Timing of preparatory landing responses as a function of availability of optic flow information
Introduction
The study of landing responses has not only practical and functional implications, but also theoretical significance for understanding vestibular and visual motor control in humans [8], [9], [10], [22], [23] and animals [4], [20], [28]. Gibson [6] suggested that when people are confronted with an object in a collision course, the triggering and modulation of their motor actions is dependent on the perceptual visual input that occurs between the interaction of the moving observer and objects or nearby surfaces. However, the triggering mechanism will occur only if the observer perceives its necessity (i.e., if he/she perceives an ‘affordance’ [31]). Affordances may perceive via optic invariants such as tau (τ) [13], [14], [15], which provides information about the time to contact (Tc) with a surface in a collision course. Tau is optically defined as the inverse of the rate of dilation of the environmental layouts projected onto the retina of the eyes. Its functional significance lies in that at critical values of τ (τmargin), a coordinated set of motor actions is triggered. Support for the implementation of such a strategy during free falls came from the animal study of Lee and Reddish [16]. These authors observed that gannets plummeting into the sea avoid crashing by using visually perceived time to contact information. Their measurements showed that the birds begin their wing folding actions at τmargin. However, time to contact information may be used without necessarily implementing a τ heuristic [27]. Houseflies tune the leg extension movements before touchdown on a surface based on visually perceived time to contact information, but may not use that strategy [32]. Srinivasan et al. [28] showed that honeybees modulate landing speed on flat surfaces by implementing a ‘constant time to contact’ strategy, and not a strategy based on critical values of τ. They keep the angular velocity of the expanding visual image at a constant value by decreasing the speed of descent at a rate that is proportional to the decrease in horizontal speed.
In humans, Sidaway et al. [26] argued that a τmargin strategy based on time to contact information is implemented when subjects land from self-initiated falls of different heights. Such an early preparation for landing is based on prospective visual information about the moment of touchdown, and should allow for the anticipatory build-up of tension in the relevant musculature in order to reduce the risk of injury at collision with the ground. However, empirical support for such an assumption is rather limited for normal heights of fall. Santello et al. [25] found no differences between vision and no-vision landings in the preparatory EMG responses during the flight, but reported that joint stiffness and impact forces increased in no-vision trials after touchdown. This may suggest that a visual modulation of the preparatory muscular activity may not be used in landings from regular heights of fall in known experimental environments. Instead, the landing preparation may be structured (pre-programmed) and triggered relative to the moment of the initiation of the fall.
The possibility that the triggering response takes place at a constant timing after the start of the fall is an interesting hypothesis. We refer to this alternative as the ‘constant time from release’ hypothesis based on the assumption that the temporal muscle patterns associated with the free-fall are not dependent on the optic flow of information arriving to the eyes.
In the present study, we used a landing paradigm comparable to that of Santello et al. [25] and investigated temporal patterns of muscular activation under conditions that maximize, reduce or completely preclude the availability of optic flow information. This was done by instructing subjects about the gaze direction before the free-fall started, by maintaining the same gaze direction during the fall, and by wearing appropriate goggles that constrained gaze direction but did not prevent vision. We were particularly interested in determining what stimulus triggers the EMG activity and how this activity is modulated during the flight. An additional goal of this study was to test the assumption that early pre-landing actions contribute to the reduction of the impacts at touchdown.
Section snippets
Subjects and design
Following approval of the research protocol by the institutional review board, five healthy men (20.4 ± 0.6 years; 178.7 ± 6.5 cm: 77.4 ± 8.1 kg) volunteered to serve as subjects for this investigation. All subjects were experienced in landing performances (>100 parachute descents each), and gave written informed consent to participate in the study after receiving explanations of all the procedures, risks and benefits. During the experiment, they performed landing jumps from 10–30 cm, 30–50 cm, 50–70
EMG responses
The preparatory EMG was expected to change its timing and amplitude as a result of the increase in the height of fall [1]. However, for the questions of concern of the present study regarding visuo-motor timing [16] and to be consistent with the previous research [10], [22], only the time of onset the EMG was used to assess differences in the patterns of preparatory muscle activation as previously done in comparable investigations [17], [26]. The descriptive results are presented in Table 1 for
Discussion
The present investigation was based on the premise that landings are essential in many basic locomotive tasks. However, the interaction between visual guidance and the pre-landing motor actions remains unclear. In an attempt to investigate the relationship between vision and action, optic flow was manipulated under three different conditions and its effect on muscle preparatory actions were examined (i.e., the trigger and time modulation of the muscular activity during the flight). In addition,
Conclusion
The results of this study using a small sample of highly experienced individuals support the hypothesis that individuals use a pre-programmed set of responses. The trigger of these responses is timed with respect to the initiation of the descent. The results also suggest that the mere availability of optic flow does not guarantee a reduction of impacts at touchdown. Results from similar experiments carried out previously using normal inexperienced subjects led to similar conclusions. A
Dr. Dario Liebermann is an Assistant Professor in the Tenior track working with the Sacker Faculty of Medicine at the University of Tel Aviv since October 2000. He has received his M.Sc. in Kinesiology (Simon Fraser University). The Ph.D. degree in Applied Mathematics and Computer Sciences (Weizmann Institute of Science). Post-Doc research in Neurosciences and Kinesiology (University of Calgary) followed. He worked in the motor control area in diverse topics such as arm kinematic modeling, the
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Dr. Dario Liebermann is an Assistant Professor in the Tenior track working with the Sacker Faculty of Medicine at the University of Tel Aviv since October 2000. He has received his M.Sc. in Kinesiology (Simon Fraser University). The Ph.D. degree in Applied Mathematics and Computer Sciences (Weizmann Institute of Science). Post-Doc research in Neurosciences and Kinesiology (University of Calgary) followed. He worked in the motor control area in diverse topics such as arm kinematic modeling, the visual control of motion and inter-muscular coordination. In addition, he has been interested in the motion patterns of elite athletes as models of optimal control. He has authored over 30 peer-reviewed publications and has over 30 conferences presentations. Dr. Liebermann investigated methods and technology designed to enhance motor performance, and currently focuses on research and development of evidence-based motor strategies for the rehabilitation of young and adult clinical populations. He lectures on diverse movement science topics at the Physical Therapy Department – School of Health Professions (University of Tel Aviv), where he heads the Movement Science Laboratory.
Dr. Jay Hoffman has been working as an Associate Professor with The College of New Jersey’s Department of Health and Exercise Science since January 2000. Recent honors and awards bestowed upon Dr. Hoffman include: Educator of the Year National Strength and Conditioning Association (NSCA), 2003 and Neag School of Education, Outstanding Alumni Research Award (University of Connecticut), 2003, and Most Outstanding Junior Investigator, NSCA 2000. In addition, he has recently been elected to the board of the NCSA and is an Associate Editor of the Journal of Strength and Conditioning Research, Dr. Hoffman is a fellow of the American College of Sports Medicine, and has published over sixty articles in peer-reviewed journals. He also reviews articles for eight journals in his field. Further sharing his research and findings, Dr. Hoffman has lectured at eighty national and international conferences and meetings.