Muscles

Skeletal muscles

The skeletal muscles are the muscles that make the body move. These muscles are attached to the bones of the body. The muscles work by contracting and thereby “pulling” the bones so that the body moves. We consciously control the skeletal muscles.

Smooth muscles

Smooth muscles are muscles that we do not control with our consciousness. Smooth muscles are found around the stomach and intestines, where they cause food to move through the intestinal system.

Heart muscle

The heart muscle is a special muscle. It has the same structure as the skeletal muscles, but we don’t control it with our consciousness. Therefore, the heart muscle is neither categorized as a skeletal muscle nor as a smooth muscle.

 

Figure 14. Muscle structure

Muscle structure

The muscles are made up of special muscle cells called muscle fibers. Muscle fibers are long and extend all the way through the muscle. Muscles are made up of many thousands of muscle fibers, which are packed in bundles (see Figure 14).

When you train to get bigger muscles, you don’t increase the number of muscle fibers, instead the individual muscle fibers become thicker.

There are different types of muscle fibers; slow and fast muscle fibers.

The slow, but enduring muscle fibers are called red muscle fibers, and the fast muscle fibers are also called the white muscle fibers. This is because you can see a color difference in a microscope.

How do muscles contract?

Inside the muscle fibers are two important proteins, actin and myosin. Below is an illustration of how actin and myosin sit together and form chains across the muscle fibers.

 

Figur 5.2 - Muskelsammentrækning: Sammentrækning af muskler sker ved at aktin og myosin pakker sig tættere og derved trækker musklen sammen. Øverst illustreres aktin og myosin i en afslappet muskel. Nederst illustreres aktin og myosin i en sammentrukket muskel. Det ses også, at reaktionen, hvormed aktinen og myosinen trækker sig sammen, kræver ATP.

Figure 15. Contraction of muscles occurs when actin and myosin pack tighter, thereby contracting the muscle. At the top, actin and myosin are illustrated in a relaxed muscle. At the bottom, actin and myosin are illustrated in a contracted muscle. It is also seen that the reaction by which the actin and myosin contract together requires ATP.

When the muscle contracts, the myosin pulls the actin proteins closer together. This process requires energy. The proteins get the necessary energy from ATP.

Figure 15 shows how actin and myosin are packed more tightly by using energy from ATP. When actin and myosin are slid in between each other, the muscle shortens.

Strength

You can divide strength training into three different phases that result in different physiological changes. The phases are divided according to how many times you can repeat an exercise at most. The maximum number of repetitions for a specific exercise is referred to as the number of RM (repetition maximum).

Phase 1 – between 12 and 20 RM

Training in phase 1 is equivalent to performing an exercise that you can repeat a maximum of 12-20 times. Training in phase 1 acclimatizes muscles and tendons to load and trains motor skills in relation to performing the exercise.

Phase 2 – between 10 and 15 RM

If the maximum number of repetitions is between 10 and 15 that you can perform a specific exercise, then it is a question of training in phase 2. Phase 2 training strengthens muscles and tendons, builds muscle structure and stimulates the nervous system.

Phase 3 – between 1 and 6 RM

When training in phase 3, the maximum number of repetitions is 1-6. This type of strength training causes hypertrophy (increased muscle volume) and explosiveness. Explosiveness means that the muscles are good at doing a lot of hard work in a very short time.