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Bente R. Jensen, Line Hovgaard-Hansen, and Katrine L. Cappelen

Running on a lower-body positive-pressure (LBPP) treadmill allows effects of weight support on leg muscle activation to be assessed systematically, and has the potential to facilitate rehabilitation and prevent overloading. The aim was to study the effect of running with weight support on leg muscle activation and to estimate relative knee and ankle joint forces. Runners performed 6-min running sessions at 2.22 m/s and 3.33 m/s, at 100%, 80%, 60%, 40%, and 20% body weight (BW). Surface electromyography, ground reaction force, and running characteristics were measured. Relative knee and ankle joint forces were estimated. Leg muscles responded differently to unweighting during running, reflecting different relative contribution to propulsion and antigravity forces. At 20% BW, knee extensor EMGpeak decreased to 22% at 2.22 m/s and 28% at 3.33 m/s of 100% BW values. Plantar flexors decreased to 52% and 58% at 20% BW, while activity of biceps femoris muscle remained unchanged. Unweighting with LBPP reduced estimated joint force significantly although less than proportional to the degree of weight support (ankle).It was concluded that leg muscle activation adapted to the new biomechanical environment, and the effect of unweighting on estimated knee force was more pronounced than on ankle force.

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Yeshayahu Hutzler and Devora Hellerstein

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Bradley P. Wyble and David A. Rosenbaum

Smeets et al. (2016) suggested that motor adjustments may be quick because they don’t require stimulus detection. We agree that these rapid adjustments probably reflect rapid perceptual processing rather than rapid motor execution, but we question whether the absence of detection is the best way to explain the effect. We suggest that it is unclear what mechanisms would be involved in detection and why detection would be required in some of the cases discussed by Smeets et al. Instead, we suggest that ultra-fast motor adjustments require very little competition among possible stimuli or responses. We suggest that escaping competition rather than avoiding detection may be the cause of the very short reaction times that Smeets et al. identified.

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Alexandra Reichenbach

Most studies investigating movement preparation report substantial longer reaction times than studies probing response times to adjust an ongoing movement. However, both behaviors constitute control processes that transform sensory information about the environment into motor commands. Smeets, Oostwoud Wijdenes, and Brenner (2016) base their review on the hypothesis of a qualitative difference between the two sensorimotor control processes. By ruling out that the three latter movement stages proposed by Donders (1969)—identification, selection, and execution—are responsible for the difference, they argue for a specific role of the detection stage. Specifically, they reason that the detection in (target) change is only necessary for movement initiation and thus constitutes the qualitative difference between the two behaviors. In contrast to this view, I will advocate that change detection is not a qualitatively discriminating feature between the two processes. Instead, the difference in response time might rather stem from different decision criteria (Wolpert & Landy, 2012) set on the detection threshold. This threshold is rather conservative for movement initiation and more relaxed for online adjustments. However, both are states on the same continuum. Thus, movement initiation and online adjustments might even not be qualitatively different at all.

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Robert L. Sainburg and Pratik K. Mutha

The target article (Smeets, Oostwoud Wijdenes, & Brenner, 2016) proposes that short latency responses to changes in target location during reaching reflect an unconscious, continuous, and incremental minimization of the distance between the hand and the target, which does not require detection of the change in target location. We, instead, propose that short-latency visuomotor responses invoke reflex- or startle-like mechanisms, an idea supported by evidence that such responses are both automatic and resistant to cognitive influences. In addition, the target article fails to address the biological underpinnings for the range of response latencies reported across the literature, including the circuits that might underlie the proposed sensorimotor loops. When considering the range of latencies reported in the literature, we propose that mechanisms grounded in neurophysiology should be more informative than the simple information processing perspective adopted by the target article.