. The whole subject is pretty straightfarward actually, and you will benefit a lot from understanding a few core concepts.
A few bits of physics and math
Before we start the analysis, we firstly need to clear out one simple fact: your muscles do not know the difference among weights, levers and cables at all. They have no eyes, so they can’t tell the object opposing their efforts to contract. The only thing any of your muscles is able to perceive is how much resistance there is to fight at any given moment while trying to bring together its two neighboring bones.
The second fact is that bones do not slide but rotate with respect to one another. Whenever you perform a certain movement, you do it by making some bones in your limbs rotate around fixed anchoring points called joints. For example, when walking you rotate your thigh bones with respect to the pelvis, shin bones with respect to thigh bones and bones inside your feet with respect to shin bones. Any particular muscle involved in walking is able to simply rotate one of the bones with respect to some other bone. Your quads rotate your lower leg bones with respect to upper leg bones so that your legs straighten out at the knees; your hamstrings do the opposite. All those bone rotations are very precisely coordinated from moment to moment, resulting in you pushing your torso forward linearly with minimal bobbing up and down.
Why is this necessary to point out? Because your muscles are actually there to produce torque rather than linear force. If the only thing that bones do is rotate with respect to one another, then all we want our muscles to do is to “force” the bones into some useful rotations around the joints. And “rotational force” is technically called “torque”. Therefore the resistance that you feel when performing a movement under tension i.e. when exercising, is the torque caused by external mechanical force acting on a bone, around a joint as an anchoring point.
An important fact to note is that external forces of same magnitude can produce very different torques around a particular joint, if they act on a bone at different “angles of attack”. The more perpendicular a force is with respect to the bone it acts upon, the greater the torque. We are certain that being a weightlifter you are a smart and brave person so we will not try to hide the fact that there is a mathematical formula expressing this relation:
T = F · L · sin(A)
F is the magnitude of the external force, L is the length of the bone, while sin(A) is a trigonometric function called “sine”: you give it the value of the angle A between the external force F and the bone L it acts upon, and it gives you back some numeric value ranging from 0% for zero degree angle (force in parallel to the bone) to 100% for 90 degree angle (force perpendicular to the bone). Thus, no matter how great the external force is, if it acts in parallel to the target bone it will be multiplied by zero, and there will be no torque; a muscle connected to such bone wouldn’t feel anything to resist at all. But if force acts perpendicularly to the bone, 100% of it will be used to produce the torque T. Who would know that math is so useful and elegant 🙂
Think about opening a house door – your intuition tells you that the easiest way to make the door open i.e. to rotate it around the hinges is to push perpendicularly on it (sin(A) = 100%). You wouldn’t try to open the door by pulling the knob towards or away from the hinges because such force would be “wasted” due to its ineffective angle of attack (sin(A) = 0). If you get really stubborn and keep insisting on pushing/pulling the knob in absurd directions, hinges will eventually succumb to the abuse and you will be able to enter the house, but you’ll certainly need to call a technician after you get sober…
Keep this picture in mind when picking exercises, as some of them are quite natural and logical as is the way we open a house door, but others might let you mess up your joints slowly over time, and yet there are nasty ones that can bite you hard when you least expect. In the next part of this article we will analyze the risks and benefits of exercising with free weights. In the third part we will see whether cables and machines can be used to mitigate the risks and surpass the shortcomings.
As a small homework, try to spot the problem in the next picture.