Energetics of the muscle contraction.
For every muscle contraction a certain amount of energy needs to be spent. This energy is provided due to the metabolic processes in the muscle cell.  About 20 % of the muscle cell volume can be taken up by mitochondria, which are the main organelles, related to energy supply from the oxidizing processes in the cell. Production of energy from the oxidizing processes demands the presence of oxygen so these processes are aerobic. In the lack of oxygen the muscle cell can supply energy in an anaerobic way but then the process is less effective. The main substances that are oxidized to supply energy in the muscle cell are glucose and butyric acids. The muscle cell contains certain reserves of glucose in the form of glycogen, which is stored as small granules in the cell. Under anaerobic processes glucose is not fully decomposed and lactic acid is produced instead of water and carbon dioxide.
Muscle contraction is performed through slipping of the actin towards the myosin threads and the result is shortening of the muscle cell.
When the muscle is relaxed the actin and myosin threads partially overlap. When the muscle is being contracted, the actin threads slip in between the myosin threads. The energy for that slipping is provided by the decomposition of adenosine three phosphate (ATP), derived from the metabolism of the muscle cell.
This process is regulated mainly by the concentration of the calcium ions in the cell.
With a relaxed muscle the concentration of the calcium ions is low, and the ions themselves are concentrated in special tanks in the endoplasmic network.  When the muscle cell is stimulated, the calcium ions leave the tanks and their concentration in the cytoplasm increases. As a result the actin and myosin threads get in touch and slip in between one another. The muscle is contracted. The return of the calcium ions in their tanks puts an end to the contraction.
Under normal conditions the contractions of the skeletal muscles are not spontaneous, but start as a result of a certain stimulation. This is the main difference between them and the smooth muscles and the cardiac muscle, where the contraction begins without stimulation from the outside and is even repeated periodically at a certain frequency, as is the case with the heart. Normally, the stimulating agent for the contraction of the skeletal muscles is the nerve impulse which starts from the central nervous system and reaches the muscle via the respective motor nerve. The spot where the impulse is passed from the nerve to the muscle cell is called a nerve-muscle synapse. As one and the same neuron is responsible for several muscle cells simultaneously, normally these cells are contracted simultaneously and in one and the same way. It is so because at its place of branching the nerve sends identical impulses to all of its terminations. Thus a neuron and all the muscle cells innervated by it form a functional community known as a motor unit. The number of cells in a motor unit determines the maximum strength it can develop, and the total number of units that are active at a given moment determine the force of the muscle contraction.
Types of muscle contractions
While contracting, muscles develop certain strength which, if it remains constant, is called isotonic contraction. An example of such is the contraction of the arm muscles when lifting an object. In some cases the weight can prove bigger and the contraction will not be enough to lift it. Then the muscle is contracted without changing its length and does not do any mechanical work. Such contractions are called isometric. With them the muscle reaches its maximum strength.
A muscle can contract as a result of one or a series of nerve impulses. In the first case we have a single muscle contraction. In the second, if the impulses come very shortly one after another, the successive contractions can overlap and fuse into one joint contraction – a titanic muscle contraction. Titanic contractions develop greater strength and most of the muscle contractions in the organism are of this kind.
Types of muscle fibers
Depending on a number of characteristics, the fibers that build up the skeletal muscles can be divided in general into fast twitch and slow twitch ones. In man most muscles contain fibers of the two kinds, with some of them having more fast twitch ones and others built up mainly of slow twitch ones. The fast twitch fibers contract quicker, develop comparatively great strength during contraction and get tired sooner. The slow twitch fibers can maintain a certain effort for long without showing any sign of getting tired. Slow twitch fibers are equipped with many blood vessels and contain more myoglobin, which makes them red. Myoglobin is a protein compound similar to hemoglobin in the red blood cells. Unlike hemoglobin it demonstrates greater oxygen affinity and needs lower concentrations of it. The “red muscles” take part in doing slow but prolonged contractions, as is the case with the muscles supporting posture. Fast twitch fibers are less blood supplied and contain less myoglobin. They build up the “white muscles”, which take part in performing quick movements of short duration.
Muscle tone
Passive movement of the different body parts meets certain resistance. Part of it is due to the mechanic properties of the muscles and joints which perform the movement. However, even at rest the body muscles are not fully relaxed but are slightly contracted.  This condition is called muscle tone. It is determined by how active the nervous system is and the impulses constantly reaching the muscles via the respective motor nerves. The skeletal muscle tone is very important as it allows us to maintain the normal posture of the body and the relation between its parts. It provides the opportunity to perform the movements faster and more precisely as well.