A study by Slater, et al., [1995] indicated that naive subjects in an immersive virtual environment experience a higher subjective sense of presence when they locomote by walking-in-place (virtual walking) than when they push-button-fly (along the floor plane). We replicated their study, adding real walking as a third condition. Our study confirmed their findings. We also found that real walking is significantly better than both virtual walking and flying in ease (simplicity, straightforwardness, naturalness) as a mode of locomotion. The greatest difference in subjective presence was between flyers and both kinds of walkers. In addition, subjective presence was higher for real walkers than virtual walkers, but the difference was statistically significant only in some models. Follow-on studies show virtual walking can be substantially improved by detecting footfalls with a head accelerometer. As in the Slater study, subjective presence significantly correlated with subjects' degree of association with their virtual bodies (avatars). This, our strongest statistical result, suggests that substantial potential presence gains can be had from tracking all limbs and customizing avatar appearance. An unexpected by-product was that real walking through our enhanced version of Slater's visual-cliff virtual environment (Figure 1) yielded a strikingly compelling virtual experience - The strongest we and most of our visitors have yet experienced. The most needed system improvement is the substitution of wireless technology for all links to the user. Copyright ACM 1999.

/pdf/walking-walking-in-place-flying-in-virtual-environments-5e18hc0psi.pdf

Walking > walking-in-place > flying, in virtual environments

There are many different techniques for allowing users to specify locomotion in human-scale, immersive virtual environments. These include flying with a hand-controller, using a treadmill, walking-in-place, and others. 
Real walking, where the user actually and physically walks in the lab, and virtually moves the same distance and in the same direction in the virtual scene, is better than flying. It is more input-natural, does not require learning a new interface, results in a greater sense of presence, and theoretically results in less simulator sickness. 
One serious problem with real walking, however, is that the size of the virtual scene is limited by the size of tracked area. For example, for an architect to really walk in a virtual prototype of a house, the tracked area must be as large as the house. This requirement makes real walking infeasible for many facilities and virtual scenes. 
To address this limitation, I have developed Redirected Walking, which by interactively and imperceptibly rotating the virtual scene around her, makes the user turn herself. Under the right conditions, Redirected Walking would cause the user to unknowingly walk in circles in the lab, while thinking she is walking in a straight and infinitely long path in the virtual scene. 
In this dissertation I develop Redirection, discuss its theoretical and physiological underpinnings, and presents results to show that it can be used: (1) to make the user turn themselves, (2) without causing the user to be aware of Redirection, (3) without unacceptably increasing the user's level of simulator sickness and, most importantly, (4) to useful effect: (A) In head-mounted display systems, the user can experience a virtual scene larger than the lab while also having the benefits of real walking. (B) In an open-backed, three-walled CAVE, users can have the increased presence and input-naturalness normally associated with a fully enclosed CAVE. 
I also present guidelines for VE practitioners wishing to use Redirection, based on the theory and observations reported in this dissertation.

Redirected walking

This target article presents a critical survey of the scientific literature dealing with the speed/accuracy trade-offs in rapid-aimed movements. It highlights the numerous mathematical and theoretical interpretations that have been proposed in recent decades. Although the variety of points of view reflects the richness of the field and the high degree of interest that such basic phenomena attract in the understanding of human movements, it calls into question the ability of 'many models to explain the basic observations consistently reported in the field. This target article summarizes the kinematic theory of rapid human movements, proposed recently by R. Plamondon (1993b; 1993c; 1995a; 1995b), and analyzes its predictions in the context of speed/accuracy trade-offs. Data from human movement literature are reanalyzed and reinterpreted in the context of the new theory. It is shown that the various aspects of speed/accuracy trade- offs can be taken into account by considering the asymptotic behavior of a large number of coupled linear systems, from which a delta- lognormal law can be derived to describe the velocity profile of an end-effector driven by a neuromuscular synergy. This law not only describes velocity profiles almost perfectly, it also predicts the kinematic properties of simple rapid movements and provides a consistent framework for the analysis of different types of speed/accuracy trade-offs using a quadratic (or power) law that emerges from the model.

/pdf/speed-accuracy-trade-offs-in-target-directed-movements-54xzivkka1.pdf

Speed/accuracy trade-offs in target-directed movements.

