The activation of back muscles during locomotion in the developing rat

The development of posture during locomotion was studied in rats from the 1 1th day until adulthood. The EMGs were recorded and analyzed of the left an d right longissimus muscles at caudal, intermediate and rostral levels as w ell as of the gastrocnemius, the tibialis and the vastus medialis muscles a nd movements were simultaneously recorded on videotape. Results indicate th at from the 12th day of life, burst activity occurs in the longissimus musc les which is phase-related to the stepcycle. Until the 21st day these muscl es are most strongly activated during burst activity in the gastrocnemius m uscle in the contralateral hindleg but thereafter this activation coincides with bursts in the ipsilateral gastrocnemius muscle. At adult age such act ivation in the LL is restricted to fast walking or to accelerations. Latenc ies between bursts in the longissimus muscles and the gastrocnemius muscles vary around 100 ms until the 25th day, but thereafter they decrease to adu lt values of less than 10 ms. The large variations in these phase-relations at all ages suggest that supraspinal influences and afferent input are imp ortant factors in this coupling. The shift from a contra- to an ipsilateral coupling between bursts in the longissimus and in the gastrocnemius muscle s might indicate that an ontogenetically older pattern of locomotion with t he trunk muscles playing a major role in propulsion, is replaced by a newer pattern, mainly effected by extremity movements. (C) 1999 Elsevier Science B.V. All rights reserved.