Clearing the air with strength
Treading heavily
Open Instagram, Onlyfans or whatever you kids use and you’d be hard pressed to not find a workout video with a fire emoji captioning it. In fact you’ll find several videos, each one looking drastically different from the next. Some may wow you while others may sow Seeds of doubt (if you know, you know). Regardless of which one you stumble upon, one thing lies in common. All participants are producing force in some way, shape or form. This article talks about what really happens when force is expressed in a particular way. Depending on your goals, some forms of training can be more beneficial than others and I’ve used tennis as an example to highlight this.
Strength is not what you have, it’s what you display:
There are various ways in which we can produce force. How we go about doing this depends on the task at hand. Let’s define this display of force as strength. Strength exists on a continuum and nobody delineates this better than Popeye. On one hand we see him pushing apart a castle (maximum strength) and on the other we see him field an entire baseball game by himself (speed). Somewhere in the middle lies power which Popeye displays by punching Bluto around the earth. All of us have a mental image of strength that exists somewhere along this continuum. Irrespective of how you perceive strength, there are three fundamental mechanisms that affect how it is produced. These can be thought of as rules.
At what length is the muscle being asked to produce force. Muscles attach to bones via connective tissues called tendons, and both are capable of lengthening and shortening. When you see joints bend and straighten, it is because the muscles that attach to them are changing in length. Sometimes you can produce force without even having to move a joint. As per this rule, we are stronger at some muscle lengths than at others. This explains why we are weakest at the bottom of a squat and strongest at the top.
Is the muscle being stretched to very long lengths. We’ve all attempted a push up at some point. We know from experience that it is a lot easier on the way down than on the way up. This is not only because gravity is on our side, but also because muscles produce greater force when lengthening than when shortening.
How quickly is force required to be produced. When it comes to muscles, force and velocity have an inverse relationship such that the faster the speed of a contraction, the less force produced and vice versa. You can never throw a medicine ball as fast as you can throw a tennis ball simply because more force is required.
How is force mainly expressed in tennis?
Tennis is a unique sport in that force is required to be produced very differently depending on the situation you find yourself in. However, an overwhelming majority of the time a tennis player will find themselves having to display strength in a very distinct way, i.e., during changes of direction.
“Tennis players make an average of 4 directional changes per point but can range from a single movement to more than 15 directional changes on a very long point. In a competitive match, it is common for players to have more than 1,000 direction changes.” - Kovacs
What constitutes direction change is the ability to apply the brakes, overcome inertia, gravity and ground reaction forces, and rapidly reaccelerate. Here is what happens in the body:
When decelerating, muscles and their tendinous attachments are stretched to longer lengths rapidly to aid absorption and subsequent force production.
At ground contact when muscles are still at long lengths, there is a high degree of force being generated to help maintain stable positions. The more stable we are, the faster we can reaccelerate.
When reaccelerating, muscles recoil and shorten to produce force quickly.
Therefore, we can deduce that strength is mainly expressed in two ways. First when muscles are at long and very long lengths, and second when muscles are shortening at high speeds. Given that we know how force is required to be expressed in tennis and the rules that govern it, we can reverse engineer the type of training needed to be done. Enter plyometrics.
What are plyometrics?
A movement is plyometric if it satisfies two criteria:
There is a landing and takeoff sequence.
This has to happen in rapid succession, typically under quarter of a second.
Think of a sprint. Firstly, sprinters spend more time airborne than with their foot on the ground. Elite level sprinters are known to strike and lift their legs off the ground in 80-90 milliseconds. We blink at approximately 100-150 ms. Take that for context. Secondly, there is a landing and takeoff sequence seen from one stride to the next. Sprinting therefore satisfies the criteria and can be termed plyometric. Other forms of plyometrics include bounding, hopping, leaping, skipping, depth jumping, etc.
If we dissect movement in tennis, there are times when plyometric actions occur. Think of a return of serve split step. On the pro tour, a returner has roughly 600-700 milliseconds to react to a serve. This includes reacting to the ball (split-step) and racquet preparation. Therefore, it is safe to assume that training to get better at explosive movements will positively affect the way we display strength in our sport.
What kind of strength do we gain with plyometrics?
Strength can be gained in as many ways as it can be expressed. The principle of specificity states that strength gains are specific to the type of training we perform. With plyometric training we see improvements in primarily 2 things:
Increase in a muscle’s ability to produce force while lengthening.
Increase in high velocity strength when the muscle subsequently shortens.
We know that a plyometric activity has to include a landing and takeoff sequence. This sequence includes what is known as the stretch-shortening cycle (SSC). When we land, muscles and tendons elongate (stretch) and when we takeoff, muscles and tendons recoil (shorten). As per this cycle, a muscles’ ability to produce force when shortened is enhanced when preceded by its lengthening. Moreover, we know that movement in tennis contains a host of landing and takeoff sequences that make use of this SSC. Therefore, it makes sense to take advantage of plyometric training since we gain strength in precisely the same way it is required to be expressed.
How does this strength gain happen?
Let us investigate the hidden changes that occur with plyometric training. These are broadly classified into two:
Changes that occur in the nervous system. Most short term adaptations are likely to be neural in nature and can be observed a couple of weeks into training. What are they?
