P1.13. TIME NECESSARY TO RESTITUTE EXTRA INITIAL FORCE IN FAST MOTOR UNITS IN RAT MEDIAL GASTROCNEMIUS MUSCLE
Katarzyna Kryściak1, Hanna Drzymała-Celichowska1,2, Joanna Majerczak3, Jan Celichowski1, Jerzy A. Zoladz3
1 Poznan University of Physical Education, Department of Neurobiology, Królowej Jadwigi 27/39, 61-871 Poznań, Poland.
2 Poznan University of Physical Education, Department of Physiology and Biochemistry, Królowej Jadwigi 27/39, 61-871 Poznań, Poland.
3 Jagiellonian University Medical College, Faculty of Health Sciences, Chair of Exercise Physiology and Muscle Bioenergetics, Skawińska 8, 31-066 Kraków, Poland.
INTRODUCTION: The beginning of fast motor units (MUs) tetanic contraction is characterized by highly dynamic changes in force including 200-500 ms period of very efficient initial force production (boost) followed by a subsequent force decline (reported as “sag”).
AIM(S): When contractions of fast MUs are rhythmically repeated in series at short time intervals (one per second), the boost appears only in the first of them, but it may be restituted after a rest period. However, the minimum inactivity time required to restitute this phenomenon is not known and this study aimed to fill this gap.
METHOD(S): Triplets of 500 ms tetanic contractions (delivered once per second) were recorded at progressively shortening time intervals - from 90 to 2 s. The reduction of boost was assessed as a decrease of the sag amplitude after the peak force related to the boost.
RESULTS: At the beginning of recordings the boost was visible in the first contraction of successive triplets, until the time intervals become critically short. After that, as time interval shortens, boost started to diminish, and finally, completely disappeared. Moreover, the boost amplitude initially increased in parallel to the development of force potentiation. For fast resistant to fatigue (FR) MUs the estimated interval duration necessary for reduction of the sag by 50% ranged 8-29 s. For fast fatigable (FF) MUs this interval was longer as ranged 35-75 s.
CONCLUSIONS: To conclude, the boost phenomenon in unfused tetani of fast MUs is strongly dependent on activation history and type of MU. The peak force within boost is potentiating after preceding activity delivered even more than 1 minute earlier but when the intervals are shorter than ~30 s for FR and ~70 s for FF MUs the boost is reduced.