Stronger, Faster, or Both? Physics and Physiology…

Jumps, Plyos, and Squats...OH MY.

It is often asserted that in order to jump better people should use squats and box jumps or jump training. The specifics of how, why, and what order or frequency we execute these movements with are often left out. A brief tour around any gym and you’ll see folks squat super fast with light weights, squat with really heavy weights, jump to boxes a million times, or jump with weights. But what are these methods doing…really? 

JUMP VS PLYO TRAINING

Somewhere along the line, some poor trainer started calling all jumps plyometrics and the telephone game of poor translation to poor translation began. What many think of as plyometric training is merely jump training, which has it’s purpose in athletic development, it just isn’t synonymous with plyometrics. So, what are jumps? What are plyometrics? How do we use these methods of training combined with strength training to become faster or stronger

Let’s dive in…

The term “plyometrics” wasn’t in circulation until Olympian Fred Wilt coined the term in 1975 and it wasn’t the original name for this kind of training (Yessis, 2009). In fact, before jumping until you puke started appearing in box gyms across the world, Yuri Verkhoshansky, a Russian sport scientist was busy developing “The Shock Method” in the 1960’s. The idea being that, when we do explosive movements like making ground contact during a jump or having an object thrown at us, we are “shocked” by a stimulus that elicits a rapid stretch in the tissue.  This rapid stretch causes a rapid production of force to oppose this incoming shock. This happens due to several underlying mechanical and neurological mechanisms. Importantly, these shock method exercises required short contact times (i.e. absorbing the landing and reversing it as fast as possible), were very elastic in nature, and often had relatively high eccentric loading due to gravity.  For an example of eccentric loading, think about how you have to squat slightly as you land in a jump off of an object – your muscles and other tissues lengthen to absorb the force – this is eccentric loading and it happens very forcefully and very quickly in the “shock method.” 

But the shock method looks like it uses jumps, doesn’t it? Yes, some exercises are jumps. So, aren’t all jumps plyometric? No! Why? The difference has to do with ground contact time, the force we are developing, the time we develop that force in, and the speed of something called the Stretch Shortening Cycle (SCC) or the Myotatic Reflex. In essence, the SSC allows our muscles to be stretched and to rapidly produce force as a rebound from that stretch, (like a spring) – this has to be fast! The thing is, not all jumps happen fast. For example,  the simple squat jump without countermovement doesn’t rely on a pre stretch, and neither does the seated jump. The box jump often uses a countermovement with the arms, so theres a degree of SSC involved, but the landing is stuck, not rebounded from so it is less elastic in nature than the “shock” method plyometric jumps such as depth jumps or rebound jumps that are highly reactive and elastic. In short, jump training generally relies on longer ground contact times and is less elastic in nature than the “shock method” style plyometrics of Verkhoshansky.

RESISTANCE TRAINING

Weight training can be an excellent tool for improving tissue stiffness, when done with heavier weights or for high speed intent it can be useful for developing the fast twitch fibers of a muscle, and when trained with eccentric or and isometric emphasis it can have very positive effects on maximum power as well (Stone et al. 2004)(Aagaard et al. 2002)(Wilson et al. 1994). If we want to produce more force or produce force faster, well planned resistance training is imperative!

 

Physics and Physiology

Please don’t skip this section, you won’t be graded, you don’t need to do math, and there isn’t homework, but if you follow along with the concepts below, why’s behind jumps, plyos, and resistance training will make a lot more sense. 

Let’s talk about impulse! The second law of motion  states that Force = Mass * Acceleration.  

Force = mass * acceleration

Acceleration = vf-vi/ tf-ti

This can be understood as Force = m(v final – v initial)/tf-ti

A little rearranging gives us:

Ft = mvf – mvi

This equation represents impulse and it’s relationship to change in momentum where:

 Ft = impulse

Impulse is expressed in Newton Seconds (N.s)

mass*velocity final – mass * velocity initial = Change in momentum 

Momentum is a vector meaning it has direction, just like velocity.

