Not all standard free weight exercises actually work the muscles they were designed to strengthen, and some can even be harmful. However, by using the principles of biomechanics, trainers can design free weight exercises that are safe and effective.
I got started years ago writing about biomechanics and what's good or not so good about certain exercises, when I wrote a Letter to the Editor (not this Editor, of course) about an exercise that a particular magazine called the Cobra. To perform the Cobra, exercisers would lie down on their side near the low pulley of the cable cross machine. (This was way back when the only cable machines in a standard gym were the lat pulldown and, maybe, a cable cross.) Then, exercisers performed what was basically a single-arm lat-pulldown-type of motion.
How this differed from simply sitting upright on the lat pulldown machine and performing a one-arm pulldown using a single handle, I don't know - and the magazine never said. But that's not the main issue. The problem is, the magazine claimed that exercisers could perform the same exercise in the same body position using a dumbbell. However, with the cable, exercisers pulled somewhat horizontally against a horizontal resistance. But, if they used the dumbbell option, they'd be pulling horizontally against a downward resistance. Not only are these not at all the same exercise, but the dumbbell version becomes a nonsensical "exercise," with absolutely no resistance to the alleged target muscle.
Even after all these years, poorly designed free weight exercises like this still exist. The good news is that further examination of the biomechanics of many exercises has led to some "new" techniques that can improve results. Not all dumbbell exercises are equal. Some are inherently better than others. That's not just my opinion; you can prove it with biomechanics. Some of your favorites may be ineffective and even potentially dangerous. And, some standards can be made even more effective with the proper application of basic biomechanics.
Moment armIn physics, the moment arm is defined as the perpendicular distance between the line of force and the axis of rotation. To apply this to an exercise, you first draw (mentally, at least) what's called a "free body diagram." Don't worry, with free weights, it's easy. With a dumbbell, the line of force is always straight down and directed straight through the hands. Technically speaking, you can pick any joint you want as the axis of rotation, but typically you're interested in looking at the main joint targeted in the exercise.
Let's use lateral raises as an example (see Figure 1). The axis of rotation here is the shoulder; more specifically, the gleno-humeral (GH) joint. (Of course, there are other joint motions taking place.) Draw a straight line down from the GH joint. Now, look at any point in the range of motion, and draw a line straight down from the dumbbell or hand. The distance indicated by the red block in the figure is the moment arm. The smaller the moment arm, the less the resistance to movement about the axis in question. The longer the moment arm, the greater the resistance.
Strength curves and resistance profilesEach joint movement has an associated strength curve that plots the joint angle and the corresponding maximum weight that can be handled in that position. Biomechanically speaking, curls are a near-perfect free weight exercise. The strength curve and resistance profiles match up well. Lifters get stronger up to about 90 degrees, then get weaker as they continue upward. In technical terms, the moment arm is maximal right around the point where the biceps are strongest. The moment arm (and resistance) are low where strength is inherently low.
The only "issue" is that there is absolutely no resistance to the biceps at the very bottom (which is okay if the exerciser has actually earned that tiny rest). There's also a small amount of available range of motion in elbow flexion at the top past the point where the exerciser is no longer getting resistance from the dumbbell.
Although it's impossible to precisely match strength curves with resistance profiles (even with the fanciest cam-based machines) - since the strength curve changes not only from person to person, but also from rep to rep in a given person - a well-designed exercise should at least reflect the typical strength curve for the intended movement. An exercise is inherently less effective for optimally stressing targeted muscles throughout the range of motion if the strength curves and resistance curves are opposite.
PulloversI'm old enough to remember when virtually every man in the weight room performed pullovers because it "expanded your rib cage," as reported in virtually every bodybuilding magazine of the day. Let's look at the moment arms (see Figure 2) for this exercise. In the bottom position, the moment arm - and, therefore, the resistance - of the barbell is maximized. Unfortunately, the biomechanics of the body are working against the exerciser. The lats and pecs are in a weaker position there.
In the top position, exercisers get just the opposite effect. Since the moment arm is zero relative to the shoulder, the resistance is zero. At the same time, the lats and pecs are in a great position to create torque. Worse yet, the bottom position is at or near the limit of the range of motion of shoulder flexion. Fail here - again, where resistance is maximum and strength is minimum - and exercisers can be at risk for an injury.
Triceps kickbacksExercisers face similar problems when performing triceps kickbacks as they do for pullovers, but at least kickbacks are safer (see Figure 3). In the bottom position, the triceps are in a strong position, yet the resistance is zero because the moment arm is zero. At the top position, the triceps are at a mechanical disadvantage; but, at the same time, the moment arm and resistance are maximized.
With kickbacks, the tendency is to choose a heavy weight, so exercisers have to basically throw to the top position to compensate for the inherent weakness at that end of the motion. Or, they can pick a light weight, but this is not at all challenging through the rest of the motion.
