Relationship between the tension-time area and the frequency of stimulation in motor units of the rat medial gastrocnemius muscle.
The tension-time area is an estimation of the work performed by contracting motor units. The relationship between tension and frequency of stimulation and between tension-time area and frequency have been studied on 148 single motor units of the rat medial gastrocnemius muscle, under isometric conditions. Motor units were classified as fast fatigable (FF), fast resistant to fatigue (FR) or slow (S). Trains of stimuli of increasing frequency and constant duration were used. For all motor units a half of the maximum tetanic tension corresponded to lower frequencies compared to frequencies at a half of the maximum tension-time area. Moreover, the slopes of tension-frequency and area-frequency curves (change of tension or area per 1 Hz rise in frequency) were higher for slow than for fast motor units. The tension-time area per one pulse was calculated for different frequencies of stimulation. For slow units the maximum area per pulse corresponded to significantly lower frequencies than for fast ones, especially of FF type. However, for all three types of motor units this optimal frequency corresponded to sub-fused tetani with a tension of about 75% of the maximum tension, and with the fusion index slightly over 0.90. The absolute values of the maximum tension-time area per pulse revealed that in one contraction within the tetanus, slow units are generating greater work than FR units. The work performed by FF units is nearly two times larger than for S units, although the tension of slow units is over eight times lower. The presented results reveal that the contraction of slow motor units is much more effective than was suggested based on their low tension.
Celichowski
J
J
Department of Neurobiology, University School of Physical Education, Poznań, Poland. celichowski@awf.poznan.pl
Grottel
K
K
Bichler
E
E
D000818:Animals; D004558:Electric Stimulation; D005260:Female; D007537:Isometric Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors