ATP: Adenosine triphosphate. ^#: amount of work performed per unit of ATP consumed. +: relative expression of a given characteristic of the muscle fibre. A single myosin heavy chain isoform is typically expressed within an adult skeletal muscle fibre. Fibres classified as type I, IIa, IIx and IIb express myosin heavy chain isoform I (or slow), IIa, IIx and IIb, respectively. Type IIx fibres have been reported in peripheral muscles of humans and animals and in the diaphragm of animals. Type IIx fibres have not been reported in the human diaphragm. More than one myosin heavy chain isoform is expressed in a few fibres (∼14% of adult rat diaphragm co-expresses myosin heavy chain isoforms IIb and IIx and <1% co-expresses myosin heavy chain isoforms I and IIa). While the velocity of muscle contraction depends primarily on the myosin heavy chain isoform, the velocity of muscle relaxation is mainly determined by troponin C calcium binding and release and by calcium re-uptake by the sarco-endoplasmic reticulum calcium–adenosine thriphosphatase (SERCA). Several SERCA isoenzymes have been identified: SERCA 1 is expressed in type II fibres (fast calcium re-uptake) and SERCA 2a is expressed in type I fibres (slow calcium re-uptake). The density of pumping sites largely accounts for different rates of calcium uptake in fast- and slow-twitch muscle fibres. Despite this separation of tasks, velocity of contraction and velocity of relaxation tend to parallel each other; type II fibres contract and relax with a greater velocity than type I fibres. Slower velocity of relaxation allows fusion of repetitive twitches at lower frequencies of stimulation compared with fast relaxations. Impairment of SERCA activity has been implicated in the development of fatigue and in disease states, including heart failure and corticosteroid myopathy. Reproduced from 15 with permission from the publisher.