Mechanical Advantage
Mastery is managed disadvantage
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Mechanical advantage occurs when a force acts against resistance at a perpendicular angle to a lever. Specifically, the force has a mechanical advantage relative to the load.
Imagine lifting a manhole cover with a crowbar.
We shove the business end of the crowbar under the edge of the plate, and now the crowbar is angled 45 degrees to the ground.
As a simple lever, the plate is the load, the point where the crowbar makes contact with the pavement is the fulcrum, and our hand provides the force.
It's up to us to provide not just the force, but also the direction of the force. What is the most efficient direction for us to push against the crowbar in terms of input/output?
This is the difference between mechanical advantage and disadvantage. The more mechanical advantage we have, the less force we need to produce to move the load.
If we exert our force 90 degrees to the arm of the crowbar (135 degrees to the ground), we have mechanical advantage.
If we push in a direction above or below 90 degrees to the crowbar, we have some degree of mechanical disadvantage, and we have to increase our force to move the load. Remember the mass of the manhole cover has not changed.
Also this process is dynamic. Say we start with a mechanical advantage, and our line of force does not change direction. As the crowbar moves, our relative angle to the lever changes, and our mechanical advantage is lost.
If we want to keep the advantage we have to change our line of force relative to the angle of the lever, essentially creating an arc that maintains a 90 degree force angle to the crowbar. This is known as torque—a force that rotates around an axis.
Similarly, mechanical advantage in the body occurs when a muscle pulls on its operating lever at a perpendicular angle.
In the above example with the crowbar, we changed our force angle to maintain a mechanical advantage throughout the entire arc of the moving lever.
When we're looking at individual muscles, mechanical advantage and disadvantage is a dynamic and changing phenomena, and is one factor among several for choosing a specific exercise.
Because our individual muscles can never change their direction of pull they are usually in a state of mechanical disadvantage relative to the bone they are pulling on.
Looking at the biceps curl, we start at a mechanical disadvantage. As we engage the biceps the disadvantage decreases until our elbow reaches 90 degrees of flexion and our forearm is perpendicular to the direction of resistance (gravity). Our biceps are now pulling on our forearm lever at a 90 degree angle and have a mechanical advantage.
As our biceps continue to pull and our forearm moves above 90 degrees we lose the advantage, and the mechanical disadvantage increases with the angle of pull.
If we remember the resistance curve, we see that the biceps hit the point of mechanical advantage right at the point where we need it most—at the peak of the resistance curve. We have the advantage when the total load is the greatest. As a double bonus we're also at the peak of our strength curve.
Also we can see that when our forearm is above or below perpendicular and we are technically at a disadvantage, the moment arm decreases and the load drops off. The load gets lighter as we become more disadvantaged.
Operating at a disadvantage is normal
It's important to note that most of the muscles in our body never achieve a mechanical advantage. This is because at no point in their range of motion do they pull on their operating lever at a perpendicular angle.
The muscles that are capable of achieving a mechanical advantage are flexion muscles. Biceps, hamstrings, calf muscles, lats, pecs, hip flexors, etc.
Extension muscles like triceps and quads pull at or near parallel with their operating lever and are always operating at a disadvantage
Every muscle in the body pulls in exactly one direction, determined by where it attaches to the bones. This line of pull shifts a bit as the bone and joint move positions, but the muscle cannot swing its angle of attack to chase the ninety-degree position the way your hand can on the crowbar. Mechanical advantage is a position we pass as we move through our range of motion, if ever.
Passing the advantage
While performing a complex motor skill involving multiple bones, joints, and muscles, it's not really appropriate to designate the individual moving parts as having a mechanical advantage or disadvantage. But understanding what mechanical advantage is helps us to understand which parts are in the most mechanically advantageous position in a given moment.
Lets look at a golf swing.
At the top of the backswing, the hips, torso, and shoulders have rotated away from the midline, loading tension into the trunk. The muscles driving that initial uncoiling — primarily the obliques and transverse abdominus — are working from a position where their pull is close to perpendicular to the line of rotation they're initiating. Not technically mechanical advantage, but still a strong mechanical position early in the kinetic chain. We can call it “the position of least disadvantage.”
As the downswing proceeds, that relative advantage doesn't stay with the trunk. The sequential segmental acceleration we covered in earlier articles passes momentum down the chain — hips, then trunk, then shoulders, then arms, then the club — and at each handoff, different muscles briefly occupy the position of least disadvantage while others fall out of it. By the time the club approaches the ball, the wrists are doing the work, and their relevant muscles are now the ones near their own perpendicular position relative to the club shaft, releasing the angle that was stored through the swing.
No single joint holds the advantage for the entire motion. The advantage itself travels, joint to joint, on a schedule set by the order of release. A golfer who tries to generate power by forcing one joint to do more than its share — muscling the downswing with the arms before the hips and trunk have finished their handoff — is fighting their own sequencing. They're asking a joint to produce force from a position of gross disadvantage that another joint, slightly earlier in the chain, was built to handle with more advantage.
This is also why "swing harder" so rarely works as instruction. More force applied at the wrong point in the sequence, from the wrong angle, doesn't compensate for lost mechanical advantage. It just produces more force in the wrong direction.
Skill has a debt
And the debt compounds. The swing, the stroke, the approach to a fence — none of these can be redesigned to put your muscles in more favorable positions. The geometry is set by the task.
Every athletic discipline creates specific mechanical biases. You repeat the same movement patterns thousands of times, always loading the same positions, always bypassing the same others, and the body adapts. In the short term these adaptations look like skill, and in the long term look like a hip replacement.
While the positions of least disadvantage are trained relentlessly by the discipline itself, the positions of gross disadvantage tend to be avoided. This asymmetry creates damage and dysfunction; the strong bits get stronger while the weak bits get weaker. It's not bad luck, or aging, but the predictable mechanical consequence of ten thousand repetitions that all go in the same direction.
Understanding mechanical advantage is what makes that asymmetry visible before it becomes a visit to the orthopedist. If you know where your discipline puts you in the position of least disadvantage, you also know where it doesn't — and those are the positions that need deliberate attention outside the discipline. Not because they're weak in some general sense, but because the discipline will never train them for you.
