Question

In experimental situations, how does skeletal muscle length determine the force of contraction? Why, in life, is this muscle length versus active tension relationship not seen?

Answer 1

The muscle-skeletal length determines the contraction:

Skeletal muscle consists of multiple bundles called fascicles that are joined collectively and forms muscle fibres. These are further surrounded by connective tissue layers called fasciae. These muscle fibres are cylindrical in nature and also consists of several mitochondria in order to fulfil the requirement of energy needed. Muscle fibres (contractile units within muscle) that are composed of myofibrils are itself composed with actin and myosin units (filaments)which are arranged as cytoskeletal elements within the cytoplasm of muscle fibres, when these filaments are repeated it forms a distinctive banding pattern via myosin and actin filaments (also known as thick and thin filaments) , this entire repeating arrangement is called sarcomere (basic functional unit of muscle fibres). This sarcomere is solely responsible for generating machinery of muscle contraction by interacting the thick and thin filament. This is explained by sliding filament theory in 1954 by two research teams, one consisting of Andrew F. Huxley and the other one consisting of Rolf Niedergerke from the University of Cambridge.

According to sliding filament theory, muscle contraction occurs when the thin (actin) filaments slide over the thick filament (myosin) and form cross-bridge causing the shortening of myofibrils. Shortening of myofibrils results to produce contraction that generates force and tension and thereby movement. This also depicts an idea for length and tension relationship.

The active tension relationship cannot be actually usually seen on muscle length in real life:

Because of the presence of titin which is the abundant form of protein present in the striated muscles. The main job of this protein is to stabilize the filaments that keeps sarcomere to avoid overstretching and also to recoil it like spring after being stretched. So, by generating the innate elasticity it helps in stabilization of muscle length. Moreover, the presence of tendons (joins bones to muscles) also keeps a required constant level of stretch within the muscle that is called a resting length. So, the muscle basically is stretched rather than shorten due to the resting length within the body. It also increases the ability of the muscles to contract when undergone stimulation.