VR legs

VR legs is an informal term in virtual reality (VR) for the tolerance a user develops to in-headset locomotion without becoming nauseous. The Urban Dictionary describes it as "the metaphorical representation of being accustomed to immersion in a virtual reality environment, particularly in regards to virtual locomotion." VR legs are therefore related to simulator sickness and to the broader phenomenon clinicians and researchers call virtual reality sickness or cybersickness, and since the release to the general public of new VR devices, users and developers have been dealing with this problem and trying to find ways to overcome it.

The term borrows from "sea legs," the adaptation sailors develop to a ship's motion. As with sea legs, VR legs are not a property of the headset but of the person: most users acquire them gradually through repeated, controlled exposure, and a minority adapt slowly or not at all. Building VR legs reduces, but does not always eliminate, the discomfort a user feels during artificial movement.

Relationship to virtual reality sickness

Moving around in a VR environment is different than in normal gaming. After only a few minutes, some users will get sick to varying degrees when exploring the virtual worlds. Others, on the other hand, will not be afflicted by nausea at all. The user is visually closed off from his surroundings when using a head-mounted display (HMD). Instead, he visually experiences the rendered virtual environment. Although the images being displayed are convincing enough to give the sensation of locomotion, the body is in fact stationary. There is a discrepancy between the visual input and the other sensory information that the body receives, which leads to simulator sickness. A somewhat similar phenomenon to this is the motion sickness in cars that some people feel.

The symptoms overlap with classic motion sickness: nausea, headache, dizziness, sweating, eye strain, and disorientation. Researchers most often measure them with the Simulator Sickness Questionnaire (SSQ) published by Robert Kennedy, Norman Lane, Kevin Berbaum, and Michael Lilienthal in 1993, which scores 16 symptoms across three subscales, nausea, oculomotor, and disorientation, to give a total severity figure. Studies have found significant correlations between exposure time and VR sickness, with longer continuous sessions increasing the risk for susceptible users.

Sensory conflict and vection

The most widely cited explanation for both motion sickness and cybersickness is the sensory conflict theory, set out by James Reason and Joseph Brand in their 1975 book Motion Sickness. The theory holds that sickness arises when motion signals from the eyes, the vestibular system of the inner ear, and the body's proprioceptors are mismatched with one another and with what prior experience leads the brain to expect. In VR the conflict is typical: the visual scene signals self-motion while the vestibular organs, sensing no real acceleration, report that the user is sitting or standing still.

The visual illusion of self-motion in a stationary observer is called vection, and it is a leading cause of cybersickness in HMDs because the user is surrounded by a full-field moving image while the body stays largely still. Work by Joel Teixeira, Sebastien Miellet, and Stephen Palmisano (2022) found that the unexpectedness of vection predicted sickness better than its raw strength: participants who experienced unexpected self-motion were far more likely to report symptoms than those whose vection matched their expectations. This fits the sensory conflict framework, in which an expectancy violation, not just a sensory mismatch, drives the response.

Building VR legs

Although the feeling of nausea can come quickly when using a HMD, users generally seem to be able to increase the time they are able to use it without feeling sick in what is called "growing the VR legs." This is achieved by persisting with the VR game or VR experience that gave the user nausea, but doing so in small bits of time for a while until he can eventually stay in VR for a long time without getting sick. This way, the maximum amount of sickness felt will get lower with time, even if at first the user might feel that he will never be able to acclimate to the VR.

The mechanism behind this is what Reason and Brand called protective adaptation: in most people, the signs and symptoms of motion sickness diminish and eventually disappear after prolonged or repeated exposure to a provocative stimulus, as the nervous system reweights its reliance on the conflicting senses. A 2022 study by Isayas Adhanom, Savannah Halow, Eelke Folmer, and Paul MacNeilage exposed susceptible, inexperienced users to a virtual labyrinth in which optic flow was ramped up over three consecutive days; sickness scores fell across sessions, and, importantly, the adaptation transferred to a separate, realistic cityscape environment, with average discomfort dropping from 3.08 to 1.71 between the first and last days.

