Muscles are made up of muscle fibers, which are long, thin cells that can contract and relax in response to electrical impulses from the nervous system. When a muscle contracts, it pulls on a tendon, which is a tough, fibrous tissue that attaches the muscle to a bone. This creates movement at a joint, allowing us to perform actions such as lifting, pushing, and pulling.
Fig. 1
The sliding fliament theory
At a microscopic level, muscles are composed of long fibers called myofibrils. Each myofibril is made up of repeating units called sarcomeres, which contain the contractile proteins myosin and actin. (see fig. 2)
When a muscle receives a signal to contract, calcium ions are released into the muscle cell, which allows the myosin and actin filaments to interact. The myosin filaments have "heads" that reach out and grab onto the actin filaments, pulling them closer together.
As the myosin and actin filaments slide past each other, the sarcomeres get shorter, causing the myofibril to shorten as well. This shortening of the myofibrils is what causes the muscle to contract and produce movement.
Fig. 2
This "way of explaining muscle contraction" are called the sliding filament theory. It is a well-established scientific theory, but there are still ongoing studies to further understand the nuances of muscle contraction!
The muscles is strongest when most myosin and actin is connected. This explains why muscles produce the highest amount of force in it's mid-range and gets weaker the more it is stretched as the actin and myosin filaments glides past each other resulting in less connections and therefore inferior strength.
The importance of strength training and how muscles and tendons adapt.
The amount of force a muscle can generate depends on the number of muscle fibers that are activated and the frequency and intensity of the electrical signals from the motor neurons.
Strength training is crucial for building muscle mass (hypertrophy) and improving muscle fiber recruitment by activating more fibers, and strengthening the nervous system's signals to muscles.
Besides that you get stronger, as a climber, it is important to know the difference between how muscles and tendons adapt to strength training - or to be more specific, how they adapt to "load/force" and "time under tension."
Tendons are tough, fibrous tissues that are designed to transmit force from the muscle to the bone. They are not as elastic as muscle tissue and do not have the same ability to adapt and grow in response to training, but often respond well to eccentric training (1) with its increased "time under tension". That being said tendons have very limited blood supply, which means that it has poor healing ability and need more rest time than muscles (72 hours)(2).
Muscles, on the other hand, are highly adaptable to both force and time under tension. When you lift weights or perform other resistance exercises, you create micro-tears in the muscle fibers. The body responds to this stress by repairing and rebuilding the muscle tissue, which leads to hypertrophy or an increase in muscle mass. Luckily there is good blood flow to muscle tissue, which also explains its good healing capabilities.
Both muscles and tendons respond to strength training, but whereas muscles adapt within 2 months, tendons seem to need longer consistent loading to respond optimally to training. (3)
The difference in adaptation to training results in an unbalanced strength progression in muscles and tendons compared. This means that you'll be able to make fast and visible growth of muscle fibers resulting in overall improved strength and hypertrophy. This is also what in many cases leads to overuse injuries as you exceed the stress your tendons can handle. Simply because they have slower adaptation and need longer rest periods between training.
Want to know more about how muscles and tendons respond to training check out this article.
Ackermann PW, Renström P. Tendinopathy in sport. Sports Health. 2012;4(3):193-201. doi:10.1177/1941738112440957
Magnusson SP, Langberg H, Kjaer M. The pathogenesis of tendinopathy: balancing the response to loading. Nat Rev Rheumatol. 2010;6(5):262-268
Bohm S, Mersmann F, Arampatzis A. Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults. Sports Med Open. 2015 Dec;1(1):7. doi: 10.1186/s40798-015-0009-9. Epub 2015 Mar 27. PMID: 27747846; PMCID: PMC4532714. this