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The Physics of Fitness: Exercise Mechanics

The “Resistance Curve” is the Most Fundamental Part of Weight Training

The basis of weight training, or any other type of resistance training, is physics – specifically, the laws of levers.  While it may not seem obvious that it’s such an integral part of weight training, virtually everything you do in the gym, involves levers and the forces that act upon them.  Understanding the laws of levers is vitally important to making sense of resistance training, and maximizing its benefits.

Every primary bone – or group of bones (like your hand, for example) in your body, is a lever, which is activated by a particular muscle.  For instance, your forearm (which is actually two bones that run parallel to each other) is a lever.  It is activated by your biceps in one direction (pulling / a.k.a. flexion), and by your triceps in the other direction (pushing / a.k.a. extension).  Your hand, as mentioned above, is also a lever (even though it is comprised of an assortment of bones), which is activated by the muscles of your forearm (flexors and extensors).  This may seem elementary, but it is not always as obvious as these two examples.

What is even less obvious – and where most people make mistakes in training – is understanding the forces which act upon those levers: namely – free weights, pulleys, and the cams of machines.

The most fundamental premise of weight training is this: Resistance must CROSS a lever, in order to challenge the muscle that activates that lever.  Let me phrase that another way: if resistance does NOT cross a particular lever, the muscle you intend to work will get little or no benefit.

Of course, challenging a muscle is what resistance training is all about.  So understanding how and when (or if ) resistance CROSSES a given lever is vitally important, if one hopes to maximize his or her success in the pursuit of fitness or bodybuilding.  Yet – ironically – it’s one of the least understood concepts by practically everyone who exercises.

First, I’ll offer this elementary example.  It may seem simplistic, but stay with me, because the next examples won’t be so obvious.

Let’s say you’re going to do a standing barbell curl.  The muscle you intend to work is your biceps.  Therefore, the lever which must CROSS resistance is the forearm, because it is the lever that is activated by the biceps.

Since you are using a free weight, gravity is the resistance at work here, and gravity always pulls straight down (on free weights).  So, before you begin your curl, you should notice that when your elbows are straight and your forearms are perpendicular to the ground, your forearms are parallel to gravity, so your biceps are not activated.  But as you begin to curl the weight, and your forearms begin CROSSING the downward direction of gravity, your biceps begin to be challenged.

When your forearms are parallel to the ground, gravity is completely CROSSING that lever.  In other words, when your forearms and gravity are forming a “ T ” position, you are getting the most resistance from that weight.  That is the point where your biceps are the most challenged – when resistance is pulling straight ACROSS the lever that is operated by the biceps.  That is the 100% resistance mark.  The curling that comes before that moment (on the way up) encounters lesser percentages of resistance (first 25%, then 50%, then 75%, etc.), and what comes after that moment (the upper half of the movement) is also lesser percentages (first 75%, then 50%, than 25%, etc.).  If you continue curling the bar upward (allowing your elbows to shift forward slightly, so that they are under the weight), your forearm will again be straight up and down (i.e. your hands directly above your elbows), the resistance will again reach ZERO – because your forearm is parallel to gravity again.  Your biceps are essentially unchallenged, in that position.

This is called the RESISTANCE CURVE.  It is the sequence of resistance changes that occurs through a range of motion, as the lever passes through varying percentages of CROSSING resistance.  Whenever a lever is completely at 90 degrees (forming a “T”) with resistance, it’s at maximum resistance.  That is when the muscle (which is moving that particular lever) has the greatest challenge.  That is when the weight seems “the heaviest”, because that is when gravity has a mechanical advantage on that lever.  But when the lever is not at 90 degrees – when it’s only at 20 degrees, or at 10 degrees, or at 5 degrees (… in other words, more parallel to resistance, than perpendicular to it) – that is when the muscle is less (or least) challenged.

