Moment Arm Exercise Chapter 2

Note from Bill DeSimone:

The first chapter is online at http://congruentexercise.blogspot.com/2013/02/how-heavy-is-ten-pound-dumbbell.html

With some formatting changes, this is the same text as originally done in 2004

Ordering information for the complete manual at the end of the article.

For Greg Anderson. 

 

2.0     Locating the Hidden Moment Arm

                        We humans remain animals.  Biology talks about us all, something that can create a lot of uneasiness-or worse.  It implies constraints on human aspirations…

                                                                                                Steven Vogel¹

            How are you strong?

          Not, “how strong are you?”, which sounds like I’m asking how many pounds you can lift.  More precisely, I’m asking, “how does strength express itself?”: how are your strong?

          Another experiment.  We’ll use the side raise again, only this time you’ll need a partner.  Your partner is going to perform the motion, only instead of with weights, you’ll provide manual pressure at the wrist.  You want to provide just enough force to prevent your partner from moving.  You’re going to note how strong your partner’s deltoids appear at certain points in the movement.

          At the bottom, with your partners’ hands directly under the shoulder, how strong does he or she feel?

          How about at midpoint, about halfway up?

          Finally, let your partner complete the motion and lower just slightly from the top.  How strong here?

          This is a much more subtle demonstration than with the dumbbells.  It’s not as visual, and relies on your perception of touch, so try not to simply overpower your partner.  Again, the partner should do the side raise slowly and steadily, and you should try to just barely stop the motion.

          Regardless of the size of the deltoid, you’re not imagining it if you notice:

                   Not too much strength to start (Figure 2.1A);

                   Significantly more in the middle (2.1B);

                   Less strength at the top (2.1C).

                     

Strength Vs. Muscle Torque

            Strength may not be the best word to use in this case, because it usually implies overcoming a resistance (actually, resistance torque, as in the previous chapter).  What we are actually describing in Muscle Torque.

          The point of this demonstration is that our muscles don’t exert a constant torque during movement.  The torque expressed by muscles, especially those that move limbs, varies predictably.  As the muscles contract from their most stretched to their most contracted, Muscle Torque first increases, then decreases, for every healthy muscle/joint complex (Figure 2.1D).

          As I’ll explain, this is simply how the machine works.  It shouldn’t be considered a controversial statement.  But when we explore the consequences of it, it challenges much of the conventional thinking about Exercise and exercises, with regard to concepts like “full range of motion”, angle training, shaping, the role of machines, free weights, etc.

 

Muscle Torque and Resistance Torque

            Muscle Torque is a tougher concept than Resistance Torque.  It can be the most frustrating part of biomechanics, because of a number of confounding factors.  We’ll examine them, but mainly in context of what we can do about them in the gym.

          Recall that with Resistance Torque, we had a variable Moment Arm and a constant force, the product of which created a variable torque.  The force in Resistance Torque is usually in the form of a weight; hard to overlook.  The Resistance Moment Arm is easy enough to visualize, as are any changes to it, once the axis and line of force have been identified.

          The Muscle side is confusing for several reasons².  First, not only do we have variations in the Muscle Moment Arm in the course of a contraction, we also have variations in the amount of Muscle Force; both components vary instead of one.  Second, neither is as visible as on the Resistance side.  We can only see the effects, and then reverse-engineer back to a model (although a well-established one).  Third, the Muscle Torque pattern is not constant.  For a given individual, it can change from training, speed of movement, fatigue, not to mention factors that are out of our control like neural factors and fiber arrangement.  It does, however, change predictably.  And finally, Muscle Torque effects overlap and interface with Resistance Torque effects, so we have to make a point of first distinguishing between the two, then manipulating the two to our advantage.

Muscle Moment Arm

            What we are ultimately trying to get at with any form of strength training is the force-producing capability of muscle; whether it’s to increase it, as in pure strength training, or to use it, as in bodybuilding or toning.  But since we’re not in the habit of cutting our limbs open, and attaching a weight directly to the tendon, we have another Moment Arm to deal with.  This Muscle Moment Arm is created at your joints, where the tendon attaches to the limb that’s going to move.  Remember, a Moment Arm is the perpendicular distance between axis and line of force.  In this case, the axis is located at or near your joints, the line of force is represented by the tendon, and the force is provided by your muscles.

          When the muscle is at its least concentric position (ie fully stretched), that line of force lies almost through the axis.  A minimal Moment Arm is created, and so minimal torque is possible.  At the most concentric position, the extended line of force also lies almost through the axis; again, a minimal Moment Arm and minimal torque.  Somewhere in the middle, the line of force lies as far away from the axis as it can, creating a Maximal Moment Arm and in turn a Maximal Muscle Torque³. (Figure 2.2)

                                                    

The Model for Muscle Force

            This change in Muscle Moment Arm would create a variable Muscle Torque, even if Muscle Force remained constant.  As it happens, however, Muscle Force itself is also subject to variation within a given contraction.  The accepted model for muscle contraction is the “sliding filament”.  Without getting too microscopic:  muscles are made up of fibers, which in turn are made up of myofibrils, which in turn are made up of sarcomeres, which is the point of the mechanism for contraction.  The pattern of force produced at each level doesn’t differ from the pattern of the previous level; so that what happens at the muscle level reflects what happens at the sarcomere.  At the sarcomere, the filaments actin and myosin don’t contract, but overlap; the contraction is the result of the filaments sliding by each other.  The force of the contraction depends on the amount of overlap.  Minimal overlap results in low force.  Maximum overlap again results in low force; there is nowhere for the filaments to go.  Somewhere in between, there is an optimal overlap where the greatest force is generated, the “favorable length” ⁴.  (Figure 2.3)

