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During dynamic contractions, high frequency muscle activation is needed to achieve optimal power. This must be balanced against an increased excitation-induced accumulation of extracellular K, which can reduce excitability and ultimately may prevent adequate responses to high frequency activation. Mean activation frequencies in vivo are often low ( sub-tetanic), but activation patterns contain bursts of high ( supra-tetanic) frequencies known as doublets, which enhance dynamic contraction in rested muscles at normal [K]. Here we examine how dynamic contractions in fast-twitch fibers stimulated by high frequency/doublets are affected during exposure to 11 mM [K] and during fatigue. Dynamic contractions were elicited by electrical stimulation in isolated rat EDL muscles incubated at 4 or 11 mM K. When stimulation frequency was maintained constant, an increase from 150 Hz to 300 Hz enhanced maximal power ( P), maximal velocity ( V), and rate of force development ( RFD) at 4 mM K, but only V at 11 mM K. Using sub-tetanic frequency trains (50 Hz) with or without an initiating doublet (300 Hz), the addition of a doublet increased maximal force ( F), P, V, and RFD at both 4 and 11 mM K. Furthermore, a work-matched fatiguing protocol was performed comparing a doublet-initiated sub-tetanic train (DT) of 60 Hz, to a constant-frequency train (CFT) of 71 Hz during 100 dynamic contractions. We found that DT produced higher power, velocity, and RFD than CFT throughout the fatiguing protocol. The results indicate that doublets enhance dynamic contraction in fast-twitch muscles stimulated at sub-tetanic frequency both during normal and fatiguing conditions.
This article was published in the following journal.
Name: American journal of physiology. Cell physiology
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Proteins which participate in contractile processes. They include MUSCLE PROTEINS as well as those found in other cells and tissues. In the latter, these proteins participate in localized contractile events in the cytoplasm, in motile activity, and in cell aggregation phenomena.
A protein complex of actin and MYOSINS occurring in muscle. It is the essential contractile substance of muscle.
Mature contractile cells, commonly known as myocytes, that form one of three kinds of muscle. The three types of muscle cells are skeletal (MUSCLE FIBERS, SKELETAL), cardiac (MYOCYTES, CARDIAC), and smooth (MYOCYTES, SMOOTH MUSCLE). They are derived from embryonic (precursor) muscle cells called MYOBLASTS.
The muscle tissue of the HEART. It is composed of striated, involuntary muscle cells (MYOCYTES, CARDIAC) connected to form the contractile pump to generate blood flow.
Prolonged failure of muscle relaxation after contraction. This may occur after voluntary contractions, muscle percussion, or electrical stimulation of the muscle. Myotonia is a characteristic feature of MYOTONIC DISORDERS.
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