There are basically two kinds of muscle fibers
(i) Red muscles (ii) White muscles
Red muscles work for a longer time continuously because
(i) These muscle fibres are dark red which is due to the presence of red haemoprotein caleld myoglobin. Myoglobin binds and stores oxygen as oxymyoglobin in the red fibres. Oxymyoglibin releases oxygen for utilisation during muscle contraction.
(ii) Mitochondria are more in number , hence they work for long periods of time.
(iii) Red muscles have less sarcoplamic reticulum.
(iv) They carry out considerable aerobic oxidation without accumulating much lactice acid . Thus, red muscle fibres can cantract for a longer period without fatigue.
(v) These muscle fibres have slow rate of contraction for long periods e.g., extensor muscles of the human back.