                                              

Muscle Torque Patterns

            As it happens, both of the primary factors in Muscle Torque change in the increase/decrease pattern, as the muscle contracts from most stretched to most contracted.  In terms of what we do in the gym, this means we are strongest somewhere in the mid-range, which begs the question, mid-range of what?  The range of muscle action probably exceeds a safe joint range, which probably exceeds the useful range of an exercise.  Or not, which is why in coming chapters we look at each muscle and joint complex separately, and design exercises to stay in a safe range for the joints while loading the range of Maximum Muscle Torque.

          For now, let’s continue to explore Muscle Torque.

          Figure 2.4A is a graph approximating Muscle Torque.  In the course of a set, the curve would shift downward, as fatigue is one of the factors affecting Muscle Force.  Is it possible to have Muscle Torque in a different pattern? A qualified Yes.

          There are some published graphs that show only increasing or only decreasing curves.  On closer inspection, you’ll see that only part of the muscle’s contraction was tested, either due to a limitation of the device or for safety.  Rumor had it that Sergio Oliva, one of the legendary bodybuilders from the ‘60s and ‘70s, had biceps and forearms so big that they collided halfway through a curl, preventing his biceps from contracting further.  In his case, the Muscle Torque curve for biceps appeared to only increase, because the size of his muscles interfered with “normal” limb movement, effectively testing only part of the muscle action.

          Right.  Like we all have to worry about that one.

                                

Passive Tension, Speed of Contraction, and Torque Curves

            There are other legitimate reasons for graphs to depict other than the increase/decrease model.  One is “passive tension”.  The increase/decrease is created by voluntary contractions.  When an external force stretches a muscle to near its limit, the active tension contributes less, and structural components provide more “passive tension” until it breaks⁵.  Technically, there’s greater potential muscle force available, but obviously there’s no point in trying to load it with weights.

          The speed of the contraction also affects the shape of the curve⁶.  Different speeds create Maximum Muscle Torque at different points; it still falls somewhere between least and most concentric.  In the gym, the speed of contraction is important, but not so much for its effect on the shape of the curve.  In the Resistance Torque chapter, we discussed that if you heave or drop the weight, you add momentum and acceleration, and so lose control of Resistance Torque.  That aside, the faster a muscle contracts, the less force it generates; the slower the contraction, the more force⁷.  If you have the technology that eliminates the effect on resistance, you may be tempted to train with faster contractions; but you still have to be careful of the muscle yanking on your own joints during the exercise.

          For most, especially those using weights, the slower contraction allows you to use more of your potential muscle force.  And at some point in training, you have to use a high percentage of that potential (if not a maximal effort).  Otherwise, your body would have no reason to maintain or increase your capacity to generate that force; which means less muscle mass and tone.  Your body would perceive the exercise just as an “activity of daily living” and not change as a result of it.

          That change is represented in Figure 2.4B.  Two curves are shown, one flatter, one more peaked.  The flatter represents untrained muscle.  The Moment Arm changes are present, the filaments behave as expected, so there is a slight increase/decrease.  The peaked curve represents potential Muscle Torque after an unspecified amount of ideal training.  As the training worked and the fibers grew larger, the muscle would generate more torque overall, but more so in the middle.  Why?  Because the “weakness” at the ends is from the reduced Moment Arms and positioning of the filaments; not from the size of the fibers.  It’s mechanical, not muscular.  Regardless of the fiber size, there will always be optimal overlap in the middle of the contraction, not the ends; and the Muscle Moment Arms will always be larger in the middle than at full stretch or contraction.  It couldn’t be otherwise, given the sliding filament and moment arm models.

Back to the Gym

            Wait a minute, you’re thinking.  That may be a nice theory, but when I do a Barbell Curl, I feel pretty weak in the middle, and pretty strong at the start and finish.  My biceps torque curve must be U shaped.  When I bench press, I feel weak at the bottom and strong at lockout, so my pecs get stronger as they contract; their torque must “only increase”.  When I row, I feel strong at the start but weaker as the weight comes to me; my lats’ torque must “only decrease”.

          To explain these apparent discrepancies, we look at the interface: the overlap of Resistance and Muscle Torque.

Notes

1.       Vogel, Prime Mover, 2001.

2.     “The Muscle side is confusing…” See Harman, 1994, pp 31-34; Brunnstrom’s, 1996, pp 136-146; Nordin and Frankel, Basic Biomechanics…, 2001, pp 160-165; Levangie and Norkin, 2001, pp95-103.

3.     “The muscle moment arm…” See Harman, pp 28-30; Brunnstrom’s, pp 61-63.

4.     All the previous references cover sliding filament and all the material on muscle function.  The explanation here is based on Vogel, Prime Mover, pp 12-18.

5.     Passive Tension:  Nordin and Frankel, pp 160-161; Brunnstrom’s, pp 138-140.

6.     “The speed at which it contracts…” See Harman, p 33 for graphs.

7.     “…the faster a muscle contracts…” See Brunnstrom’s, p 142; Nordin and Frankel, pp 158-162; Levangie and Norkin, pp 97-98.

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