Previous work has shown that hypertensive patients that undertake 8 weeks of inspiratory muscle strength training exhibit significant declines in systolic and diastolic blood pressure [Ferreira et al., 2011], and our lab has been able to reproduce these results in a healthy young adult population over a 6 week training period [unpublished data]. The mechanism underlying this change is not known. Here, we compare inspiratory muscle strength training (‐ pressure, + lung volume) against four other training protocols: deep breathing (+ lung volume), Mueller maneuver (‐ pressure), expiratory muscle strength training (+ volume, + pressure), and placebo, to determine which parameter may affect blood pressure. Fifty healthy adult volunteers (ages 18‐30) were randomly assigned to one of the five treatment groups and undertook 5 minutes of supervised training, 5 days a week, over a period of 6 weeks. After 4 weeks, preliminary findings from fifteen men and women show reductions in systolic and diastolic blood pressure for individuals in inspiratory (‐8.0/‐2.3) and expiratory (‐9.6/‐2.1) muscle strength training groups as well as the Mueller maneuver (‐4.4/‐3.4) group, compared to no change in the placebo group. On this basis we suggest that the large pressure swings associated with both respiratory resistance training and Mueller maneuvers may contribute to improvements in blood pressure. Our findings support additional studies aimed at assessing the effects of training on efferent mechanism(s), such as sympathetic outflow, that may underpin alterations in blood pressure.
Motor unit (MU) recruitment was assessed in two muscles with similar muscle fiber type compositions and that participate in skilled movements: the tongue muscle, genioglossus (GG) and the hand muscle, first dorsal interosseous (FDI). Our primary objectives were to determine in the framework of a voluntary movement whether muscle force is regulated in tongue as it is in limb i.e., via processes of rate coding and recruitment. Recruitment in the two muscles was assessed within each subject in the context of ramp force (FDI) and in the tongue (GG) during vowel production and specifically, in the context of ramp increases in loudness, and subsequently expressed relative to the maximal. The principle findings of the study are that the general rules of recruitment and rate coding hold true for both GG and FDI and second, that average firing rates, firing rates at recruitment and peak firing rates in GG are significantly higher than for FDI (P <0.001) despite tasks performed across comparable force ranges (∼2-40% of max). The higher firing rates observed in the tongue within the context of phonation may be a function of that muscle's dual role as (prime) mover and hydrostatic support element.
The nervous system regulates force via two means; rate coding and recruitment. In a previous study (Bailey et al. 2007) we reported on rate coding of genioglossus (GG) motor units (MUs) in the context of voluntary tongue movement. Here we focus on MU recruitment and compare GG with first dorsal interosseous (FDI). We recorded single MU action potentials via intramuscular tungsten microelectrodes inserted into the GG and FDI in 9 healthy adults (N=3 men; N=6 women). Recruitment was elicited in FDI via force increments associated with finger abduction (force range 4‐7% of max), while recruitment in the GG was elicited via increments in sound pressure level (loudness range 5‐34% of max) associated with articulation of the vowel /i/. We noted significantly higher firing rates in GG MUs at recruitment relative to FDI (18.9 4.8 vs. 11.0 2.8). Once recruited, GG MUs discharged at significantly higher rates than FDI MUs (20.9 4.7 vs. 12.1 2.7). Moreover, the firing rates of subsequently recruited units was also notably higher for GG than for FDI (21.3 4.8 vs. 12.2 2.7). These findings provide useful insights into motoneuron pools that regulate movements of the tongue in speech and of the hand in grasping. Higher firing rates at recruitment for GG compared to FDI, given comparable pressure/force ranges, suggest hypoglossal motoneurons have a shorter duration after hyperpolarization than spinal motoneurons (Bailey et al. 2007). Grant Funding Source: Supported by the National Institute of Health Grant (DC 009587)
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