Test it Yourself

In order to get a thorough understanding of the resistance curve, do this:  pick up a 10 pound dumbbell, and – while holding it in your hand – put you arm flat on a table top, with your arm fully extended.  Now, begin curling the weight upwards (bending your elbow), and observe how much force you have to use to initially lift the weight (or your hand) off the table.  Then, observe that when the weight gets to the point that is directly over your elbow (with your forearm straight up and down), it feels like there is no more “resistance” – even though you still have a 10 pound weight in your hand.  You can easily balance that weight over your elbow, without effort or strain to your bicep.  With your free hand, poke the bicep of the arm holding the weight, and you’ll see that it’s un-flexed and un-challenged, by the weight.

If you were to continue “pulling” (or curling) beyond that point, you’ll notice that the weight actually begins falling toward your shoulder.  In other words, you don’t need any bicep effort to continue curling your arm.  This is because gravity is no longer crossing your forearm from the side of the biceps; it is now crossing the forearm from the other side.  Imagine rolling a large, heavy ball to the top of a hill, and then – once it gets to the very top – it suddenly begins rolling on its own, without any force required by you.

If this little experiment hasn’t yet made the “resistance curve” perfectly clear, do it again with a heavier weight.  Try to 20 or a 30 pound dumbbell.  You’ll see that no matter how heavy a weight you use, it will always offer ZERO resistance to the working muscle, when the lever (in this case, the forearm) and gravity, are parallel.  And it will always offer the MOST resistance when the lever is perpendicular to gravity.  Naturally, this means that resistance will offer percentages between ZERO and 100%, when the lever is between parallel to resistance, and perpendicular to resistance.

Analyzing Exercises

The goal of an exercise should be to utilize the optimum lever advantage, thereby bestowing the greatest benefit to the target muscles, with the least amount of wasted effort.  In other words, avoid exercises that bestow marginal benefit to your target muscles, while requiring a huge amount of work by muscles that are not your target, and unnecessary strain to joints.  Let’s see how some common exercises rate.

Analysis # 1

Let’s analyze Parallel Bar Dips.  Assuming your objective is to do a good triceps exercise, your goal should be to do an exercise during which resistance CROSSES the forearm, from the side of the elbow (the back side).  Keep in mind that – somewhere during the range of motion – resistance should reach a “T” position with your operating lever (in this case, your forearm), in order to get optimum triceps benefit.  However, if you were to observe someone while they were performing Parallel Bar Dips, you will easily see – as they dip down and push up – that at no point during that range of motion, do their forearms EVER reach a “T” position with gravity.  When they are at the top position, their forearms are parallel to gravity.  Then, as they begin lowering themselves, their forearms begin to cross gravity only a little bit, but they never get past about 15 or 20 degrees (from perpendicular).  That’s not good.

But – perhaps more interestingly – if you observe the upper arm bone (a.k.a. the “humerus”) during a Parallel Bar Dip, you will notice that IT certainly moves through the “T” position with gravity – but it’s not operated by your triceps.  In other works, whichever muscle is operating THAT lever, is the muscle that’s getting MOST of the work.  Which muscle is that?  It’s the frontal deltoid, in this particular movement, given the path through which the arm is passing (i.e. with the elbows close to the sides).  Your primary objective is probably NOT to work your frontal deltoids with Parallel Bar dips, and yet that is what’s getting most of the work.  In fact, if you were to assign percentages of effort, you could estimate that about 65% of the work is being done by the frontal deltoids, about 15% is being done by the pectorals, and about 20% is being done by the triceps.

Now, you may ask, “well then why do I feel it in my triceps when I do them?”.  Answer: because 20% of your bodyweight is a lot, for your triceps.  But, while your triceps are pushing 20% of your bodyweight, other muscles are having to work beyond their safe limits.  In fact, Parallel Dips move your upper arm bones into an extreme front deltoid stretch (far beyond a safe stretch position), and does so with (approximately) 65% of your bodyweight – ouch! (i.e., unless you’re using a machine that offers a counter-weight … but the movement is still being done primarily by your front deltoids).

