New Perspectives in Human Movement Variability

Movement variability is defined as the normalvariations that occur in motor performance acrossmultiple repetitions of a task.2 Bernstein1 describedmovement variability quite eloquently as ‘‘repetitionwithout repetition.’’ Traditionally, movement vari-ability has been linked to noise and error, being con-sidered to be random and independent. Thistheoretical approach blends well with traditional sta-tistical and assessment methods of movement vari-ability that assume randomness and independence ofobservations. However, numerous studies have indi-cated that when movement is observed over timevariations are closely related with each other neitherbeing random nor independent. Practically, traditionalmethods can mask the temporal structure of movementvariability and contain little information about howmovement changes over time.

Importantly, a growing body of literature hasshown that the variations seen in a wide variety ofphysiological systems may offer insight into the controlof these motor systems. For example, they can enhanceour understanding of the effect of various pathologieson movement allowing us to develop better prognosticand diagnostic biomedical engineering tools. Thus,many new methods have been presented recently toassess the temporal structure of movement variability.These nonlinear methods can also allow us to developalternative theoretical models to guide experimentaldesigns and scientific inquiry.

Therefore, in this special issue we provide a thoroughview of the state-of-the-art methods currently used in themainstream biomechanics for evaluating and quantifyinghuman movement variability. These new perspectives inhumanmovement variability are further elaborated in thelight of the temporal structure of movement variability. Itis our hope that this special issue will provide a thoroughbackground in understanding of the fundamental con-cepts of human movement variability and their utility insolving particular problems in motor control, and relatedperipheral topics and fields in the biomedical engineering.Our collection of 14 papers from leading laboratorieshighlights these advances in distinct human movementarenas.

The articles featured in this special issue addressseveral different motor control systems utilizing variousnonlinear dynamical systems tools. The first article dis-cusses the theory of optimal movement variability(Kaipust, McGrath, Mukherjee, and Stergiou) investi-

gating the complexity andpredictability of time seriesdata of gait characteristics byassessing fractal indexes in adual task paradigm (detrend-ed fluctuation analysis). Sim-ilarly, the second and thirdarticles explore the fluctua-tion dynamics of cycling(Warlop, Bollens, Creveco-eur, Detrembleur, and Leje-une) and eye saccades using aregime-switching approach(Wong and Shelhamer).Fractal fluctuations of cen-ter-of-pressure data weredetermined in the fourthpaper to better understandthe cascade dynamics (Pal-atinus, Dixon, and Kelty-Stephen). The fifth (Ihlen andVereijken) and sixth (Kuz-netsov, Bonnette, Gao, andRiley) articles propel us intothe new frontier of the mul-tifractal scaling analyses per-hapsmore fitting for posturalstability modeling than the monofractal scaling meth-ods. The seventh article investigates the utility of fractalscaling analysis in biofeedback training during walking(Tirosh, Cambell, Begg, and Sparrow). The next threearticles employed varied linear and nonlinear methods(maximal Lyapunov exponents using Wolf’s and Ro-senstein’s methods, approximate entropy, and detrend-ed fluctuation analysis) to explore the notions ofcomplexity and stability. The eighth article (Huisinga,Mancini, George, and Horak) probed the movementvariability of walking using a trunk accelerometer dataof patients withMultiple Sclerosis and healthy controls.The ninth article (Nessler, McMillan, Schoulten, Shal-low, Stewart, andDeLeone) elaborates on the nonlineardynamics of desynchronized gait with an interestingwalking protocol—side-by-side treadmill walking. Thetenth article (Myers, Johanning, Pipinos, Schmid, andStergiou) evaluates gait variability changes post vascu-lar occlusion using complementary linear and nonlinearanalyses. The next set of articles describes independentlinear and nonlinear methods to assess characteristics

Thurmon Lockhart

Nick Stergiou

Annals of Biomedical Engineering, Vol. 41, No. 8, August 2013 (� 2013) pp. 1593–1594

DOI: 10.1007/s10439-013-0852-0

0090-6964/13/0800-1593/0 � 2013 Biomedical Engineering Society

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related to movement variability. The eleventh article(Kurz,Arpin,Davies, andHarbourne) elaborates on thestochastic and deterministic components within theCOP movement variability of children with balanceimpairments using a Langevin equation methodology.The next article (Ramdani, Tallon, Bernard, and Blain)discusses an interesting approach of differentiating olderfallers and non-fallers utilizing the recurrence quantifi-cation analysis. The thirteenth article (Roerdink, Rid-derikhoff, Peper, and Beek) exploits informationaltiming and neuromuscular properties in the control ofrhythmic movements. In the last article of our specialissue, footfall placement variability is investigated usinga novel Fourier based variability concept (Socie,Sandroff, Pula, Hsiao-Wecksler, Motl, and Sosnoff).

We hope that you will find these articles stimulatingand informative, and timely in your research inunderstanding human movement variability.

ACKNOWLEDGMENTS

We acknowledge grant support from the NSF—Information and Intelligent Systems (IIS) and SmartHealth and Wellbeing—1065442 and 1065262 (Lock-hart), and NIH R01AG034995 and NASA EPSCOR(Stergiou).

REFERENCES

1Bernstein, N. A. On dexterity and its development. In:Dexterity and its Development, edited by M. L. Latash, andM. T. Turvey. Mahwah: Lawrence Erlbaum Associates,1996, pp. 1–244.2Stergiou, N., and L. M. Decker. Human movement vari-ability, nonlinear dynamics, and pathology: is there a con-nection? Hum. Mov. Sci. 30(5):869–888, 2011.

Thurmon Lockhart

Department of Industrial and Systems Engineering,Virginia Tech,Blacksburg, VA, USASchool of Biomedical Engineering and Sciences,Virginia Tech/Wake Forest University, VirginiaTech, Blacksburg, VA, USAElectronic mail: lockhart@vt.edu

Nick Stergiou

Nebraska Biomechanics Core Facility and theCenter for Research in Biomechanics, College ofEducation and College of Public HealthUniversity of Nebraska at Omaha and University ofNebraska Medical Center,Omaha, NE, USA

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