Momentum is usually expressed in kg.m/s

And for a jump calculation, since we can generally assume our athlete’s mass will stay the same, the equation can be further simplified into: 

change in momentum = mass * velocity

So if Ft = impulse and mv = momentum

Ft = mv

impulse = change in momentum momentum

This is called the Impulse Momentum equation, and it tells us that:

 impulse = change in momentum.

Effectively, all explosive sporting movement could be thought of using impulse. For example, when we are braking against an incoming force, or producing force on an object or against the ground (like when we’re stopping someone tackling us backwards), catching something heavy, or jumping to dunk a basketball we are changing our momentum or the momentum of some other object or person. 

When graphed, Impulse is represented by the area under the curve. For example, if all of these jumps have the same impulse, they will have the same area under the curve, and all the same jump height, but some jumps are done with less force over more time where as some are more force over less time. I didn’t do the exact math for the image below, this graph is conceptual only – but let’s pretend the total area under each curve is the same for the sake of argument.

 

In many sports, being slow is simply not an option. So improving the speed at which we can produce force becomes a priority. If I can achieve more total force in the same time, my area under the curve is bigger, thus is my impulse, and thus is my movement!

Summary: Big Impulse = Big Jump, Little Impulse = Little Jump

We can improve our maximum force output and decrease the time we do it in through training that uses resistance training, jumps, and true shock method plyometrics too. In other words these methods can improve our impulse for movements we train with these methods. 

RATE OF FORCE DEVELOPMENT & TIMING

So now we know impulse is the area under the curve on a force time graph. Impulse reflects total force all added together, but doesn’t tell us when that force is produced in respect to the time constraint. So, in addition to thinking about much total force we develop in a time constraint, we can also consider the where the maximum expression of force happens in a movement and how soon or late this maximum force is produced during a movement itself. 

For example, the two jumps below have similar total force, but the athlete in red produces more of their force SOONER during the movement. Depending on sport, this may be advantageous. 

The type of training we focus on influences how we adapt to how soon we develop force vs how much total force we develop pictured a little differently we can imagine strength vs speed adaptations as: 

 

POWER AND THE FORCE VELOCITY RELATIONSHIP 

By now we’ve repeatedly emphasized the fact that the athlete that can produce more force and that can move fast will be superior to the athlete that produces less force and doesn’t move as fast. That being said, needs are sport specific too. For example, powerlifters generally move high loads over a longer period of time time than olympic lifters who must also be very fast in addition to lifting high loads. Still, powerlifters typically move more total poundage at upper levels than do olympic lifters, both are expressing power that can be represented on the force velocity curve.

Why?

Power, is defined as “the rate at which work is done (Urone 2020).”

The traditional equation for power is

Power = Work / Time

Where power is expressed in Watts

Since we are only moving a load in one direction we can use the instantaneous power equation

Here, we can use the equation for instantaneous power, where one object is moving in a constant direction with constant velocity.

Pinstantaneous  = Force * Velocity

We can graph most exercises on a Force x Velocity curve such as the one below

The powerlifter has higher force, but slower velocity = “Max Strength”

The olympic lifter has lower force*, but higher velocity = “Strength Speed”

*lower compared to the high 800-1000 lb loads used by competitive powerlifters, though let it be known that I’m no way implying competitive olympic lifters are weak in any sense of the word. 

They are both, literally, powerful athletes but their tissues and neurological control are biased for slightly different tasks. This differentiation happened, not by accident, but through planned training! Powerlifters can benefit from explosive movement undoubtedly, but their specific sport doesn’t demand high speed as much as it demands absolutely high values of force over a longer time that plyometrics take place in, this would influence the hows, whys, and whens of using explosive exercises in the training of powerlifters. 

The kind of training we focus on will determine the kinds of adaptations we make in relationship to the Force Velocity Curve. Picture below, the differences in ability based on speed vs strength training:

The person who focused on speed training improved their force at near max and max speeds, and the person who focused on strength training improved their speeds at near maximal and maximal force. 

