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Determinants of Strength Performance

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 and talk about the determinants of strength performance.

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, 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.

Fig Newtons
Not a unit of force. But still delicious.


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. To keep this simpler, 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 while a lower bar squat means leaning over more.  And this makes simple conclusions about which is more low back stressful well…simple. And simple is usually wrong.  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.

Biceps Brachii


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.

The SSC and Strength Performance

Although it kind of fits in with the impact of muscular factors on strength performance, I want to discuss the stretch shorten cycle (SSC) separately.   This refers to a situation where a muscle is first stretched (an eccentric muscle contraction) before shortening (a concentric muscle action).

There is also a brief isometric muscle action where the muscle doesn’t change length in-between the two.   When this happens, a greater amount of force is generated than would occur otherwise and this improve strength performance.

You can demonstrate the existence of the SSC for yourself by comparing jump height for a squat jump (where you jump as high as possible from a crouched position) to a countermovement jump (where you squat down and immediately jump up).  In the first there is no SSC since there is no initial lengthening of the muscle (it starts from an isometric position) and in the second there is; this increases force output.

The basic reason that the SSC exists is to make movements more efficient or effective since more total force is being generated, often with less total effort. In the most general sense, force can be generated through two major factors: muscular/metabolic and elastic.

Elastic here has to do with the presence of connective tissues such as ligaments, tendons (and I suspect things like titin) that can stretch or compress and then spring back, producing force.  Any force that can be generated through elastic forces is less that is required to be generated by muscular forces.

So let’s say you need to generate 100 units of force in some movement.   If the elastic component generates 20 units of force, the muscle only has to produce 80 units of force.  Relative to strength performance, if muscle still contributes 100 units of force and the elastic component contributes 20 units, the total output is 120 units of force.

Sure, you could achieve the same with 120 units of muscular force but if you’re capable of generating that, adding 20 units of elastic force still takes you to 140.   Which is better depends on the goal.  For endurance type activities using less muscular force overall means fatigue happens more slowly; for maximal strength or power, adding the elastic contribution to muscle force increases total force output.

Mechanisms Behind the SSC

There are at least four potential mechanisms that contribute to the SSC which are  force potentiation (this has apparently been dismissed as a contributor so I won’t discuss it), reflex muscle actions, time to generate force and elastic contributors.

There is some debate over which mechanisms or combination of mechanisms is at work and it may depend on the movement and how it is being performed (i.e. how long the movement itself is, how long a delay occurs between the stretch and contraction and whether the muscle is being stretched rapidly or slowly).

Reflexes refer to, well, reflexes.  We’ve got lots of them in the body (such as the knee tap relex that works by stretching the patellar tendon which stretches the quad so that it reflexively fires) but the impact of this is debatable.

It seems to depend on the length of the movement since it takes time for the muscle to be stretched, send a signal to the spinal cord, get a signal back and fire.  Apparently 130 milliseconds is the cutoff and it’s interesting that most sports have about a 200 millisecond duration for maximal force production so reflexes probably play a role.

The next, and more relevant mechanism, has to do with the time to generate force.  As I discussed in some detail in a previous series, muscle doesn’t generate maximal force instantaneously (and the speed with which it does so is called Rate of Force Development or RFD).

So if the muscle has proportionally longer to generate force over, more force can be generated.   Most of this has to do with the muscle already generating force during a controlled eccentric so that it can produce more when the movement reverses.  Part of this is muscular pre-activation, the muscle is contracted to start which shortens it’s time to peak force.

But this has a secondary effect which is that even in a technically isometric contraction causes a small amount of shortening in the muscle.  But a lot of this is pulling out the “slack” in the tendons.  Even if this doesn’t impact on how the muscle is generating force, it impacts on how the muscle transfers force to the bones (remember that tendons attach muscle to bones).  If the slack isn’t taken out, there is a delay before the force from the muscle is translated to movement.

And this brings in to play perhaps the most well-established factor in the SSC which has to do with the elastic component I mentioned above, force generated by elastic connective tissues such as tendons and ligaments that when stretched can rebound by generating force (like a rubber band).

You’ll see these tissues referred to as the Series Elastic Component (or SEC) due to the fact that they run in series (rather than in parallel) with the muscle.   So when the ankle bends, this stretches the Achilles tendon which stores force which is returned, the same happens at the patella and the hip.  And of course it can happen in the upper body.

The impact of the elastic contribution to the SEC is more pronounced with increasing tendon length.  An odd example of this is that kangaroos, who have enormously long Achilles tendons and who, during hopping, generate 92-97% of the force generated from rebound (i.e. muscle are contributing only 3-8% of the total force).  This makes them insanely efficient.  Animals with shorter Achilles tendon lengths don’t get nearly this much effect.

