P1.08. CONCOMITANT EFFECTS OF FATIGUE AND POTENTIATION IN RAT MEDIAL GASTROCNEMIUS FAST MOTOR UNITS
Angelos Angelidis1, Hanna Drzymała-Celichowska2,3, Katarzyna Kryściak2, Rene Vandenboom1, Jan Celichowski2
1 Department of Kinesiology, Centre for Bone and Muscle Health, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, ON, L2S 3A1, Canada
2 Department of Neurobiology, Poznan University of Physical Education, 27/39 Królowej Jadwigi, 61-871 Poznan, Poland
3 Department of Physiology and Biochemistry, Poznan University of Physical Education, 27/39 Królowej Jadwigi, 61-871 Poznan, Poland
INTRODUCTION: During contractile activity of muscles the force of their motor units is unstable. Namely, the force can be reduced (i.e., fatigue/performance fatigability) or increased (i.e., potentiation) in a stimulation frequency and task-dependent manner, through the parallel action of various potentiating and fatiguing mechanisms. These two effects can be observed concomitantly, with the most characteristic example being increased submaximal force in parallel with maximal tetanic force reduction.
AIM(S): The majority of these observations come from isolated whole muscle models, with less data available at the motor unit level. Therefore, the study aimed to investigate and compare the dynamics of the development of force potentiation and fatigue in both types of fast motor units.
METHOD(S): We investigated in situ the dynamics of fatigue and potentiation development in fast motor units of rat medial gastrocnemius, fast fatigable (FF) and fast fatigue resistant (FR) ones, using a 180-s repetitive isometric contractile protocol including twitches, unfused tetani of different fusion degrees (30 Hz, 40 Hz) and maximal tetanic contractions (150 Hz).
RESULTS: Potentiation of twitch and unfused tetanic force was apparent in both motor unit types in parallel with reduced maximal tetanic force (stronger for FF motor units). The potentiation depended on the fusion degree and was the strongest for moderately fused tetanic contractions at 30 Hz. Considerable overlap in potentiation magnitude between FF and FR units was observed, across stimulation frequencies. However, FF units exhibited potentiation more rapidly compared to FR units.
CONCLUSIONS: These observations directly indicate that force potentiation and fatigue coexist in both types of fast motor units most probably due to different intracellular mechanisms. Furthermore, they illustrate the progressively narrowing range of possibilities for regulating fast motor unit force by changing the stimulation frequency during activity.