Analysing the one-leg jump, part VII: The tendons

Over the course of this entire series we’ve analysed pretty much the entire kinetic chain (we haven’t talked about the core, and briefly touched the pelvic tilt), and we’ve really focused on the active, contractile elements of human kinetics. In other words, on the muscles. They generate tension, so obviously – we’re interested in what they do, how they act in between them, how they communicate, what joints they control and how a weakness in one can overdevelop or overload another and wreck havoc in the entire movement, disguising itself as something it is not.

That’s all good and dandy, but here’s the thing: there’s such a thing called a muscle-tendon unit. Why? Because, you guessed it – the tendon is part of this unit. It’s not a muscle that’s sitting there doing stuff – that muscle has to express its tension (actually moving the skeleton) through something – and that something is the tendon.

Think of it like this – the muscle generates tension, the tendon transmits tension (it connects muscle to bone) and the ligaments provide stability and support (they connect bone to bone).

So, the muscle needs great tendon “transmission” in order to actually provide you with movement. Now, you can have the best muscle strength in the world – if the tendons are “weak” and don’t transmit that strength to the bone/skeleton, you won’t move powerfully, and most importantly – you won’t jump powerfully.

Furthermore, if you look at the one leg jump and its ground contact time (GCT), you’re going to find some very, very low GCTs. For example, I consistently hit 0.16s (4 frames of video at 25 fps) in my amortization phase on my one leg jumps. That’s 0.16s to generate power in with your muscles. Doesn’t sound like a lot of time to do it, does it? On the two-leg jumping side, the amortization phase time increases (I’ve timed mine’s to as much as 0.4s on some very bad jumps, but usually good, reactive two-leg jumpers spend between 0.20s (great reactivity, less voluntary power) and 0.30s (less reactivity, more voluntary strength). In a standing vertical jump, the “amortization phase” (read – from the moment of the beginning of the eccentric dip to the moment of taking off) is the longest. Also, the knee bend tends to be the most of any jumps. Why? Because tendon contribution decreases and voluntary power contribution increases. In other words, you have to compensate for the lack of tendon contribution in the MTU by generating more power (muscular strength). A reactive jump is more tendon-muscle, while a power jump is more muscle-tendon.

Considering this information, I think we can agree how much having great tendons matters. But is tendon “greatness” something genetic or something that can be trained for? Well, like anything else, is each of both. To understand this, we must talk about the qualities of the tendons, or their attributes:

  1. Tendon length
  2. Tendon stiffness
  3. Tendon hysteresis

Now, first off, we have to understand that the tendons are really behaving like elastic bands. You stretch them and they accumulate energy more or less (depending on their properties). How do you stretch them? By performing movements and loading them eccentrically (lengthening them, or stretching them). Once you do that, they accumulate an energy potential. That energy potential can and will be expressed in two ways – some will be recoiled in the shortening phase, adding to the power that the muscle is generating, and some will be lost as heat.

1. In terms of tendon length, it is generally considered that a longer tendon has a better potential to recoil with more power. However, this really depends. In a standing bilateral vertical jump, a longer tendon is better because that length really helps in an amortization phase that is long – you can stretch it a lot, with a lot of knee bend, and then use that accumulated tendon recoil, plus your muscular power (which becomes more important in the standing vertical jump) to get a ton of power in the jump.

Conversely, while it was long believed that the same thing should be happening in the one-leg jump, in reality – a shorter tendon (by the way, we’re talking about the calf tendon, or the Achilles tendon) actually is the best. Why? Because a shorter tendon doesn’t need all that wind-up (knee bend or ankle dorsiflexion) to be stretched, and therefore helps you be both more hip dominant and at the same time takes less time in the amortization phase to be stretched, so a lower GCT can be attained with a more powerful recoil over a shorter ROM (range of motion). So with a short Achilles you can do a quick snap of the ankle in a very powerful way, and very quick. Some of the best high jumpers (think Stefan Holm) have a short Achilles tendon. How do you determine if you have a short or long Achilles? By looking at your calves. If they are a “ball” up towards the knee, you have a long Achilles. If they are longer and tend to go all the way down to your ankles, then you have a short Achilles.

2. As far as tendon stiffness is concerned, depending on the tendon we’re talking about, you want it to be either stiff or compliant. Now we have to understand that stiffness has nothing to do with flexibility or range of motion. What stiffness is referring to is how much force is needed to change the length of the tendon. The more force is needed, the stiffer the tendon. So if you need 100 Newtons to lengthen a tendon 10mm, and 500 Newtons to lengthen another tendon 10mm, then the 2nd tendon is 5 times stiffer than the first tendon.

Generally, you want most of your tendons to be stiff. When I say most of them, for a one leg jump I’m talking about the Achilles and the quadriceps tendons. On the other side of the spectrum you have the hip flexor tendon which you want to be compliant.

So how did we establish all this? Well, let’s start with the compliant tendons.

A compliant tendon gets stretched easily over a large range of motion and returns little power (because it’s compliant and gets easily stretched, so it doesn’t really accumulate too much recoil power). Therefore, we want compliant tendons for these large ROM movements under a light load (specifically because it doesn’t contribute with a lot of recoil, so it wouldn’t be useful when a lot of weight has to be moved).

