Posted on

What Determines Strength Output?

In a long-ago written article (that was written while I was doing a lot of endurance training, go figure) I wrote about the primary determinants of endurance performance and today I want to do sort of the equivalent article to that for strength production (of no surprise, I’m doing mostly lifting now).

Now, if you want to get technical, you can define different kinds of strength.  What is often measured in the lab is isometric strength using some kind of tensiometer (that will give you force output in Newtons, not the Fig kind, or whatever the units are) but in practical sense most will be more concerned with how much weight they are lifting in some gym movement. Even that can be subdivided and some folks might really get up their butts by worrying about concentric strength (how much weight can be lifted), isometric strength (how much weight can be held at some position in a movement) and eccentric strength (how much weight can be lowered under control).

The weights would go up from concentric to isometric to eccentric (i.e. you can lift less than you can hold and hold less than you can lower) but for the purposes of this article, I’m only going to worry about concentric strength.  Most of what I will write still applies but there are some slight differences that I can’t be bothered to talk about.  So concentric strength, how much weight can be lifted through the range of motion for some exercise is how I will define strength here.

Muscles, Bones and Force Production

Without getting into a big physics wank about the forces acting on the body let me talk briefly about how muscles generate force.    Muscles are simply bundles of individual fibers that, when they contract, attempt to move the bones that they are attached to.  By doing so they translate what is linear movement (muscles contract linearly) into rotational motion (all joints move in a rotational fashion).

So when the biceps contract in a straight line, they cause the forearm to curl upwards as it rotates around the elbow (I’m not getting here into torque, axis of rotation or lever arms here; at some point I want to do an overwritten series on that topic alone but this is not that time).

So the muscle shortens and pulls on the bones, the bones rotate and this generates movement.    But assume that there is a weight in the hands.  If the force produced around the elbow is high enough, that is exceeds the force being exerted by the weight (usually due to gravity which points DOWN) the weight will be lifted.  If the force output isn’t high enough the weight can’t be lifted.

But this means that how much force is actually produced involves both the muscle itself and factors related to inherent anatomy and mechanics.  And it turns out that muscle force has two determinants which are the muscle XSA and neural factors.    And it is those three factors that I will examine.

Anatomical Factors and Strength Performance

First let me make it clear that anatomical factors here is referring to inherent anatomical factors.  Clearly muscle size/XSA is an anatomical factor but I’ll address it separately in the next section.  Here I am talking about two primary factors which are limb length/mechanics and the actual physiology of how muscle attaches to those bones.

By mechanics here I’m talking about biomechanics and I’m going to use this to refer to two different things.   The first is the easy one, the length of the bone in question.  This gets into the physics of this that I don’t want to get into huge details about.

To keep it simple just think of the amount of force that is required to lift a given weight as being related to the weight itself and the length of the limb being moved.  It’s not that simple but I’m just not getting into lever arms and torque here.  So for the purposes of this discussion, just remember that the further away from a joint a given weight is, the more force will be required to lift it and the closer it is, the less force it will take.

I’d note that it also gets way more complex when you start looking at multi-joint movements since multiple muscles are contracting around multiple joints in complicated and fascinating ways.  But I’m not getting into that.  For now, I’ll just focus on a single joint movement activity such as a barbell curl where all that is really happening is that the elbow is bending.  Yes, I know that there can be shoulder movement and wrist movement but I don’t care.

So imagine two people curling the same weight.  The one with shorter arms has to produce less force than the one with longer arms.  Alternately, the one with shorter arms can lift a heavier weight while generating the same amount of force as the one with longer arms.  Alternately again, if the person with longer arms could find a way to move the weight closer to their joint, they would need to generate less force.  I’ve attempted (badly) to draw this below.

Force Production Comparison

This is meant to represent the upper arm (vertical line), forearm (horizontal line), the elbow (blue circle) and a weight in the hand (number inside circle) that is being curled.  The arrow is the biceps and the number above it is the amount of force in arbitrary units that is required to complete the lift.

So in the upper left is a long-armed person is lifting 10 lbs and it takes them 20 units of force to lift the weight.  Upper right is a shorter armed person, the same 10 lbs takes them 15 units of force.  Bottom left is a situation where the long-armed person moves the weight in (maybe a wrist cable to apply the force) to the same distance as the short-armed person; the same 15 units of force can move 10 lbs.  Finally is the lower right where the short-armed person can lift 20 lbs for the same 20 units of force that it takes the long-armed person to lift 10.  All numbers are made up for illustration; focus on principles, please.

So that’s a primary factor, limb length as it impacts on how far from a joint a given weight will typically fall.  The further away from that joint, the more force any given weight will require to lift.  A secondary effect of this is that longer limbs mean that any given weight has to be lifted through a longer total distance.  Long-armed people have to move the weight further in a flat bench and move through a longer distance when they squat for example.

But there’s a secondary factor and this gets a little bit dense.  Muscles attach to bones via tendons and these tendons can attach at difference places along the bone in different people.  Some people have longer muscles and others have shorter muscles or longer or shorter tendons.  For example, common to African Americans are calves that insert very high on the bone; this is fantastic for jumping (for reasons I won’t get into) but terrible for calf growth.  You can find people with pecs that simply don’t meet that close together (they lack cleavage), people with shorter or longer biceps, etc.  And the end effect of this is that shorter muscles generate less force around a joint (thought they generate it faster) than longer muscles.

Altering Mechanical Factors

Unfortunately, neither of the above factors can be altered without some rather serious interventions.   There are rumors that the Soviets experimented with detaching tendons and reattaching them further down the bone but it supposedly screwed up people’s motor patterns if it was done at all.  The very occasional story of someone popping a tendon and having it reattached surgically further down the one and seeing strength go up exist.  But for the most part, the above can’t be changed.  If your born with long arms and long or high tendons, that’s just the way it goes.  Note that people who are good at a given lift very often have very similar mechanics and those with other poorer mechanics often suck at a given lift.

