Although strong muscles are important for health and athletic performance -the key to high level athletic performance is power which is the ability to produce a lot of force quickly. This takes more than regular strength – the secret is being able to store and release elastic energy to create more explosive movement. By understanding how power is produced, we can keep people walking, running and jumping into older age and how to walk again after injury or illness.
Storing and releasing energy is known as the stretch-shortening cycle or muscle-tendon elasticity. These cycles are the foundation of ALL human movement, and this is not just about muscles – tendons are key! Tendons attached muscle to bone are very strong.
In fact, in the most powerful, explosive athletes – their tendon strength is much higher than the average person and this is key for power production as we will see.
Muscle Strength and Contraction Speed
Although muscles can be very powerful (calf muscles can produce upwards of 1,000lbs of force) they have one big limitation. Muscles can only shorten/contract so fast – even in the fastest athletes. In fact, at high speeds muscles can produce very little force at all while shortening. This is important because muscle shortening, known as a concentric muscle contraction, is how force is applied to external objects including the ground!
To make it even more confusing - when athletes move at the fast speeds their muscles cannot produce force fast enough to help while shortening – so how do they move so fast?
Tendon Stiffness
Humans, like most other animals on Earth, make use of something called the stretch-shortening cycle aka muscle-tendon elasticity which allows us to store and release energy rapidly when needed.
The stretch-shortening cycle relies on our relatively long, stretchy tendons attached to a strong muscle and bones. Tendons are what attaches muscle to bones. When the muscle produces force it stretches the tendon, storing elastic energy. The subsequent recoil of the tendon generates way more power than our muscles are capable of producing. Our tendons act as power magnifiers!
To take advantage of this ability people intuitively learn that they need to move in the opposite direct of desired movement (called “counter-movement”). For example: if you ask anyone to jump as high as they can you will see them squat down and then rapidly change directly to jump up! This results in energy being stored in the tendons as the muscles lengthen which is then released as the muscles stop lengthening and shorten rapidly as the person jumps up!
Stiffer Tendons and tendons that are stretched further can store more energy and generate more power during recoil. Because most of the power used in running and walking comes from the ankle joint – it should be no surprise that the Achilles Tendon of Runners are stiffer than the average person!
Eccentric and Isometric Muscle Contractions
The key to fast, explosive movement is the stiffness of tendons combined with the proper use and timing of isometric and eccentric muscle contractions. Optimizing this technique is one of the keys to optimal athletic performance.
Isometric contractions are used to prevent movement such as during the plank exercise where the core is working to prevent sagging of the low back. There is no muscle movement during isometric contractions.
Eccentric contractions are used to absorb force like landing from a jump and muscles are lengthening under tension.
We engage eccentric contractions during the counter-movement which occurs immediately before we produce a strong concentric (muscle shortening contraction). For example, in running when we land the ankle, knee and hip joints flex and the muscles around these joints lengthen under tension to absorb the impact of landing.
Then we couple this with isometric contraction at the bottom of the counter-movement to stop briefly. Then finally we rebound back up using the energy stored in the tendon to generate MOST of the force needed. Over 70% of the energy for efficient human movement is produced through this and process and NOT produced by concentric/muscle shortening contractions!
The faster we run the more energy we store and the more recoil energy is released from the tendons. This occurs even in walking and allows us to move more efficiently and at high speed!
How to Improve the Stretch-Shortening Cycle and Power Output
Part of our ability to optimize the stretch-shortening power cycle is determined by genetics, but it’s also something we can improve through training. Technique training helps, but also the use of heavy strength training and eccentric dominant training can both help to improve tendon strength and stiffness.
As we age, our tendon stiffness and power output decrease, and this means we lose movement efficiency and must use more energy for activities like walking because we get less recoil energy from our tendons.
People with less stiffness in their Achilles’ tendon (and the accompanying lower strength in the calf muscles) have slower walking speeds. Also walking speed is directly related to mortality and morbidity in the elderly so maintaining tendon stiffness is important as we age.
The greatest force for walking, running and jumping is produced at the ankle joint. This is an important target for athletes, but also for anyone who wants to maintain their walking, running, jumping capacity as they age.
How to Build and Maintain Stronger Tendons
They key to building tendon strength is focusing on the eccentric/muscle lengthening aspect of strength training. This can be challenging to do, and one of the best ways to build this strength is the use of the reACT Trainer – www.reacttrainer.comNothing does a better job of safely improving eccentric lower body strength and endurance and accompanying tendon strength and stiffness.
It is also good to specifically keep your ankle muscles conditioned by doing calf raises with resistance. The calf and ankle joint can produce enormous amounts of force so as the exercise gets easier add more resistance to continue to build muscle and tendon strength and stiffness.
Another great exercise to build tendon stiffness and power is repeated jumping at a relatively fast pace ideally in bare feet on a stable flat surface. Like your ankle your foot and foot muscles are specifically designed to allow you to store energy in the foot muscles when you land and the reuse that energy to project more energy as you push-off.
It is key that you land ball of the foot first and then allow your heels to touch the ground before you rebound. Start with very small jumps at a quick pace – done properly you will literally feel the stretch-shortening cycle and feel “springy”. Slowly build up to being able to do this for 5 full minutes – it is harder than it sounds and you will notice your feet and calves both get tired at first!
If you have ever watched children in bare feet you will notice that they frequently spontaneously do this type of jumping because it feels good and teaches them to how to store and release energy from the Achilles Tendon.