The hip flexor tendon fits the bill here, because it is stretched during hip extension and then the knee comes in and up for another stride during sprinting. So the hip flexors and their tendons really have to lift the weight of the leg over a long ROM, and that weight is not that much to lift. Also, in a sprint this is a repetitive movement, and we want the tendon to be compliant so that we can repeat this movement a lot and be efficient at it (considering you take a lot of steps in a sprint). That’s why we want compliant hip flexor tendons. Compliant tendons are usually long and thin.

A stiff tendon is harder to stretch, but returns a lot of power. So it takes more force to stretch it, but once stretched, the power that you get back from it is much more than for a compliant tendon. In this situation, we want stiff tendons for small ROM movements under a heavy load. Since the ROM is small, that means the movement at that point will be quick and therefore the muscular contribution will be limited. How do we compensate? By having the tendons doing the job, and to compensate for the lack of muscle contributions, we want these tendons to be very stiff. What are the tendons in question here? The Achilles and the quad tendons. We want these to be stiff. The ROM in a one-leg jump at these joints will be small, the muscular contribution will be less (as voluntary contributions, the muscles still have to be strong to lock up isometrically very fast, prevent collapse and allow the tendons to deform and recoil) and the tendon contributions have to increase. Also, the load on these joints is very heavy, as in a high speed one-leg jump plant forces can get to even as high as 10G, so if you weigh 70 kg the forces at these joints reach 700 kg. This is why you need stiff Achilles and quad tendons. Stiffer tendons are usually short and thick.

To recap – you want a compliant hip flexor tendon for efficiency and repetition under a light load and a large ROM, and stiff Achilles and quad tendons for a powerful recoil under a heavy load and a short ROM.

So how exactly does tendon stiffness respond to training? Well, heavy weight training will stiffen the tendons. It is well known in the one-leg jumping world that the one-leg jump responds to partial movements. Think step-ups and 1/4 squats. Why? Well, I think a big reason for that is because they actually increase tendon stiffness over a short ROM. Doing all these heavy 1/4 squats and stepups will affect the quadriceps tendon and stiffen it, allowing it to return more power in a short ROM, instead of making you bend a lot at the knee to generate that power that is missing from a stiffer tendon by muscular action (by actually using the quads).

And if you think about it, it must mean that 1/4 squats and step-ups will work, yet again, to prevent the knee collapse. Not only do they promote a shorter knee travel and stiffen the quad tendon, but they also lessen the quadriceps voluntary contribution by providing the needed power through the recoil of the quad tendon instead. So they actually make you less quad dominant. Think full squats for muscle building and heavy 1/4 squats and stepups for specificity. Large volumes of plyo work also increases stiffness.

Flexibility work with light loads, on the other hand, increases compliance.

3. In terms of tendon hysteresis, things become even more interesting. Now, the research in this domain has been pretty limited. There isn’t enough available information on the subject (at least to me), but nonetheless, here we go:

What is hysteresis? It is the property of a tendon to release elastic energy. More precisely, it is the amount of energy loss between the stretch and the recoil that is independent of stiffness or compliance. So obviously, this must be very important. We want a lot of recoil and as little energy loss as possible for high performance. How is the energy lost? The energy is lost as heat.

Low hysteresis = a low amount of energy loss (what we want) => more power with less muscular work. This even translates into actual efficiency. If you have a stiff tendon with a low hysteresis, then it must mean that the muscle itself has less work to do, and can focus on isometrically contracting when needed (and then relaxing), and this in turn means that you can do more repetitions of that particular movement since the muscle will stay fresh. That’s really what movement efficiency is all about. THIS is where elite level athletes have the upperhand over regular people in athletic events. Not only do they have the strength, but they also have the right tendon properties to display that strength efficiently and to be reactive at the same time. In the elite athlete, the MTU (muscle tendon unit) functions the way it’s supposed to – the muscles do their role isometrically and the stiff tendons recoil with great power due to their low hysteresis. Sounds great, right?

A low hysteresis is important for BOTH stiff and compliant tendons. We don’t really want our elastic energy to be lost as useless heat, do we?

So how exactly does hysteresis work? Well, the tendons have a dual nature – they are both viscous and elastic. We want to become more elastic. How do we do that? By warming up. Increasing the temperature lessens viscosity and improves the elasticity, and LOWERS hysteresis. In other words, it lowers the loss of accumulated elastic energy as heat. And if that loss is lowered, then that energy will be displayed as actual recoil gain (in other words, more powerful tendon contributions in the jumps, sprints etc).

Considering this, the more tendon dominant you are, and the weaker muscularly you are, the more important the warm-up becomes. Re-read it. Considering this, the more tendon dominant you are, and the weaker muscularly you are, the more important the warm-up becomes.

This is why the older you get, and the less stronger you become, the more and the better warm-up you need.

Proof?

Remember: dynamic flexibility + plyo work reduces tendon hysteresis. Dynamic + static stretching reduces both stiffness and hysteresis (another reason why stretching is important). A good idea to warm-up well and do those leg swings before jumping, right? (lowering the hysteresis and increasing the hip flexor tendon compliance). This is also why a dynamic stretching routine and a low level plyo workout can be beneficial, by lowering the hysteresis. Over time, you will adapt your tendons, considering you strength train with heavy weights and do the right plyos and dynamic warm-ups, to have low hysteresis and stiff AND compliant tendons where you need them. Combine this with strength and you can’t help but be athletic!

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