Burnley, Mark

Challis, John H.

Forrest, Sarah M.

Winter, Samantha L.

English

Kent Academic Repository

ISB 2015Neurological and Motor ControlISB 2015-1036COMPLEXITY OF ISOMETRIC FORCE PRODUCTION IS ASSOCIATED WITH TIME TO ACHIEVE STEADYSTATE WHEN MOVING TO A NEW FORCE LEVELSamantha L. Winter* 1, Mark Burnley1, Sarah Forrest2, John Challis31School of Sport and Exercise Sciences, University of Kent, Chatham Maritime, 2Sport and Exercise Science,Aberystwyth University, Aberystwyth, United Kingdom, 3Department of Kinesiology, The Pennsylvania State University,State College, United StatesPreferred Presentation: Oral PresentationIf your abstract is not accepted as an oral do you wish to be considered for a poster?: YesClinical Biomechanics Award: NoDavid Winter Young Investigator Awards: NoEmerging Scientific Award sponsored by Professor J De Luca: NoPromising Scientist Award sponsored by Motion Analysis: NoIntroduction and Objectives: Motor output and function has been shown to decline in later life, for example older adultshave increased stimulus response times [1]; reduced strength [2] and changes in the variability of force production [2]. The loss of complexity hypothesis [3] suggests that a decrease in complexity is associated with ageing as the integrationof complex physiological systems declines, and that this reduced complexity is associated with constrained behaviourand a loss of adaptability or flexibility in reponse. The function consequences of this loss of complexity in the fluctuationsseen in isometric force records have not been investigated.  However, it is possible that the reduced complexity, andmore slowly repeating and constrained fluctuations in force seen in older adults [4] would limit ability to adapt to alteredforce requirements.  It was hypothesised that reduced complexity for isometric force records in older adults would beassociated with an increase in the time taken to achieve a steady state when moving to a new isometric force level.Methods: Young (18 to 25 years) and older (65 to 72 years) neurologically healthy adults produced isometric abductioncontractions of the index finger using the First Dorsal Interosseus.  Participants established a steady state force at agiven effort level (a percentage of maximum voluntary contraction force - %MVC) by matching a target displayed visually. A second feedback condition used both the visual display and an audible tone that varied in pitch when subjects movedbelow or above the target.  The target was then suddenly and unpredictably increased or decreased at a randomlydecided time point and participants were instructed to match this new target as quickly as they could, and to hold the newcontraction intensity as steadily as possible (Figure 1).  The conditions investigated were: increasing force from 5% to25% MVC, 25% to 50% MVC, 25% to 75% MVC, and decreasing the force from 25% to 5% MVC, 50% to 75% MVC and75% to 25% MVC.  The functional reaction time was taken to be the point at which the force exceeded 3 standarddeviations (SD) from the mean force in the direction of the new force level.  The time to steady state was the time fromthe end of the functional reaction time to the start of the 3 second window with the lowest standard deviation for force. The complexity of the force record before the target change, after the target change, and during simple continuous forceproduction with no target change was determined for the different effort levels (5%, 25%, 50%, 75% MVC) usingApproximate Entropy (ApEn) [5].Results: Confirming previous results older adults exhibited reduced complexity indicated by reduced ApEn values ateffort levels >25% MVC (p<0.001) and this was the case for both the simple continuous force task (p<0.001) and thesection of the force record after the change in target force (p=0.0438).  There was no difference in the MVC abductionforce for older and younger adults (p=0.885).  There was also no difference between older and younger adults in themagnitude of the fluctuations in force, as quantified by the standard deviation of force (p>0.05).  Functional reaction timewas increased by around a third in older adults (p<0.001) in both feedback conditions.  Fuctional reaction time wasreduced for both age groups by the addition of the audible signal. Confirming the hypothesis, the time to steady stateafter the function reaction time increased from a mean of 2.83s in younger adults to 3.23s for older adults (p=0.025), andthis difference was unaffected by feedback condition.  Post-hoc tests showed that the time to steady state differed for oldand young adults at starting effort levels >5% MVC.Figure:Caption: Figure 1: Display of force record showing a trial moving from an initial target of 50% MVC to a target of 25%MVC.Conclusion: Force levels associated with reduced complexity in older adults compared to younger adults wereassociated with an increased time to steady state when moving from an isometric contraction at one effort level to anisometric contraction at a new effort level.  This suggests that there is a change in the neuromuscular system with ageevidenced by the reduced complexity that constrains response to altered force requirements. These findings haveimplications for the control of muscle force production in later life.References: [1] Fozard et al., J. Gerontol., 49: P179-189, 1994.[2] Marmon et al., Med. Sci. Sports Exerc., 43: 560-567, 2011.[3] Lipsitz et al., J. Am. Med. Assoc., 267: 1806-1809, 1992.[4] Vaillancourt et al., J. Appl. Physiol., 94: 903-912, 2003.[5] Pincus, PNAS, 88: 2297-2301, 1991.Disclosure of Interest: None Declared

Complexity of isometric force production is associated with time to achieve steady state when moving to a new force level.

http://kar.kent.ac.uk/53127/1/ISB%202015-1036-v3%20%283%29.pdf

Complexity of isometric force production is associated with time to achieve steady state when moving to a new force level.

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