Final answer:
Isometric contractions increase muscle tension while the muscle length remains the same due to the internal shortening of sarcomeres. This is a result of the sliding filament mechanism that does not lead to movement because the muscle cannot overcome the external resistance.
Step-by-step explanation:
In isometric contractions, there is an increase in muscle tension, but the muscle length remains constant. The change in sarcomere length in question refers to the shortening of the sarcomere within the muscle during contraction, even though the overall muscle length does not change.
This occurs due to the sliding filament mechanism, where the myosin heads in the thick filaments bind to the actin in the thin filaments and pull the actin filaments towards the center of the sarcomere, resulting in the zone of overlap increasing.
The H zone and I band shorten while the A band's length stays constant, and the Z discs move closer together, which creates muscle tension without changing the muscle's overall length because the load is too great for the muscle to move.
The length-tension relationship explains that the degree of sarcomere shortening affects the force generated; however, in isometric contractions, the force generated is not sufficient to produce movement against the resistance.
Therefore, the result is an increase in muscle tension with no movement of the load, as in maintaining posture or trying to lift an immovable object.