We investigated the effect of a one-time application of elastic constraints on movement-inherent variability during treadmill running. Eleven males ran two 35-min intervals while surface EMG was measured. In one of two 35-min intervals, after 10 min of running without tubes, elastic tubes (between hip and heels) were attached, followed by another 5 min of running without tubes. To assess variability, stride-to-stride iEMG variability was calculated. Significant increases in variability (36 % to 74 %) were observed during tube running, whereas running without tubes after the tube running block showed no significant differences. Results show that elastic tubes affect variability on a muscular level despite the constant environmental conditions and underline the nervous system's adaptability to cope with somehow unpredictable constraints since stride duration was unaltered.Key points class="unordered" style="list-style-type:disc">The elastic constraints led to an increase in iEMG variability but left stride duration variability unaltered.Runners adapted to the elastic cords, evident in an iEMG variability decrease over time towards normal running.Hardly any aftermaths were observed in the iEMG analyses when comparing normal running after the constrained running block to normal running. class="kwd-title">Key words: Electromyography, adaptation, performance class="head no_bottom_margin" id="sec1-1title">IntroductionVariability in human (motor) behavior and performance is still a two-sided affair. Although the advantage of variability has already been mentioned as early as in the 1960's by Russian pioneer Nikolai Bernstein, , variability in the domain of sports (especially during movement production) has only recently been considered as an essential requirement. Traditional approaches saw variability or inconsistency in movement as noise or a problem to be reduced with training and practice (Bartlett et al., ; Davids et al., ). That neglected, however, its important functional role in motor behavior (i.e. variability that is beneficial to outcome performance) (Hamill et al., ; Hatze, ).The development and successful integration of different perspectives (e.g. synergetics or dynamic system approaches, stochastic resonance, as well as natural and artificial neural networks) eventually contributed to a reconsideration of variability. The transfer of knowledge of, for example, information gained from networks within biological and computational science to sports highlights the importance of different experiential contents ensuring adaptation and flexibility (i. e. generalization) in task execution (Riley and Turvey, ; Schöllhorn et al., ). Research in other domains such as disease or aging (e.g. loss of variability in gait due to aging and Huntington's disease (Van Emmerik and Van Wegen, )) further supports the positive characteristics of movement-production variability as being not only non-interfering, but rather fundamental to achieving a consistent movement outcome (Heiderscheit et al., ; Schöllhorn et al., ).Given that most sport skills involve a large number of muscles and joints (i.e. many degrees of freedom), variability became an indicator reflecting readiness of these degrees of freedom to covary to achieve a required higher order macroscopic movement outcome (Handford et al., ). This dynamic variability is the consequence of variations from the underlying nonlinearities in the system and emerges due to shape new emergence of coordination and control (Davids et al., ; Hatze, ; Van Emmerik and Van Wegen, ). Due to this indeterminacy within sublevels in repetitive movements, the movement outcome as the result of a complex interplay of forces acting on the body (non-muscular forces or according to Bernstein, reactive phenomena) and those forces actively produced by the person itself (i.e. internal or produced muscle forces) will always inhere a certain level of variability (Hatze, ).In sports, analyses of even closed movements such as a free throw in basketball or treadmill running (e.g. Button et al., href="#ref8" rid="ref8" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887687">2003; Verkerke et al., href="#ref41" rid="ref41" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887694">1998; Wheat et al., href="#ref42" rid="ref42" class=" bibr popnode">2005) illustrate that top athletes do, in fact, show variability in their execution levels (e.g. release angle in basketball (Button et al., href="#ref8" rid="ref8" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887675">2003)). Despite this fact, they are still able to achieve the same movement outcome. This indicates that variability may be essential for producing skilled behavior (Bartlett et al., href="#ref2" rid="ref2" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887680">2007; Wilson et al., href="#ref44" rid="ref44" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887662">2007) and functionality for highly skilled athletes (pointing to an ability to co-vary) (Handford et al., href="#ref17" rid="ref17" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887698">1997).For training and development, this would further implement that adding variability by engaging athletes in complex and time-varying situations and settings may be adaptively advantageous in situations of environmental unpredictability (Fontanini and Katz, href="#ref14" rid="ref14" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887705">2008). Several studies in various sports (e.g. volleyball (Spratte et al., href="#ref37" rid="ref37" class=" bibr popnode">2007), soccer (Trockel and Schöllhorn, href="#ref38" rid="ref38" class=" bibr popnode">2003), speed skating (Savelsbergh et al., href="#ref34" rid="ref34" class=" bibr popnode">2010), indoor hockey (Beckmann et al., href="#ref3" rid="ref3" class=" bibr popnode">2008; Birklbauer et al., href="#ref6" rid="ref6" class=" bibr popnode">2006) or athletics (Schöllhorn et al., href="#ref35" rid="ref35" class=" bibr popnode">2010)) support the positive effect of adding variability.However, if, on the one hand, variability, induced by the set constraints or different executions or tasks, is too broad, the exercises may no longer be supportive for the actual task (i.e. no transfer of the exercises to the actual movement is possible); on the other hand, if there is no variability, the athlete is tightly constrained and it may be difficult to find the individual optimum (Schöllhorn et al., href="#ref36" rid="ref36" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887672">2009). Hence, the magnitude of variability must be attuned to remain within a functional bandwidth of variability (Birklbauer et al., href="#ref6" rid="ref6" class=" bibr popnode">2006; Handford et al., href="#ref17" rid="ref17" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887696">1997). Combining the aforementioned variability aspects, it is assumed that the induced variability (at least at a certain skill level) should remain within the movement skill or at least within its immediate vicinity, to maintain the basic structure of the movement pattern.Against this background, the application of elastic tubes to provide resistance to the lower extremities was assumed to meet the requirements to create variability within an optimal boundary. As the given elastic constraint (due to its property) may influence loading of lower extremities and the resulting alteration in the moment of inertia of the leg contribute to the movement outcome (Martin, href="#ref26" rid="ref26" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887668">1985), the upcoming question is how elastic tubes influence a well-established behavior as for instance running.It is obvious that running with different tube positions increases variability during movement production and perhaps results in a variable movement outcome. That is because the permanently changing environmental constraints may require adaptation in muscle synergies to achieve the desired movement outcome and perform the requested movement pattern; however, it would be of interest to identify whether one single application can increase variability and if so, the extent to which it is increased and how performance can be adjusted to changes through this constraint.Up to now, the field of application of elastic tubes was resistance and conditioning training (e.g. athletics) (Corn and Knudson, href="#ref10" rid="ref10" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887658">2003) but not as a technique (and coordination) training device as is the case in this study. The tubes (attached between the hip and heel) influence the freedom of movement of the lower extremities and alter the forces and, thus, lead to variation within the reactive phenomena. The imposed perturbation would result in supportive and counterproductive forces and for that reason muscle activations change accordingly with respect to an optimal pattern of coordination and performance. Consequently, it has been assumed that such a “variability” constraint increases variability on a muscular level. Running was chosen to be the investigated task because it is a routine and one of the most common types of locomotion (Abe et al., href="#ref1" rid="ref1" class=" bibr popnode tag_hotlink tag_tooltip" id="__tag_327887688">2007).Therefore, the aim of this study was to assess the acute effect of a one-time application of elastic constraints on variability of muscle activation variability during running. We wanted to determine whether it is possible to influence variability within the muscular components when running with elastic tubes. Specifically, it may be expected that there is a change and adaptation in variability over time during running with elastic tubes. Since participants were novices with the tubes, we expected to see an increase in muscle variability through the application of elastic tubes (and their sensitivity of initial condition) and also an after-effect, when switching back to running without tubes.
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