A hypothesis about sensorimotor integration (the λ model) is described and applied to movement control and kinesthesia. The central idea is that the nervous system organizes positional frames of reference for the sensorimotor apparatus and produces active movements by shifting the frames in terms of spatial coordinates. Kinematic and electromyographic patterns are not programmed, but emerge from the dynamic interaction among the system s components, including external forces within the designated frame of reference. Motoneuronal threshold properties and proprioceptive inputs to motoneurons may be cardinal components of the physiological mechanism that produces positional frames of reference. The hypothesis that intentional movements are produced by shifting the frame of reference is extended to multi-muscle and multi-degrees-of-freedom systems with a solution of the redundancy problem that allows the control of a joint alone or in combination with other joints to produce any desired limb configuration and movement trajectory. The model also implies that for each motor behavior, the nervous system uses a strategy that minimizes the number of changeable control variables and keeps the parameters of these changes invariant. Examples are provided of simulated kinematic and electromyographic signals from single- and multi-joint arm movements produced by suggested patterns of control variables. Empirical support is provided and additional tests of the model are suggested. The model is contrasted with others based on the ideas of programming of motoneuronal activity, muscle forces, stiffness, or movement kinematics.

The origin and use of positional frames of reference in motor control

For the last 20 years, it has been hypothesized that well-coordinated, multijoint movements are executed without complex computation by the brain, with the use of springlike muscle properties and peripheral neural feedback loops. However, it has been technically and conceptually difficult to examine this "equilibrium-point control" hypothesis directly in physiological or behavioral experiments. A high-performance manipulandum was developed and used here to measure human arm stiffness, the magnitude of which during multijoint movement is important for this hypothesis. Here, the equilibrium-point trajectory was estimated from the measured stiffness, the actual trajectory, and the generated torque. Its velocity profile differed from that of the actual trajectory. These results argue against the hypothesis that the brain sends as a motor command only an equilibrium-point trajectory similar to the actual trajectory.

https://science.sciencemag.org/content/sci/272/5258/117.full.pdf

Equilibrium-Point Control Hypothesis Examined by Measured Arm Stiffness During Multijoint Movement

The objective of this study was to determine the extent to which subjects modulate their elbow joint mechanical properties during ongoing arm movement. Small pseudo-random force disturbances were applied to the wrist with an airjet actuator while subjects executed large (1 rad) elbow joint movements. Using a lumped parameter model of the muscle, tendom and proprioceptive feedback dynamics, a time-varying system identification technique was developed to analyze the phasic changes in the elbow joint's mechanical response. The mechanical properties were found to be time-varying, and well approximated by a quasi-linear second-order model. The stiffness of the arm was found to drop during movement. The arm was always underdamped, with the damping ratio changing during movement. Inertia estimates were constant and consistent with previous measurements. Overall, the moving arm was found to be very compliant, with a peak stiffness value less than the lowest value measured during posture, and a natural frequency of less than 3 Hz. Changing the speed of movement, or the load from gravity, changed the stiffness measured, but not in strict proportion to the change in net muscle torque.

Time-varying stiffness of human elbow joint during cyclic voluntary movement

The inertial force due to the acceleration of a locomotion interface is identified as a difference between virtual and real-world locomotion. To counter the inertial force, inertial-force feedback was implemented for the Treadport, a locomotion interface. A force controller was designed for a mechanical tether to apply the feedback force to the user. For the case of the user accelerating forward from rest, psychophysical ex periments showed that subjects preferred inertial-force feedback to a spring-feedback force proportional to position or to position control, where the force feedback maintained a force of zero on the subject.

/pdf/inertial-force-feedback-for-the-treadport-locomotion-4e91zady8w.pdf

Inertial-Force Feedback for the Treadport Locomotion Interface

https://asmedigitalcollection.asme.org/IMECE/proceedings-pdf/doi/10.1115/IMECE2000-2446/6792193/1293_1_imece2000-2446.pdf

Design Specifications for the Second Generation Sarcos Treadport Locomotion Interface

This paper investigates whether torso force feedback on treadmillstyle locomotion interfaces can substitute for treadmill tilt to simulate walking on smooth inclines. The experimental platform is t...

Torso Force Feedback Realistically Simulates Slope on Treadmill-Style Locomotion Interfaces

This paper describes a perturbation method for the identification of linear time-varying systems with an unknown input (voluntary joint input) using ensemble data. The method separates the unknown input and the perturbation through high-pass filtering and recasts the multi-input single-output system identification into single-input single-output system identification. The method is robust to intertrial variation, and can track changes of system dynamics up to 5 Hz. Analysis and simulation are given for the conditions similar to those for the human arm experiments. Experiments show that mechanical properties of the human elbow joint change with voluntary movement speed and that the mean stiffness with voluntary movement is in the range of the posture and is higher than reported before.

/pdf/a-robust-ensemble-data-method-for-identification-of-human-pzx45ht4un.pdf

Yangming Xu

Papers

Time-varying stiffness of human elbow joint during cyclic voluntary movement

Inertial-Force Feedback for the Treadport Locomotion Interface

Design Specifications for the Second Generation Sarcos Treadport Locomotion Interface

Torso Force Feedback Realistically Simulates Slope on Treadmill-Style Locomotion Interfaces

A robust ensemble data method for identification of human joint mechanical properties during movement