How might considering your sporting goal’s location on the force-velocity curve influence your choice of exercise at different times throughout the year or training process?

Lastly, let’s talk Conservation of Energy. Every time we complete almost any jump movement there are three phases:

  • eccentric (muscle lengthening)
  • amortization (isometric contraction)
  • concentric (the muscle shortening)
Visualized as a jump this is as follows:
 
 
We can also visualize our jump as a spring such as:

When our connective tissue is pre-stretched, it has an elastic effect – like a rubber band being stretched and snapping back. In the muscle and connective tissue, this is called the myotatic reflex or the Stretch Shortening Cycle.

In the Stretch Shortening Cycle (SSC) elastic energy (EE) is absorbed and stored in the body during the eccentric phase, if this storage is followed by rapid transition to amortization or isometric contraction (catching and reversing the movement at the bottom of the jump) then to the concentric muscle action it is used to make the movement less metabolically demanding and assist the force of contraction. If too much time passes between the eccentric transition to isometric to concentric, some of the stored EE will be lost. More specifically, there are two SSC time ranges we can focus on. These are:

Fast SCC: <250 milliseconds

For example: Sprinting 

Slow SCC: >250 milliseconds

For example: Countermovement jump 

Why two different Stretch Shortening Cycle Times?

Ground contact time! Sprinters are in contact with the ground for much less time between their rapid and elastic leg movements when compared to someone completing a countermovement jump who has their feet on the ground for much longer between repetitions than the sprinter does. 

The elastic energy storage happens mostly in the tendons and during the rapid stretch of a movement be it landing from a previous jump or dropping the hips rapidly in preparation for an initial jump. However, how much EE is stored in a tissue may also be proportional to how deformed that tissue is by the stretch. Whichever tissue is deformed the most by a stretch (tendon vs muscle) will likely store the greater amount of  EE (Turner & Jeffreys 2010). Tissue aside, elastic energy is stored and released to assist in the movement. This Elastic Energy can only be stored for so long, in fact this storage has a half life of roughly 850 milliseconds (Wilson 1994). In other words our intent to rapidly express this stored EE is important when we’re training.

We need to consider time available, force vs. speed improvements needed, and athlete/position/direction specific needs when programming strength work, jumps, and plyometrics (Verkhoshansky 2009). 

Practical Applications

To recap the previous sections, we know that athletes have varying needs but that producing high degrees of force with time constraints are a common factor amongst many sports. Understanding the underlying physics and physiology of the stretch shortening cycle, where movements fall on the force velocity curve, and how rate of force development is important to sporting movements. We also know that plyometrics work because of the way they teach the body to produce high quantities of force rapidly and that part of this improvement is due to changes in the nervous system, and part of it is due to the elastic qualities and adaptations of our muscles and soft tissue. 

In practical application, training programs should begin with low-moderate skill and low-moderate training loads  to condition the tissues and to teach our nervous system how to organize movement in a controlled environment. We also want to increase our general strength, lifting with weights in the 75-85% range where we are able to create hypertrophy as well as neurological improvements for increasing our strength without the high drain on the nervous system for 90%+ loads (Yessis 2009). After adequate skill and strength are built with basic resistance training, low skill jump training can be an excellent precursor to higher demand plyometric work. We do these things in sequence over time from least demanding to most demanding because we need to have a robust tissue base, in other words, we want soft tissue that can generate/ tolerate the increase increase in force from our nervous system, the absorption of higher impact plyometrics, and over all uncreased impulse of our movement and we want to develop our coordination skill. Once we’ve developed strength and basic skill, we can continue to train our ability to produce maximal force, improve rate of force development, and develop our tissue capacity to tolerate even higher forces through a combination over time of strength work in different intensity zones, slower SCC work, true plyometric training with fast SCC focused work, and sport skill specific training (Verkhoshansky & Siff 2009). 

For more information on planning the training of athletes I highly recommend Supertraining by Verkhoshansky and Siff as well as Triphasic Training by Cal Dietz. 

Sources

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