Events of the excitation-contraction-relaxation (E-C-R) cycle in fast- and slow-twitch mammalian muscle fibres relevant to muscle fatigue

The excitation–contraction–relaxation cycle (E–C–R) in the mammalian twitch muscle comprises the following major events: (1) initiation and propagation of an action potential along the sarcolemma and transverse (T)‐tubular system; (2) detection of the T‐system depolarization signal and signal transmission from the T‐tubule to the sarcoplasmic reticulum (SR) membrane; (3) Ca2+ release from the SR; (4) transient rise of myoplasmic [Ca2+]; (5) transient activation of the Ca2+‐regulatory system and of the contractile apparatus; (6) Ca2+ reuptake by the SR Ca2+ pump and Ca2+ binding to myoplasmic sites. There are many steps in the E–C–R cycle which can be seen as potential sites for muscle fatigue and this review explores how structural and functional differences between the fast‐ and slow‐twitch fibres with respect to the E–C–R cycle events can explain to a great extent differences in their fatiguability profiles.