Part 3: So much power comes from tendons.
Tendons have the tensile strength ranging from 50 to 150 MPa, bones by comparison have a tensile strength of 150MPa1. The achilles tendon can bear load stress of 8 times the body weight while running2.
Functionally, the tendons must be pliable and elastic to elongate and let the joints move without restriction. But they must also be able to lock in rigidity at the joints when the body is delivering force into the ground. That is, for example, while sprinting the tendons and joints must be rigid in lock stiffness, absorbing and returning the kinetic energy that is being produced from the momentum of the body and the muscles creating the movement. While running the tendons are able to return 40% of the energy from the body, into the ground and back into the body3.
This varies depending on your running style. With the “eyeball test” we can visually see someone that is able to run with elasticity in their tendons, they look like they’re bouncing off the ground, as if they’re running on a trampoline. While another running style, looks like their legs are pumping, driving like pistons into the ground, they look like they’re running with all their “big” muscles to sprint – quads, hamstring, glutes, etc. The second method not requiring as fast and elastic tendons as the first.
How fast and elastic a tendon is depends on the tendon’s pliability, that is the ability for how quickly the tendon can go from released state to flexed stiffened state. In this capacity not all tendons are trained equally. Visualize the pliability action of a tendon on a bell curve with the peak of the curve being ground contact. Jogging, let’s say, is a wide bell curve and sprinting requires a narrow bell curve. The tendon’s reflexivity in jogging doesn’t require a narrow bell curve, because the ground contact is longer than sprinting and the leg has more time to stiffen and release as it cycles around. While sprinting require a narrow bell curve because the ground contact is so concentrated in a smaller fractional time and must go into flex and get back into the leg cycle almost instantly as it springs up from the ground.
Take another example of the importance of the tendon’s pliability, the ability to decelerate to a stop quickly depends on how fast and strong the tendons can hold that joint position, injuries will occur if the tendon is too weak and involuntarily gives way. Just because you can squat a heavy load doesn’t necessarily translate to being able to stop on a “dime” when sprinting at your max speed.
The pliability of the tendon is dependent on its length, strength and mobility. But also muscle control mechanics – the ability to completely control the release and contraction from the central nervous system. That is why the pliability requirement of jogging is not crossover compatible to what the equivalency would be as a sprinter. The length and mobility requirement is different, a jogger’s legs are cycling in a mid-range motion, and even short-range, never pushing toward the end-range positions. The strength component is different, the speed in jogging does not reach the force and velocity of sprinting, which produces greater loads. And the muscle control mechanics are different, jogging is training the tendons to reflex in the wider bell curve, going from wide to narrow bell curve reflex requires re-trained neural pathways, new muscle control abilities, and increased strength and length requirements.
References
1 https://link.springer.com/chapter/10.1007%2F978-1-4757-2968-9_8
2 https://en.wikipedia.org/wiki/Achilles_tendon
3 https://www.ncbi.nlm.nih.gov/books/NBK513237/