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Determinants of Strength Performance Part 2

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Continuing from last week where I looked at the contribution of inherent mechanics and muscle cross sectional area, I want to finish up this week by looking at the next factor that determines overall strength performance.  This was originally meant to be a fairly short bit as I was trying to keep it line with the mechanical and msucular factors I discussed last chapter but it turned out to be far more involved.  So it’s long.  Too long.  And while I will try to sporadically link out to references (which I know most don’t care about), most of it will be coming from two primary sources which are Enoka’s Neuromechanics of Human Movement and Strength and Power in Sport edited by Komi.

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 peopel 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 it’s 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 disuss 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 3



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