So the question is why would you want to do an exercise that maximizes the leverage-resistance for your front deltoid (thereby creating risk of injury), while offering only partial leverage-resistance for your triceps, when you can – more easily, safely and effectively – do an exercise like triceps pushdowns with a cable?  With “pushdowns”, your forearms move through that perpendicular (“T”) position, thereby crossing resistance (i.e. the cable), and your triceps receive a maximum leverage advantage, without strain to other body parts, and without excessive (and unnecessary) effort.

Equally bad mechanics as Parallel Bar Dips, is “Dips on a Bench” (with your feet up, or not).  Next time you see someone doing that exercise, observe how their forearms stay almost perfectly parallel to gravity, rather than CROSSING gravity.  And know that unless their forearm crosses resistance (moves through a “T” position), their triceps are not getting the full benefit of the exercise, while other body parts (namely the front deltoids) are at risk of injury due to excessive load and stretch.

Analysis # 2

Let’s examine Supine Triceps Extensions (sometimes called “skull crushers”) – either with dumbbells or with a barbell (often done with an EZ Curl bar).  Typically, a person starts with arms straight, holding a bar or dumbbells over their chest.  At that position, their forearms are parallel to resistance, and therefore offer no challenge to the triceps.  Then, the person begins bending his or her elbows, and his forearms begin CROSSING gravity (so far – so good), and encountering increasing percentages of resistance as they approach the position of having their forearms perpendicular to gravity.  When they reach that position (the “T” position), they reach maximum resistance (their elbows would be bent at about 90 degrees at that point).  They might bring the dumbbells down a little farther, and then push them back up to the top.  As you can see, the triceps were taken from a ZERO resistance position at the top, and then – as the weight was lowered, resistance went from 25%, 50%, 75% and 100% resistance levels, and then – beyond that (with an elbow bend of about 60 degrees) – reaching about 75% resistance again, and then back the other way, up to the top.  That’s fine.  The triceps are getting the full benefit of the resistance, and there’s no strain to any joints, nor excessive stretch or demand on any other muscles.

But what happens when – towards the end of that set – the person decides to “finish off” with a modified version of that movement, whereby they bring the dumbbells down to their ribcage (instead of toward their forehead), and then back up again?  Well, since the forearms are then parallel to gravity, the triceps get almost no benefit.  Instead, the upper arm bone begins CROSSING gravity, and – again – the front deltoids begin doing the work.

If you’ve done this, your rationale might have been along the lines of: “…since my triceps are so exhausted, I’m just squeezing the last bit of juice from them”.  Foolhardy.  If you read my previous blog article (“To Burn or Not to Burn”), you would understand that there is essentially no productive reason to “squeeze the last bit of juice” out of any muscle.  This aberration of a triceps extension is mostly without benefit, given the lack of leverage advantage.

Analysis # 3

Hanging Leg Raises.  The target muscle one typically wants to work when doing this exercise is the abs.  The “lever” that is moved by the abs, is the spine.  Specifically, the abs pull the ribcage and the pelvis closer together, thereby creating a rounding of the spine, by bending the spine forward.  The abdominals do not pull on the legs.  Yet, as you can plainly see, when one does a Hanging Leg Raise, it is the thigh bone (femur) that CROSSES gravity, and it’s the hip flexors that perform that function – not the abs.  The abs are only involved in stabilizing the torso, while the legs move up and down, and it’s this static contraction (with a very little bit of isotonic contraction) that accounts for the burning sensation you feel when doing this exercise.  But the vast majority of the work is being done by other muscles, never mind that you also have to use your arms from which to hang.  The bottom line is that of all the muscles that are working while doing Hanging Leg Raise, the abs are the least involved because the spine is mostly parallel to gravity, and never approaches a perpendicular position (“T”) to it.  Plus, hip flexion (raising the knees up) has nothing to do with the abs.  So it’s a lot of work, with little reward (i.e. abdominal benefit).

