Energy for Muscles
Muscles use ATP from aerobic respiration to contract. However, depending on muscle type (below), anaerobic respiration may be carried out to provide energy very quickly. When this is the case, a reaction involving both ATP and PCr (phosphocreatine) occurs. The trick here is that unlike typical anaerobic respiration, no lactate is produced.
Glycogen stored in muscle is the next port of call for energy during muscle contraction, as it can be broken down into glucose. In the absence of sugar-based energy, cortisol is released which stimulates protein breakdown as another source of energy.
Types of Muscle
There are two main kinds of muscle fibre: slow twitch and fast twitch. Hmm, I do wonder what they do. Perhaps they joyously twitch slowly… or fast.
Slow twitch: slow contraction, uses ATP from aerobic respiration, can work for prolonged periods of time e.g. back muscles holding us up throughout the day.
Fast twitch: fast contraction, uses ATP+PCr from anaerobic respiration, can work for short periods of time e.g. leg muscles moving us out of the way of an incoming van.
Therefore, slow twitch muscle fibres contain plenty of mitochondria for aerobic respiration and capillaries for the oxygen required to do it. Fast twitch fibres contain few mitochondria and aren’t as richly vascularised.