Enhanced pre-activation - The sooner a muscle can be activated, the faster it is capable of producing force. If our leg muscles are activated even before landing, the more effective we are at utilising potential and elastic energy in the subsequent takeoff phase. The more elite you get, the further away from the ground you begin pre-activating. In a sport like tennis where margins are extremely small, this could mean beating the ball to the bounce or letting it whizz past you.
Faster muscle fibre firing rate - The brain sends a signal to the muscle in order to activate it. The speed of the signal determines how quickly this happens. The faster the signal, the sooner we have access to all our muscle fibres to produce high speed strength.
Inhibiting an innate protective mechanism - In our tendons lie a receptor called the Golgi Tendon Organ (GTO) that is sensitive to the tension (force) placed on a muscle. When the GTO senses too much tension, it reflexively relaxes the muscle in order to protect it. This is counterproductive to our want to produce very high levels of force in as short a time as possible. It is commonly assumed that regularly exposing the body to plyometric training inhibits this GTO response which lets us maximise high velocity force production. However this topic is still in contention, so don’t quote me on this.
Changes that occur in the muscles and tendons. These adaptations are likely to take a couple of months to manifest.
Increase in muscle stiffness - Landings place high forces on muscles and tendons. A force applied to a compliant muscle will cause it to change in length a lot more than if it were stiff. A muscle that lengthens more also has to shorten more, all of which takes valuable time. In tennis we know that even being a hundredth of a second too late can cost us. An increase in muscle stiffness can therefore make us more efficient movers.
Improved elastic ability - Just like muscles, tendons are also capable of increasing and decreasing in length. When tendons are lengthened (in the landing phase) they store elastic energy. Consequently when tendons shorten (in the takeoff phase) they release this stored energy which has an additive effect on force production.
From this we gather that plyometric training is responsible for a host of adaptations that seek to improve strength in a very specific manner, i.e., the ability to produce large amounts of force, very rapidly, as a muscle is lengthening.
Can conventional strength training also improve strength in this manner?
To some of you, this may all seem a little DUH! Want to get better at movements that involve harsh landings and quick takeoffs? Then train with movements that do precisely just that. How is strength gained from lifting heavy weights any different from strength gained from plyometrics? Let’s compare a heavy quarter squat to a depth jump. Using the three rules outlined in the beginning of the article, we can highlight similarities and differences between the two.
Rule 1 is concerned with the length at which a muscle produces force. Most tension is achieved at moderate lengths. Both the depth jump and quarter squat look very similar in terms of the way our body segments are positioned at the bottom. Both involve bending and extending at the ankles, knees and hips to a fairly similar degree. Therefore, the muscles of these joints must also lengthen and shorten to similar amounts and produce similar forces, right? Not exactly.
Given that the depth jump happens at a fraction of a second while the heavy quarter squat can take a couple of seconds to grind out, this can affect the behaviour of muscles and tendons differently such that they produce very different adaptations. Muscles and tendons are in a game of Tug of war with one another. The one that is stiffer (resists deforming) is the one that wins. When performing heavy quarter squats, tendons gain in stiffness while muscles gain in compliance. This means for a given change in tendon length, muscles length changes to a greater extent.
Contrast this to a depth jump in which muscles gain in stiffness while tendons gain in compliance. This means for a given change is tendon length, muscle length changes to a lesser extent. What this tells us is that even though these two movements look similar, one relies more on the tendon to produce tension, while the other relies more on the muscle. The depth jump places the muscle at a more favourable length to produce tension.
Rule 2 deals with the way muscles behave when taken to long lengths. We just read that a muscle lengthens more in a squat than in a depth jump. When taken to long lengths, muscles resist being stretched which contribute to its ability to produce force. Since depth jumps don’t lengthen muscles much, they don’t benefit much from this mechanism but have to rely more on elastic energy from the tendon. Again, this tells us that the way in which muscles and tendons interact with one another is the way in which we preferentially get stronger.
Rule 3 talks about the speed of contraction. Visually this is the most obvious difference between the two. We then observe invisible changes that occur in the nervous system when training with fast speeds, as outlined in the previous section. Just know when training with heavy weights, we get better suited to producing force at slow speeds and when moving light weights quickly, we get better suited to producing force at fast speeds.
Finally, another way to demarcate the two moves is the challenge of coordination. Coordination is a skill that is very sensitive to load and speed. Our brains learn to coordinate a heavy back squat very differently than it learns to coordinate a depth jump. Coordination influences strength to a vast extent. Improvements in high speed coordination don’t transfer well to slow movements and vice versa.
Therefore, the answer to “can conventional strength training also improve strength in this manner?” is more nuanced than a yes or no. They are similar in some ways while being dissimilar in others. It is in these dissimilarities that lie their suitability. Up to a point (especially for beginners) you can expect a good deal of transfer from heavy strength training to plyometric training. Since tennis players have to express strength in various ways and possess the necessary adaptations to do so, heavy strength training should be used as an adjunct to plyometric training.
Conclusion:
There is more than one way to skin a cat but also one that is most efficient. You can use a butter knife to cut steak but there are far easier ways to get the job done. Strength training is much the same. Pick the type of training that best serves your purpose. If your purpose is to make great strides on court, look to incorporate plyometrics.