The function of the long back muscles during postural development in the rat.

The development of the EMG of the multifidus (MM) and longissimus lateralis (LL) muscles was studied in 16 rats between the 6th day (P6) and P45 and related to behavioural development. The EMG changes gradually from irregular and spiky towards a regular interference pattern and simultaneously, the general activity level increases. From P11, tonic background activity occurs with phasic activity superimposed. During locomotion, a relation between increasing activity in the long back muscles and the swing phase of both hind paws develops, but only from P15 the activity modulates consistently with the step cycle. From the end of the second week of life, the activity in the long back muscles precedes hind paw movements. The development of the EMG of the LL and MM is closely followed by the development of adult and fluent motor patterns. This suggests that the stabilisation of the trunk is essential for the development of these patterns.