In sports, this overload is actually wanted. Stimulating the muscles to work harder than in daily life enables the athlete to improve his performances. Muscles are build up in several layers. From the inside out we see individual muscle fibres, covered by a layer of connective tissue. Individual fibres are bundled together in another layer of connective tissue, and the whole muscle is covered by yet another layer of connective tissue. Back to the individual fibres the contraction takes place at an even lower level. Within the muscle fibres, which are long shaped cells, are strings called myofibrils. These strings are made of two different protein filaments, namely the actin filaments and the myosin filaments. These protein filaments can either move towards each other or move away from each other. This moving of the actin and myosin filaments with regard to one another is what a muscle contraction actually is.
In order for a muscle to grow the muscle fibres either multiply in number, or grow in cross-sectional area, or grow in length. The first process (hyperplasia) is still unclear in its working and plays a less-prominent part in the growing of muscles. The last two processes are called hypertrophy and can be explained by looking at the position the protein filaments are in when the muscles are in rest. If the filaments are positioned further apart form each other in length, the fibre has grown in length. If they also have removed themselves from each other in other directions, the cross-sectional area of a fibre has grown. At this point the type of muscle fibre plays a crucial part in which kind of growth takes place and whether the growth is actually visible on the outside of the body. .
Muscle fibres are divided into two categories: slow-twitch and fast-twitch. In older studies these were divided as red and white muscle tissue (based on the visible appearance of the two), but the difference turned out to be more defined than scientist thought at first.
