Human body has the most complicated structure and intelligence as well. Nature has put brilliantly resistive and endurable aspects in human body. These mammals can survive at certain harsh conditions with really marvelous adaptions and capabilities.
There are different modes where our body transits its energy reserves to maintain smooth exercise.
Our body uses different processes for supplying the energy to be used by muscles during the exercise.
Muscles primarily need ATP (Adenosine Triphosphate), which is the energy packet for muscle cells and for movement which is supplied by different means as described below.
Phosphocreatine (Creatine Phosphate)
Creatine Phosphate is the quickest form of energy delivered to muscle cells as these have high-energy phosphates which help in producing ATP rapidly.
After the breakage of phosphates of creatine phosphate is converted into creatinine which is, then, excreted through urine.
Muscles already store creatine phosphate for rapid energy usage for short period of exercise or any other activity. There is 120g of creatine present within the muscular tissues of 70kg man out of which 40% is non-phosphorylated creatine and 60% is Creatine Phosphate.
Creatine phosphate can power strenuous muscle contraction for, at least, 5 to 6 seconds.
Glycogen (Anaerobic Respiration)
Muscles store carbohydrates like glucose and fructose into Glycogen by a process called glycogenesis. Glycogen is converted into glucose by another process called gluconeogenesis, to meet up the need of energy during intense exercise. Glucose is, then, used to produce ATP for energy by muscles.
Glycogen, as the energy source, is slow but still quite faster and durable as it provides power uptake of 90 seconds approximately.
More interestingly, usage and conversion of muscle glycogen does not need oxygen to carry out reactions (while liver glycogen may require so) and, hence, this is also called anaerobic respiration.
Lactic acid is a substance produced as a result of glycogen breakdown in the absence of oxygen, this lactic acid makes the muscles sored and tired.
Aerobic Respiration (Oxygen Using Reactions)
When glycogen and glucose level in the blood plasma declines, epinephrine from adrenal cortex jumps in to induce ketosis (metabolism of fats). So, the body, then, moves towards other means of energy sources when the strenuous activity is prolonged.
Our body moves towards other energy reserves like fat, liver glycogen and remnant glucose in muscles. Fats are fundamentally made for the purpose of energy storage. Fats are triglycerides present in the adipose tissues; they metabolize into fatty acids and glycerol upon oxidation in the presence of oxygen.
These products are, then, converted into energy packets, ATP, in the mitochondria of catabolic cells through various processes.
Fat metabolism during exercise is slower process than the other sources of energies but, once fueled, it can allow muscle movement for several minutes to hours. Yes, obese people have more energy reserves.
Other Energy Assets
At the extreme level of exercise, protein of skeletal muscles is broken down into amino acids which is further metabolized for energy and this happens only if the whole glycogen and fat reserves are majorly depleted. Protein is used to buildup, maintain and repair several tissues and it can provide about 15% of the bodily energy needed for exercise.