Our body muscle tissue can be classified as skeletal, cardiac, and visceral.
– Skeletal muscles in all instances are attached to osseous tissues (bones).
– Cardiac muscles form the muscular body of the heart.
– Visceral muscles are present in all hollow viscera such as gastro-intestinal tract, blood vessels, ducts of glands, respiratory, urogenital and lymphatic systems of the body.
Certain changes occur in the functions of various organs when exercise is repeated over a period of time. The magnitude of change depends on many factors, the most important being the intensity and frequency of exercise. Age and heredity also play a role. The nature of the change depends on the type of exercise, the muscles used and the previous training of the individual.
Changes that are produced by training disappear after some time if the person stops training. The primary effects of training occur in skeletal muscle. There is an increased number of capillaries in muscle tissue, leading to increased blood flow, and therefore more oxygen is brought to the muscle cells.
There is also an increase in asteriovenus oxygen difference, which means more extraction of oxygen by muscle cells and lower lactate concentration (lactate is a product from anaerobic oxidation of glycogen of our body during insufficient supply of oxygen by blood to the muscle cells which leads to the phenomenon called muscle cramp) in muscle and blood at a given work load. This indicates that the muscles depend more on aerobic mechanism – a mechanism that uses up oxygen to oxidize glycogen to carbon dioxide and water and yield energy.
Myoglobin content: Myoglobin content stores oxygen in a manner similar to that of haemoglobin inside RBC. Training increases the Myoglobin content of skeletal muscle.
Energy Stores: There is up to a 100 per cent increase in the glycogen storage fuel of our body. A high carbohydrate diet enhances the storage glycogen in muscle. The amount of glycogen is an important factor in endurance sports eg: long distance running. It is also found that the activity of the enzyme systems required for oxidative metabolisms are also increased. This results in about a 45 per cent increase in the rate of oxidation.
Mitochondria: Size and number of mitochondria in skeletal muscle cells increases. Also there is an increase in the concentration of enzymes needed for utilization of fuel substances to obtain energy.
Therefore, there is an increased capacity of the muscle cells to oxidize carbohydrate and lipids. Because of this greater oxidative capacity of the cell, more fat can be used. This is why we usually have a lower respiratory quotient (RQ) with training. RQ is a method employed to measure metabolic rate of our body. It has been found that the rate of energy metabolism is directly related to oxygen consumption. The uptake of oxygen by the subject is used in measuring the metabolic rate. The production of CO2 varies with the fuel being utilized. Thus, when glucose is the only source of energy being used at a certain time RQ is equal to 1 Co2/O2=1). This ratio (RQ) is known as the respiratory quotient.
C6H12O6 (Glucose) + 6 O2 = 6 Co2+ 6 H2O + energy.
RQ = 6/6 = 1.00.
Muscle strength and muscle mass:
Both the muscle mass and the force produced by the muscle increase as a result of training. The increase in muscle mass particularly occurs after heavy overload training (eg: weight-lifting). There is an increase in the amount of contractile proteins (actin and myosin).
The number of muscle fibres is not affected but the individual fibres increase in size. More protein synthesis and less protein breakdown occurs, Amino acid (building block of protein molecules) uptake by muscle cells is enhanced, probably due to increased permeability of muscle cells.
The central nervous system seems to adapt by increased excitation and decreased inhibition of alpha motor neurons. As a result of this, a nerve impulse causes more actin-myosin cross-bridges after training (actin-myosin cross-bridges occur with the expense of energy during muscle contraction phenomenon). Muscle connective tissue, tendons and ligaments become stronger with training.