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      Relationships Between Neuromuscular Function and Functional Balance Performance in Firefighters

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          Abstract

          The purpose of the present study was to examine the relationships between neuromuscular function and functional balance performance in firefighters. Fifty career firefighters (35.1 ± 7.5 yr) performed isometric leg extension and flexion muscle actions to examine peak torque (PT), and absolute (aTQ) and normalized (nTQ; %PT) rapid torque variables at 50, 100, 150, and 200 ms. A performance index (PI) was determined from the functional balance assessment completion time. Partial correlations were used to examine the relationship between the PI and the maximal and rapid TQ variables for each muscle and the composite value, while controlling for demographic data related to the PI. Multiple regression analyses examined the relative contributions of the maximal and rapid aTQ variables, and demographic data on the PI. After controlling for age and %BF, the majority of the later aTQ and nTQ variables (100–200 ms) and PT were associated with the PI ( r = −0.501–−0.315). Age, %BF, and aTQ 100 explained 42–50% of the variance in the PI. Lower rapid strength, increased age, and poorer body composition were related to worse performance during the functional balance assessment. Strategies to improve rapid strength and %BF, especially in aging firefighters may impact dynamic balance abilities in firefighters.

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          Increased rate of force development and neural drive of human skeletal muscle following resistance training.

          The maximal rate of rise in muscle force [rate of force development (RFD)] has important functional consequences as it determines the force that can be generated in the early phase of muscle contraction (0-200 ms). The present study examined the effect of resistance training on contractile RFD and efferent motor outflow ("neural drive") during maximal muscle contraction. Contractile RFD (slope of force-time curve), impulse (time-integrated force), electromyography (EMG) signal amplitude (mean average voltage), and rate of EMG rise (slope of EMG-time curve) were determined (1-kHz sampling rate) during maximal isometric muscle contraction (quadriceps femoris) in 15 male subjects before and after 14 wk of heavy-resistance strength training (38 sessions). Maximal isometric muscle strength [maximal voluntary contraction (MVC)] increased from 291.1 +/- 9.8 to 339.0 +/- 10.2 N. m after training. Contractile RFD determined within time intervals of 30, 50, 100, and 200 ms relative to onset of contraction increased from 1,601 +/- 117 to 2,020 +/- 119 (P < 0.05), 1,802 +/- 121 to 2,201 +/- 106 (P < 0.01), 1,543 +/- 83 to 1,806 +/- 69 (P < 0.01), and 1,141 +/- 45 to 1,363 +/- 44 N. m. s(-1) (P < 0.01), respectively. Corresponding increases were observed in contractile impulse (P < 0.01-0.05). When normalized relative to MVC, contractile RFD increased 15% after training (at zero to one-sixth MVC; P < 0.05). Furthermore, muscle EMG increased (P < 0.01-0.05) 22-143% (mean average voltage) and 41-106% (rate of EMG rise) in the early contraction phase (0-200 ms). In conclusion, increases in explosive muscle strength (contractile RFD and impulse) were observed after heavy-resistance strength training. These findings could be explained by an enhanced neural drive, as evidenced by marked increases in EMG signal amplitude and rate of EMG rise in the early phase of muscle contraction.
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            Influence of maximal muscle strength and intrinsic muscle contractile properties on contractile rate of force development.

            'Explosive' muscle strength or contractile rate of force development (RFD) is a term to describe the ability to rapidly develop muscular force, and can be measured as the slope of the torque-time curve obtained during isometric conditions. Previously, conflicting results have been reported regarding the relationship between contractile RFD and various physiological parameters. One reason for this discrepancy may be that RFD in various time intervals from the onset of contraction is affected by different physiological parameters. The aim of the present study was to investigate the relationship between voluntary contractile RFD in time intervals of 0-10, 0-20, ..., 0-250 ms from the onset of contraction and two main parameters: (1) voluntary maximal muscle strength and (2) electrically evoked muscle twitch contractile properties. The main finding was that voluntary RFD became increasingly more dependent on MVC and less dependent on muscle twitch contractile properties as time from the onset of contraction increased. At time intervals later than 90 ms from the onset of contraction maximal muscle strength could account for 52-81% of the variance in voluntary RFD. In the very early time interval (<40 ms from the onset of contraction) voluntary RFD was moderately correlated to the twitch contractile properties of the muscle and was to a less extent related to MVC. The present results suggest that explosive movements with different time spans are influenced by different physiological parameters. This may have important practical implications when designing resistance training programs for specific sports.
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              Increased risk for falling associated with obesity: mathematical modeling of postural control.

              Recent epidemiological studies report that obesity is positively related to fracture incidence. In the present experiment, a model of postural control was used to examine the impact of an abnormal distribution of body fat in the abdominal area upon postural stability. Obese and lightweight humanoids were destabilized by imposing a small initial angular speed from a neutral standing position. To avoid a loss of stability yielding a stepping reaction or a fall, an ankle torque is necessary to counteract the perturbation. Three torque parameters--ankle torque onset, time to peak torque, and muscular ankle torque--were entered in a program to simulate the intrinsic variability of the human postural control system. A loss of stability was detected when the center of pressure exceeded stability margins. The most striking observation is the nonlinear increase of torque needed to stabilize the humanoid when the motor response was characterized by delayed temporal parameters. The effect was more pronounced when an anterior position of the center of mass was included in the simulations. This suggests that, when submitted to daily postural stresses and perturbations, obese persons (particularly those with an abnormal distribution of body fat in the abdominal area) may be at higher risk of falling than lightweight individuals.
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                Author and article information

                Contributors
                edryan@email.unc.edu
                Journal
                Sci Rep
                Sci Rep
                Scientific Reports
                Nature Publishing Group UK (London )
                2045-2322
                17 October 2018
                17 October 2018
                2018
                : 8
                : 15328
                Affiliations
                [1 ]ISNI 0000000122483208, GRID grid.10698.36, Department of Exercise and Sport Science, , University of North Carolina at Chapel Hill, ; Chapel Hill, NC USA
                [2 ]ISNI 0000000122483208, GRID grid.10698.36, Human Movement Science Curriculum, , University of North Carolina at Chapel Hill, ; Chapel Hill, NC USA
                [3 ]ISNI 0000 0001 2151 958X, GRID grid.420282.e, United States Army Research Laboratory, ; Aberdeen Proving Ground, MD USA
                [4 ]ISNI 0000 0004 0456 4954, GRID grid.450232.2, Under Armour, ; Baltimore, MD USA
                [5 ]ISNI 0000 0001 2185 8768, GRID grid.53857.3c, Department of Kinesiology and Health Science, , Utah State University, ; Logan, UT USA
                [6 ]ISNI 0000000122483208, GRID grid.10698.36, Department of Allied Health Sciences, , University of North Carolina at Chapel Hill, ; Chapel Hill, NC USA
                Article
                33555
                10.1038/s41598-018-33555-z
                6193010
                30333503
                ca8ca13f-39a7-418b-954e-07e7d2dd73f7
                © The Author(s) 2018

                Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

                History
                : 20 June 2018
                : 27 September 2018
                Funding
                Funded by: FundRef https://doi.org/10.13039/100000125, U.S. Department of Health &amp; Human Services | CDC | National Institute for Occupational Safety and Health (NIOSH);
                Award ID: T42OH008673
                Award Recipient :
                Funded by: EDR is the principle investigator of a service contract with the North Carolina Department of Public Safety.
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