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Motor Unit Recruitment And De-Recruitment Asymmetry Increases With Increased Frequency Of Sinusoidal Force Modulation: 1668Board #158 May 30 2:00 PM - 3:30 PM

Martens, Christopher; Poojari, Dhiraj; Bellumori, Maria; Knight, Christopher A.

Medicine & Science in Sports & Exercise: May 2007 - Volume 39 - Issue 5 - p S267–S268
doi: 10.1249/01.mss.0000274027.32566.34
B-29 Free Communication/Poster - Neuromuscular Control, Balance, and Gait: MAY 30, 2007 1:00 PM - 6:00 PM ROOM: Hall E

University of Delaware, Newark, DE.

(Sponsor: Farquhar, FACSM)

The amount of advance between neural input and muscular output is partly determined by electromechanical delay and the temporal aspects of muscle contractility. The PURPOSE: was to explore this neuromuscular advance within the specific context of motor unit recruitment and de-recruitment. Considering the asymmetry of muscle twitches, it was hypothesized that during symmetrical increases and decreases in muscular force, the force level of recruitment (RF) would be less than that of de-recruitment (DF). Also, it was hypothesized that as the rate of change in force increases, so would the mismatch between RF and DF.

METHODS: 9 adults performed isometric index finger abduction tasks (36s duration). Subjects produced sinusoidal force oscillations surrounding 20% of their maximal force (MF) level. To manipulate the rate of change in muscular force, subjects performed 6 force-matching conditions consisting of combinations of 2 amplitudes (+/−3, 6%MF) and three frequencies (.3, .6, .9-Hz). Motor unit action potentials were recorded from a tetrode consisting of four fine wires that were inserted in the first dorsal interosseous muscle. Motor units producing repeating bursts of action potentials across multiple cycles were selected for analysis. RF and DF were taken as the force level at the first and last spike within a burst. Data from multiple bursts within a single force-matching trial were averaged. One summary variable describing the asymmetry was computed as the difference between DF and RF (DF-RF).

RESULTS: DF-RF was significantly greater than zero (p<05) in the .6 and .9-Hz task frequencies but not in the .3-Hz task frequency. There was a significant increase in average (SE)DF-RF across frequencies [1.1 (.4), 1.9 (.6), 4.0 (.8) %MVC at .3,.6 and .9-Hz respectively).

CONCLUSION: Consistent with the hypotheses, these results indicate that with increases in the frequency of sinusoidal force oscillations, motor units are de-recruited earlier with respect to changes in force. The nonlinear increase in asymmetry across frequencies suggests a threshold frequency between .6 and .9 Hz at which the task dynamics and recruitment dynamics are more interactive.

© 2007 American College of Sports Medicine