Muscle Contraction Under Opposing Forces

Doesn't the Z line in the sarcomere have 2 contacting forces acting opposite from adjacent sarcomeres? It may look like a biology question, but it's more a physics question: If we consider one sarcomere, the actin filament pull them towards them and the sarcomere contracts. fine. But if we consider another sarcomere adjacent, the actin in that adjacent sarcomere would want to pull it towards itself.... so isn't it that the common Z-line has 2 forces acting opposite to each other making the net zero?? if so what makes the muscle contract then?
- Stefano (age 27)
Lecce, Italy

That is an interesting biophysics question. Let us focus on a striated mucle bundle, such as biceps. The origin of the force is myosin motor proteins, that are between each Z lines and pull them towards the middle line (H). The force is due to the simultaneous pulling of many myosins, but since they do not work in phase, the force may be considered quasi-constant. So yes, each Z line would be under roughly zero net force, so nothing should move anywhere - under static equilibrium. This is what happens when your dog pulls the leash to no avail. But contraction is not static.

You have about 10⁵ of them connected in series, but the ends are eventually pulling on your bones via tendons. So the ends are free to move, and under the tension of the muscle, they will come together shortening the total gap between them (and the length of the muscle fiber). This system is analogous to a stretched rubber band. While you are holding the ends, the tensile force along each point along the band is the same and equal to what you apply, so nothing moves. But if you release one (tail muscle) or both ends (biceps), it will shrink. There will be a quite chaotic oscillatory motion starting from the endpoints, but the friction will dissipate all energy resulting in a static equilibrium rapidly. Muscle contraction deviates from this model in that force will gradually build up and the system will be close to equilibrium throughout the process.

Tunc

(published on 04/15/2015)