What can be changed, however, at least within limits is how a lift is performed.  That is, while the body’s inherent anatomical mechanics can’t be changed, the mechanics of a lift can be changed.  Grip width or elbow position on bench press can be changed, people with longer legs can use a wider stance on squats which effectively shortens the lever arm at the hip (just take this at face value please).

Sumo DL is often superior for people with a very long torso since their low back often gets beaten up or is limiting in the movement: the length of their spine means their low back muscles have to generate more force for the same weight lifted.  By making the torso more upright, Sumo eliminates this particular weakness (and the wider stance may additionally benefit long legged folks).

The same occurs by changing the bar position in the squat since it changes the relative distance from the weight to the hip/spine.   This interacts with torso position which complicates this but this is why a lot of very simplistic analyses of high- and low-bar squats on the web are hilariously wrong.  Moving the bar higher on the neck is usually accompanied by a more upright torso and vice versa, lower bar squat means leaning over more.  And this makes simple conclusions about which is more low back stressful well…simple.  Eventually I’ll do a full piece on this.

Hopefully you get the idea and again, I will eventually expand on all of the above in a separate series.  Just accept that, outside of changing movement technique, inherent anatomical factors can’t be changed.  If you have long arms you have long arms; if you have short muscles and either longer tendons or tendons that attach closer to the joint, you can’t impact those.  At best you can pick movements suited to your mechanics or alter lifting technique within certain limits.

Muscle XSA and Strength Performance

Ok, I want you to imagine a muscle, go hit a most muscular in the mirror if you must (and have muscles to most in the first place).  Imagine what it looks like (time for another pose and maybe a little stroke or kiss).  So the biceps runs from one end of the upper arm to the other and in one sense it’s a straight line.  Except that humans are not one-dimensional beings (well, some are in personality I suppose) and a line doesn’t really describe a muscle.  So next move to two-dimensions, a muscle has both length and width or length and height depending on whether you look at it from the top or the side.  Now we’ve moved to area (hello Flatland).

But that’s still not entirely correct since we exist in three dimensions (four if you count time and a whole lot more if you get up your butt about quantum theories of the universe).  For now let’s focus on three.   I want you to try to imagine the biceps as a kind of tapered cylinder, it starts narrow, widens in the middle and narrows again but is semi-roundish.  Now we’ve moved from area to volume. It has width, length and height and while I was going to try to draw this there is no way in hell I will have the ability to do so.  Here’s a stock photo that kind of shows what I’m babbling about that kind of shows the three dimensionality of a muscle inasmuch as a two-dimensional picture can.


Ok, now imagine that you’re looking at the muscle from the front, looking at what kind of amounts to a cylinder.  Or imagine you took a slice out of it.  Like you took a cucumber and cut it in half lengthwise and now you’re looking at the end.  Draw a line straight across the thickest part.  This is the cross-sectional area (XSA) of the muscle and represents the total cross-section of all of the muscle fibers packed into the circle.  Like this, we’re looking at the muscle from the front:

Muscle Cross Sectional Area

Note: there are different types of muscles in the body with shapes and fiber arrangements that aren’t as simple as what I’ve shown (called pennate muscles) but this isn’t critically important to this piece.    Just remember that physiological cross sectional area (PCSA) is a key factor in how much force a muscle is potentially capable of generating and that this contributes to strength performance in the gym.  Because each fiber within that area can generate some amount of force (called specific tension)  which depends on the XSA of the fiber.   Bigger fibers generate more force than smaller fibers.

This is the basis of the idea of increasing muscle size to get stronger (or getting stronger to increase muscle size).    Technically the goal is to increase muscle XSA but since muscles grow in three dimensions, that means that volume is what is increasing.  And that means, to one degree or another, muscle XSA will increase.  Big picture, don’t get too hung up on this and just consider a larger muscle a potentially stronger muscle.

This actually interacts with the mechanics issues I mentioned above.  Since volume scales with area which scales with length, the fact is that someone with longer muscles (which is often but not always associated with longer limb lengths) has to gain more total muscle to increase muscle volume to increase the actual XSA.  This makes sense in my head and I hope it makes sense here.  Basically since muscle grows in three dimensions, if it’s longer to start with, it takes that much more of an increase in total muscle volume to meaningfully increase XSA.

Which is kind of two ways that people with long limbs are screwed; since their muscles may be longer, they have to gain more total muscle to achieve the same visual or volumetric size.  It’s why a lot of bodybuilders and strength athletes are either short or weight 300 lbs; it’s the only way to get that big.  And since they have poorer levers overall, that means that they will lift less on average than someone with both shorter limbs and shorter muscles.

Altering Muscle Size

Of everything involved, increasing muscle size is almost the simplest although eventually people reach their muscular genetic limits and/or have to deal with a weight class limit and can’t gain more muscle without moving up a class.   And while a many different types of training are turning out to stimulate roughly equivalent amounts of growth, I will continue to argue for the existence of a practical hypertrophy zone here.  But proper training (progressive tension overload in the context of sufficient frequency, volume and intensity) with proper nutrition leads to increased muscle size.  It may not be simple in practice but it’s simple in principle.

And this increases the potential for strength production.  Please note my use of the word potential as there is no guarantee that a larger muscle will necessarily improve performance in a given movement.  And this is due to the presence of the third factor that contributes to strength performance: neural factors.

And that’s where I’ll pick up next week.

Similar Posts:

Facebook Comments