The Direction of Resistance

Every exercise you do, offers a resistance that comes from a particular angle.  Gravity – of course – is invisible.  But, if you are using a free weight (not a pulley or machine), simply imagine an arrow pointing straight down.  It is up to you to determine whether or not that force is CROSSING the lever operated by the muscle you want to work, or not.  Ideally, in order to do this type of analysis, you need some basic knowledge of which muscles operate which levers, and along which paths.  But, assuming you have this basic knowledge, it is very easy to understand whether or not resistance is being properly applied.

When using cables, it’s entirely visible.  The cable acts like an arrow – illustrating the direction of the resistance.  For example, say you were to place a Preacher Curl Bench in front of a low pulley, about two feet away from it.  Now, as you do your Preacher Cable Curls, you should observe how the direction of the resistance is different than it would be if you were doing FREE BAR curls on that same bench.  When the cable and your forearm reach a “T” position, your biceps is getting the most resistance.  And you’ll notice that that position is half-way-through the Preacher Cable Curl, as compared with the quarter-way-up position when using a FREE WEIGHT.  Conclusion: these two exercises have different resistance curves – even though they utilize the same bench – because you’ve changed the direction of the resistance.

Cams (on weight training machines) offer a more continuous resistance.  In essence, the entire range of motion offers a “T” moment.  This isn’t necessarily good or bad.  It’s just different, and different is good.  Variety (of resistance curves) is good.

What is not good, is using an exercise that does not offer an opposing resistance to the lever that is operated by your target muscle.  In other words, if an exercise fails to encounter the “T” moment (CROSSING resistance) – as well as the slightly lesser percentages, i.e. 90%, 80%, 70% etc. – during the range of motion, it’s benefits are compromised.

The Power of RC

(…and I don’t mean “Cola”.)  Understanding the Resistance Curve not only allows you to avoid exercises that are risky or unproductive, but it also allows you to modify exercises to create variety or enhanced benefit.

For example, if you do a Standing Side Raise for your lateral deltoids, the Resistance Curve is such that you get the most resistance at the conclusion of the movement (when your arm is perpendicular to your torso), and the least resistance at the start of the movement (when your arm is at your side).  But if you do a side raise while lying on your side, on a floor mat (with one arm, of course), you “flip” that resistance curve: now you’ll have the most resistance at the start of the movement, and the least resistance at the conclusion.

Here’s another example of how you can vary the Resistance Curve on a given exercise.  While doing Triceps Pushdowns (from an upper cable/pulley), stand farther back – away from the pulley.  When you stand close, you have more resistance at the top and less at the bottom.  But by standing back, you add resistance to the bottom (where the muscle contracts) and take some away from the top.  You can easily see this, because the cable indicates the direction of the resistance, and the forearm is the lever that it crosses.  The more “across” the cable is from your forearm, the more resistance you get on your triceps.  The less “across” it is, the less resistance you get.

Is your knee bothering you?  How about your elbow?  Maybe your shoulder?  You can work around it, by modifying the Resistance Curve.  Just establish whether your discomfort is worse at the beginning of a movement, or at the end, and adjust the Resistance Curve accordingly (… by selecting exercises that provide more resistance where you have no pain, and least resistance where you do have pain).

There’s literally no end to the ways you can modify exercises, using this knowledge.  This would most definitely improve your results, and minimize – if not eliminate – any risk of injury.


You probably now realize the importance of understanding the Resistance Curve, though this article barely begins to clarify how this concept effects ALL the exercises you do in the gym.  This article might seem a bit too technical, but if you are at least able to grasp the concept of the lever, and that when resistance CROSSES it, you get the most resistance, and when it’s parallel to resistance, you get the least resistance, you have a basic understanding, with which you can begin to distinguish the good exercises from the bad ones – and modify the good ones.

I will discuss this concept again in future articles, and with more specific examples.  Eventually, I think you will be able to understand clearly why some exercises are obviously better than others, and thus you can begin to make super-productive, efficient and safe choices in the gym.

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