The SSC and Real-World Activities

Ok, enough background, let me look at some real-world implications and applications of this. When you walk or run, every time your foot hits the ground, it stretches the Achilles tendon.  This stores energy when your foot hits the ground which it then returns when you start moving forwards.

This is a clear adaptation to improve human walking and running efficiency since it decreases the amount of force that muscles have to generate (there is some evidence that humans evolved for distance running and this would be part of that).

Tangent: I’d note here that women appear to utilize more SSC when they walk and during certain lower body movements.  Limited data suggests that men may use more SSC in upper body movements.  And this makes a certain degree of logical sense if you consider the evolutionary pressures that the sexes underwent (I guess gathering vs. punching or throwing stuff).

And while there are other reasons having to do with muscular distribution and fatigue (and social stuff), I also think practically it helps to explain why men tend to prefer training upper body (they are better at it) and women love training legs (they are better at it).  But I’m getting off topic.

With a few exceptions, the SSC occurs in a huge number if not the majority of movements.   If you watch a javelin thrower, after he finishes he run, he’ll let the arm drop back before throwing; a high-jumper will plant their foot, sink onto it before the jump and this is all to take advantage of the SSC.    In most sports, the SSC is utilized to one degree or another.

Perhaps one of the odder examples of using the SSC is in my old sport speed skating.  There you glide along on one leg (for about 0.8 seconds) before pushing.  And prior to the push you use what my coach called a compression.  Basically you sink a little bit deeper before starting the push.  Not only does this let you sit a little bit higher (which limits the buildup of fatigue metabolites) while pushing from a lower position (lengthening the push), it gets you a bit of SSC effect to generate more force.

Moving to the weight room, you can see the SSC (or occasionally lack thereof) most places. Assuming they don’t do it from the start, when people start to fatigue they tend to start dropping the weight faster and try to rebound it out of the bottom.    Some training approaches have even suggested using a deliberate bounce in stretch movements to take advantage of the SSC in the bottom of stretch position movements (usually focusing on the reflex aspect of it).

In the article I linked above talking about calves, I specifically mentioned why so many people with small calves can bounce the stack and it has to do with the Achilles tendon being so good at storing and returning energy.  I also suspect that’s where the idea of doing sets of 100 or whatever came from.

If you’re bouncing and the Achilles is generating a lot of the force, the muscles are generating less so it takes more reps to get full fatigue.  If, instead you pause for 2 seconds at the bottom of every rep and allow the SSC to dissipate the muscles do more of the work and you can torch them in far less reps (it takes enormous amounts of weight off the machine too).  Try a heavy set of 8 both ways, bouncing versus pausing and see which wrecks your muscles more.

Or consider the bench press where people routinely lower the weight very quickly to get a bounce of the chest.  Certainly part of the reason this is done is to get a physical rebound off the chest but by going from a rapid lowering to pressing they can get an effect from the SSC.

Even without a drop and a bounce, a touch and go bench press is different than having to pause (i.e. in powerlifting competition).  The pause can easily take 5-10% off a lifters best touch and go poundages since the SSC dissipates or at least starts to while the bar is motionless on the chest.

The same occurs for benches started from the bottom in a power rack or what have you.  DB bench press is often very difficult to start for the same reason, you don’t get the initial lowering to load the elastic tissue or get any SSC; with practice after kicking the DB’s up you can do a quick short lowering to start the rep and this works by generating at least some SSC contribution.  At the very least, getting super tight to pre-activate the muscles goes a long way towards starting the rep.

The same is true in the squat where the initial lowering, so long as there’s no extended pause in the bottom generates and SSC.  Compared to pause squats or, god forbid, bottom position squats doing it with a lowering is much easier.

Olympic lifters try to catch a bounce out of the bottom of the squat after a clean for the same basic reason, it allows them to stand up more easily while saving their legs for the jerk.  When Ol’ers get stuck at the bottom of a clean, they will go up sightly before dropping back down to try to catch a bounce.

Now consider the deadlift which, due to the starting position, eliminates the SSC on the first repetition.  Have you ever noticed that the second rep of a set is often easier than the first even though you should technically be fatigued?  Well there are two reasons for this.

One is that people often hit a better pulling position with an initial lowering in terms of their hip and back.   But relevant to this article is that lower the bar under control generates and SSC; so long as the next repetition starts without too much time passing, it contributes to force production.  The bar doesn’t even have to be bounced, if it’s simply lowered under control to the pulling position before the next rep is started, the SSC contributes.

You can demonstrate the above pretty easily by doing deadlifts starting at the top.  You have to have a very adjustable power rack but if you start at the top and lower the bar first, you generate an SSC and you’ll find that the first rep off the floor will be easier than if it were started on the floor.  Here’s a good video from Broderick Chavez at Evil Genius Sports who I have done two podcasts (one on muscle gain and the other on fat loss) with.

