Tendons are a specialiced and conective tissue that unite muscle and bones. This allows forces transmision so we can move. Furthermore, tendons are a very usefull tissue when it comes to energy storage. Releasing that energy later in the movement and potenciating muscle contraction.
Well structured tendons can handle a lot of strain in order to transmit the forces developed by neuromuscular system. This stiffness on tendon allow that little energy is losed in force transmision. Tendon pathologies can alter this function creating less stiffness and then less capacity to transmit forces.
We know that muscle-tendon unit (MTU) is different depending on the joint. For example, in knee joint, tendon length is low compared with muscle length (Patellar tendon), wichs means higher number of in serie sarcomeres and greater capacity to produce forces at high velocity. In ankle joint, tendon length is much greater compared with muscle length allowing Achilles to storage-release a greater amount of elastic energy that potenciate muscle contraction.
Storage-Release mechanism:
When a force is applied to a limb and have some pre-activation there’s not much muscle lenghten. Indeed, it can shorten while MTU gets lengthened. This muscle lengthen reduction could serve as a protective mechanism due to the muscle damage caused by eccentric contraction. Tendon act as a buffer mechanism.
Muscle can also be lengthened but in a different phase. While tendon gets lengthen in the fast phase of the movement, muscle gets lengthened in the final phase when MTU starts shortening. You will better understand with this image.
Then, tendon storaged energy pass to muscle active fibres where it can be dissipated or used to potenciate muscle contraction:
- Eccentric action: Leads to energy dissipation
- Isometric action: Leads to muscle potentiation
HERE YOU CAN FIND THE “WHY” OF TRAINING ALL THE CONTRACTION PHASES AND NOT JUST THE CONCENTRIC.
As a consecuence of highly forces ocurring we observe injuries in MTU (Tendon lengthen decrease neededs of muscle to generate high eccentric forces)
Energy storage ocurrs in a fast process. The slower the process the lower the energy storage because the lower the forces (high eccentric forces comes from greater velocity). Despite this energy storage is slower than energy release due to tendon recoil.
Elastic energy storage-release is essential in almost all sports. Indeed, it can decrease energetic cost of muscle contraction amplifying it. Tendon recoil give back almost a 100% of the energy sotraged (90-95%). The fact is, that work to lengthen tendon can’t be greater than the recoil work.
GREATER ENERGY ABSORTION PEAK TAKES PLACE IN TENDON AND NOT MUSCLE.
We have seen now that not every tendon acts the same, and structura properties may interferes in MTU behaviour and mechanical capabilities (Force and Velocity)
STORAGE-RELEASE CAPABILITY AND STIFFNESS:
Stiffness is not only the capability of not being deformed but also the capability to recoil faster. So, for a given tendon deformation, the tendon with the greater stiffness will recoil faster loosing less elastic energy.
Tendon length is necessari to energy storage. Thiner tendons gets more strain for a given force due to the lower cross section area, wich leads to a greater energy storage. Achilles > Patellar. The greater the stiffness for a given length the greater the work to be applied and the greater the returning work.
So there must be an optimal stiffness that allows a faster elongation (greater storage capabilities) and faster recoil (avoiding energy leaks)
Indeed, thiners tendons will not equally adapt than greater cross section area tendons through training due to its necessity to storage-release. Cristi-Sánchez (2019) found no Achilles stiffness increases in experimental group vs control group while it did increase in Patellar.
This Swain Müller (2004) figure shows us how the stress affects strain in Achilles (TC/CT) and Patellar (LP/PL)
In conclusion:
- Greater stiffness leads to greater force transmision capabilites
- Lower stiffness leads to a greater energy storage capabilities.
- Elastic energy gets stored in tendon by faster lengthens.
- Greater stiffness leads to less energy leaks (elastic energy leaks as heat when tendon recoil slower)
PRACTICAL APPLICATION:
We know that maximal force is the key in sports abilities and strength training. But lets take into acount all mentioned before loads and exercise selection.
If we need to work maximal force, the load wont allow the tendon to get lengthened (greater eccentric contraction and lower tendon length) and slower isometric-concentric phase so more elastic energy is leaked. So if we use high loads on sprints and jumps we will kill storage-release capabilities of tendons thus the optimal stiffness won’t be optimal. Greater loads will increase histeresis and will lead low quality of force transmision capabilites.
This does not mean we should not train with maximal or sub-maximal loads!
Indeed, training with this loads while lifting will lead to greater gains of strength.
But…. think about the impact that would have use of maximal or near maximal loads in highly storage-release demand exercises like sprints and jumps. Is it worth to highly load them and kill the energy-storage capabilities?
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