Patterns of muscle activity during lung ventilation, patterns of innervation and some contractile properties were measured in the hypaxial muscles of green iguanas. Electromyography shows that only four hypaxial muscles are involved in breathing. Expiration is produced by two deep hypaxial muscles, the transversalis and the retrahentes costarum. Inspiration is produced by the external and internal intercostal muscles. Although the two intercostal muscles are the main agonists of inspiration, neither is involved in expiration. This conflicts with the widely held notion that the different fibre orientations of the two intercostal muscles determine their ventilatory action. Several observations indicate that ventilation is produced by slow (i.e. nontwitch) fibres of these four muscles. First, electromyographic (EMG) activity recorded from these muscles during ventilation has an unusually low range of frequencies (less than 100 Hz). Such low-frequency signals have been suggested to be characteristic of muscle fibres that do not propagate action potentials (i.e. slow fibres). Second, during inspiration, EMG activity is restricted to he medical sides of the two intercostal muscles. Muscle fibres from this region have multiple motor endplates and exhibit tonic contraction when immersed in saline solutions of high potassium content. Like the intercostals, the transversalis and retrahentes costarum muscles also contain fibres with multiple motor endplates. Thus, although breathing is a phasic activity, it is produced by tonic (i.e. slow) muscle fibres. The intercostal muscles are also involved in postural and locomotor movements of the trunk. However, such movements employ twitch as well as slow fibres of the intercostal muscles.

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