The energy needed for muscular action is provided by ATP. ATP breaks released energy and itself gets changed to ADP. ADP (Adenosine diphosphate) is formed as a result of ATP breakdown during muscle contraction. Continuous supply of glucose is needed for the activity of the muscle as when glucose is oxidised one phosphate is given to ADP which again becomes ATP. Lactic acid is formed as a result of anaerobic breakdown of glucose. Accumulation of lactic acid leads to fatigue because neuro-muscular junction being sensitive to it.