There are basically two kinds of muscle fibres red muscles and white muscles
Red muscles work continuously for a longer time because
(i) These muscle fibres are dark red, due to the presence of red haemoprotein called myoglobin. It binds and stores oxygen as oxymyoglobin in the red fibres. Oxymyoglobin liberates oxygen for utilisation during muscle contraction.
(ii) Mitochondria are more in numbers, hence they work for long periods of time.
(iii) Red muscles possesses less sarcoplasmic reticulum.
(iv) They carry out considerable aerobic oxidation without accumulating much lactic acid. Thus without fatigue red muscle fibres can contract for a longer period.
(v) These muscle fibre have slow rate of contraction for long periods. e.g., extensor muscles of the human back.