This is a big part of why a lot of deadlifters recommend standing up between sets of reps and resetting every one so the set is a series of singles.  In competition you don’t get to use the SSC on a second rep so it ends up being a series of single repetitions from a dead stop.  Since you can’t use the SSC, they argue, you shouldn’t train with it.  At the same time, at least one of the top DL’ers (I forget which) does top down DL so they can use a heavier weight.  But it’s in addition to normal dead stop reps off the floor.

Lifters who use a dynamic start, dropping the hips down before starting the pull are actually trying to generate an SSC With the rapid drop and turnaround.  It works but only so long as the lifter doesn’t get pulled out of position and shoot their hips (which most do).  Olympic lifers will often use a dynamic start for the same reason but the same thing applies.

At the very least, you will see top Dl’ers get tight from head to butthole (as I am fond of saying) prior to starting the pull and this still acts as pre-activation/taking out the slack within the muscle and tendon even if the total overall SSC contribution is small.

Improving the SSC for Strength Performance

As with the other sections, let me finish up (and I know this got way away from me) by talking about how the SSC is typically improved.  In the most general sense, like anything else, SSC can be improved by practicing it and there are various adaptations that occur that I’ll finish up with.

I mentioned that Olympic lifters will use a bounce out of the bottom of the squat and practicing it helps with the timing of the bounce in addition to some of the adaptations I’ll mention in a second. Some will also deliberately not bounce in order to train the muscles more intensely.  When the bounce is then added more weight goes up.

Powerlifters, by and large don’t really train the SSC (I suppose Westside style DE benching might count since it’s usually a quick drop to explosion) except for the squat since it’s part of the movement.   If you have to pause a bench, you don’t get much effect from the SSC although some lifters will do touch and go benches in training (to lift more weight) and switch to pauses closer to competition.

Squats inherently use the SSC although things like box or pause squats are often used to deliberately remove it; this makes the muscles work harder so that when the SSC is used, more weight goes up.  Since deadlifts don’t get to use the SSC at any point, most wouldn’t do much training using the SSC except maybe the top down thing I mentioned to allow more weight to be handled.

To be honest, I don’t see much point to deliberately use the SSC for bodybuilding.  In its strictest form, the SSC reduces stress on the muscle when the goal is typically the exact opposite.   Pausing briefly between repetitions, deliberately forcing muscles to do more work makes the most sense under most conditions.

So far non weight room activities, athletes generally do a lot of things to try to improve the SSC contribution in addition to their other training.  I imagine many readers are familiar with the concept of plyometrics.  Most frequently used for the lower body, you can sort of think of this as jump training although that spans a pretty large category of things.

Basically a plyometric exercise is any one that uses the SSC (in terms of an eccentric into a rapid concentric) in some form or another.  Skipping rope is technically plyometric (and is a great warm-up for plyometric training) but things like bounding or repeat hurdle jumps are more common (there are comparatively fewer upper body plyometric exercises but something like an explosive push-up would be an example).

Perhaps the most infamous plyometric training are depth jumps where an athlete steps off a box of some height, hits the ground and attempts to jump up as high as possible.  This got popular in the 70’s or so when it came out that the Russians were using it. Supposedly they were developed by Yuri Verkoshanksy and he called it shock training.

And it injured endless athletes since it was used without true understanding of how intense it was or how it was used (generally for short blocks once or twice a year).  True maniacs did one legged depth jumps and, well, just listen for that Achilles tendon snapping.

Plyometric training does a number of things that can improve the SSC.  One is that it actually does serve to strengthen the muscles in general due to the large eccentric component (the athlete is stopping multiples of their own bodyweight when they land).

Learning to resist the downward forces (called amortization) to rebound is part of this.  There is also a muscular stiffness issue as the pre-activation of the muscle (which is improved with practice) since this is required to help with the amortization factor.

In the long-term, there are probably adaptations within the series elastic component as well as, over long periods of time, connective tissues can strengthen and become thicker which probably makes them springier (there is a story in the fantastic book The Sports Gene about a world class high-jumper who’s Achilles had become basically a spring after 20 years of training).  This is an excruciatingly slow process and can’t be rushed.

But I actually think, getting so far off topic as to be painful that it partly explains an old weird observation which is that athletes often keep improving performance long after things like strength, aerobic capacity or whatever have stopped increasing.

And that is that elastic connective tissues are strengthening and providing even more of an elastic contribution to the movement.  This happens in tendons, it happens and ligaments and, while poorly studied, I predict it will be shown to happen in other connective tissues such as titin.

Finally and supporting the role of reflex actions in the SSC is work showing that reflexes can actually improve with training, and rather rapidly at that.   Four weeks of hopping training has been shown to improve stretch reflexes in the calves.

Which brings me in a very roundabout way to the final factor in strength performance I want to discuss which is neural factors.  But since this bit, which should have been much shorter, got way out of hand, I’ll save that for next week.

Read Determinants of Strength Performance Part 2

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