Dumbbell fliesDumbbell flies are yet another example of the resistance being maximized where exercisers have much less strength (see Figure 4). Numerous bodybuilders insist that there are legitimate reasons to perform this exercise, but that's no reason for most typical members to perform it.
Not only does the resistance profile oppose the strength curve, the bottom position - where exercisers are weakest - can take the shoulder beyond the physiological range-of-motion limit and cause injury. Besides, at the top position, there is still available range of motion in shoulder adduction, but the resistance from the dumbbells reaches zero once the hands are directly over the shoulders.
Preacher curlsWith preacher curls, the top position has no resistance, although there is still strength in that position, and exercisers get even more range of motion in elbow flexion. In fact, many people make the mistake of going beyond the "top" position to a point where their biceps can no longer do the work. Any further movement beyond the "noon" position of the forearm comes from gravity, not the contraction of the biceps.
Preacher curls are another exercise in which the bottom position can be potentially dangerous. At the bottom, there's still some resistance from the weight, although exercisers reach the end of range of motion of the elbow. The pad itself can then be a fulcrum that helps the weight take them into hyperextension of the elbow.
The See-Saw PrincipleImagine two children of equal body weight sitting on a see-saw. Theoretically, if neither moves, they will sit there perfectly balanced. This is true regardless of the size of the children. In engineering, this is called a state of equilibrium. Now, if a fly lands on the head of one of the children, the see-saw will start to move. Here's how the see-saw analogy applies to a couple of common exercises.
Dumbbell side bends. Dumbbell side bends are not an inherently poor exercise when performed intelligently. Many people, however, make the mistake of using two dumbbells - one in each hand - for this exercise. One dumbbell goes up as the other goes down. What happens is, the two dumbbells offset each other, much like a see-saw. The tiniest bit of force - as in contracting the obliques and quadratus lumborum with minimal force - will move the system. The target muscles don't have to do much work at all. Any muscle activity exercisers feel comes from moving the weight of the trunk (the head, arms and truck comprise about 2/3 of bodyweight), plus the minimal amount of force needed to get the dumbbells moving. The weight of the dumbbells, in this case, is nearly inconsequential.
To perform this exercise properly and intelligently, exercisers should use a single dumbbell.
Wrist rotation. Golf-happy country club trainers see this mistake a lot. The action here is pronation and supination, which takes place at the elbow, not the wrist. Most people grab the dumbbell in the middle and simply follow instructions like these copied from a popular website: "With your palm down, rotate your forearm until the dumbbell is parallel to the ceiling" or "this time, keep the palm face up while holding onto the dumbbell. Rotate the dumbbell until it is parallel to the ceiling."
If exercisers grab the dumbbell in the natural, middle position, the two heads of the dumbbell offset each other. In order to avoid the see-saw effect, they should grab the dumbbell near the end of the grip; otherwise, they're basically wasting their time.
Smarter dumbbell exercisesLet's take another look at the shoulder abduction/lateral raise example. People are strongest in shoulder abduction around 70 degrees. If you take a look at the diagram (Figure 1), you see that the point of greatest resistance for this exercise is at 90 degrees. So, although the strength curve and resistance profile have the same shape, they're out of phase, with strength reaching its peak around 70 degrees, and resistance reaching its peak at 90.
There are two ways to address this discrepancy, with one being clearly superior to the other.
Standing leaning lateral raises. This type of lateral raise is a quasi-classic that you can probably find in many advanced bodybuilding magazines. It's a nice try, but the biomechanics are slightly off. For educational purposes only, here's how to perform it: With a dumbbell in the right hand, grab a stable anchor point with the left hand and lean the entire body toward the right. From there, simply perform the usual lateral raise.
The usual selling point is that this changes the resistance profile, and some argue that any change is the resistance profile provides a useful new stimulus. When you look at the biomechanics, however, you'll see that the strength curve and resistance profiles are even more out of sync. The point of greatest resistance now occurs much above the 90 degree abduction point, although, again, a person's strongest range of abduction is around 70 degrees.
Let's see if we can align those two curves a little better.
Telle lateral raise. I named this option for Jerry Telle, author of Beyond 2001. Again, we'll work on the right arm, but this time, exercisers sit on a bench and lean the trunk to the left, supported by the left arm (see Figure 5). With about a 20 degree lean of the trunk, the exerciser better coordinates the resistance profile with the strength curve. Also, exercisers still have resistance, even at 0 degrees of abduction, which is purported by some to stimulate the supraspinatus more effectively. Moreover, it optimizes the tension in the deltoids throughout the range of motion. As Telle writes, "By adjusting your body during a particular exercise, you can maximize muscle tension and target the muscle much more effectively." Seemingly simple changes can make a world of difference.