Even when you’re sitting perfectly still, when meditating for example, your muscles are still contracting a bit to stabilize joints and bones. And this force that the muscles apply at rest is called muscle tone.
On the other hand, when you pick up a 10 pound sack of potatoes, the force generated by the muscle contraction is much higher than the normal muscle tone in your biceps.
The pulling force transmitted through the muscle fiber is called the muscle tension.
Now let’s dive into some basics of muscle physiology, starting with a single muscle cell or muscle fiber. Within the muscle fiber is the sarcoplasm, which is the cytoplasm of a muscle fiber.
The sarcoplasm is filled with stacks of long filaments called myofibrils.
And each myofibril consists of contractile proteins called thin actin and thick myosin filaments.
These filaments don’t extend through the entire length of the muscle fiber - instead they’re arranged into shorter segments called sarcomeres. Alright, now let’s zoom into a sarcomere.
At the center of the sarcomere is the M line made of myomesin proteins, where the thick filaments attach.
At the borders of the sarcomere are the two Z-discs made of alpha actin proteins, where the thin filaments attach.
For every thick filament, there are two thin filaments-one above and one below and the two types of filaments overlap.
The region with only thin filaments is called the I band and it appears light.
The region with thick filaments is called an A band and it appears dark.
Now, most of the A band has overlap between the thick and thin filaments, but there’s an area towards the center called the H zone where there are only thick filament, so it appears slightly lighter.
When the muscle contracts, the thick filaments pull the thin filaments above and below it towards the M line.
The Z discs attached to the thin filament also gets pulled towards the M line, and the whole sarcomere gets shorter.
Now, the A band does not change since it’s the length of the thick filament.
But the H band and I band shortens because as the overlap increases, the region that consists of only thick or thin filament decreases.
At maximal contraction, there’s an almost complete overlap of the thick and thin filament and the H band and I band are almost completely gone.
Thick myosin filament is made up of hundreds of myosin proteins, each with a tail and two small club-like extensions, which are called myosin heads.
On the other hand, the thin filament actually looks like a pearl necklace that’s gently twisted.
Each pearl represents one G-actin protein, which has an active site where the myosin head binds to during contraction.
Now, before myosin can bind actin, it first needs to power up. Part of the myosin head is an ATPase, meaning that it can cleave an ATP molecule to ADP and phosphate ion and release some energy. The energy is used to cock the myosin head backwards, into its high energy position.
Next, the myosin head binds to the active site, and this is called cross-bridge formation.
Cross-bridge formation is the trigger to release the stored energy in the myosin head, kind of like firing the catapult. When that happens, the myosin head launches pulling the thin filament along with it. This is called the power stroke.
The combined power strokes of all the myosin heads lead to sliding of the thin filament along the thick filament, and this results in the contraction of the skeletal muscle.