In online forums, users of VR devices have been sharing their experiences with different VR games and recommendations about how to decrease the simulator sickness and get their virtual reality legs. Some users advise not to prolong the playtime until the feeling of nausea becomes too uncomfortable. They recommend staying in the virtual environment just a bit after disorientation starts, in order to develop adaptation. Beyond that, a break is suggested. Other tips include ginger tea or peppermint tea to help with the nausea. Besides this, it is advised that the users adjust the settings of the device and software, like checking if the head tracking is working properly and if the frames per second (FPS) are good enough for a smooth experience. While some users got their VR legs two weeks after starting using VR devices, others needed a month, or even 5 or 6. This adaptation led to an increase of time spent playing without noticeable side effects. It is also of notice that for different players, games might affect them differently. Also, there are some reports that getting the VR legs isn't something permanent, with the VR sickness sensitivity increasing in direct relation to the period of time without using the VR equipment.

Individual variation

Susceptibility to VR sickness, and therefore the ease of building VR legs, varies widely between people. Reported factors include prior VR experience, general susceptibility to motion sickness, age, and the strength of the optic flow in a given application. Because no single comfort technique works for everyone, developers commonly offer several locomotion options and let the player choose. Meta classifies content on its Horizon Store with three comfort ratings, "Comfortable" for experiences appropriate for most people that avoid camera movement and player motion, "Moderate" for those with some camera movement or motion, and "Intense" for those not appropriate for most people, particularly newcomers; a fourth label, "Unrated," warns that content may be intense. Meta advises new users to begin with Comfortable titles before progressing.

Since VR sickness and adaptation is a young phenomenon, there isn't a general consensus about how long it takes most people to acclimate to VR. It is expected that, as VR technology progresses, these problems will be reduced, since hardware improvements such as higher refresh rates, lower latency, and wider tracking lessen the sensory conflict in the first place.

Comfort techniques

Due to the nature of VR, walking and running often isn't as comfortable as driving and flying. Movement is an essential part of games and game design, but it has been a challenge to fully overcome the problems of moving in a virtual environment while the body of the user stays still. Due to this, developers have experimented with new locomotion techniques, such as the method of "blinking" from one place to another. Although it is comfortable, this specific method is not the most immersive one. Other techniques are being explored and applied so the user can feel immersed in the virtual experience and not be nauseous with its exploration, although there isn't a global solution for locomotion in VR games. While devices like the omni-directional treadmill Virtuix Omni, or VR parks such as The VOID give a good immersive experience, reducing the sickness side effects, these technologies are still not completely accessible to the average user.

The most common software comfort techniques work by reducing or removing the optic flow that triggers vection. Teleportation instantly moves the player between positions, often with a brief fade, so that no continuous visual motion contradicts the still vestibular signal. Snap turning rotates the view in discrete steps or increments instead of smoothly, avoiding the continuous rotation that is one of the strongest nausea triggers; Meta recommends offering snap-turn options of 30, 45, or 90 degrees because users have strong preferences about how far they turn. Vignetting, also called tunneling or blinders, crops or darkens the periphery of the field of view during movement, where peripheral optic flow contributes heavily to vection; some advanced implementations dynamically narrow the view only when the camera moves fast. Smooth (continuous) locomotion is the least comfortable of the common options for new users, and matching virtual walking and running speeds to real-world rates reduces excess optic flow when it is used.

One technique is roomscale locomotion, in which the user moves around within a large-scale tracking volume. Oculus and Sony already offer large-scale tracking volumes, but are focusing on a balance between standing and seated gameplay. Another one is to use a vehicle that allows the user to assume a natural sitting position while also reducing the discrepancy between what is seen and what is felt. These cockpits are used in racing or flying games, for example. It's also possible to use a full vehicle instead of only the cockpit, provided there's enough space available. As an example, the 2016 title Hover Junkers, by StressLevelZero, is a VR game that allows users to play with the game's hover boats, as a roomscale vehicle. Teleportation also addresses some things that induce nausea in VR, like the yaw-stick problem, or when using the right stick of the controller to turn the point of view. According to an article on uploadvr.com, "YAW refers to movement along a vertical axis, such as turning the nose of an aircraft. YAW is also one of the quickest ways to get someone sick in VR. When you connect a game's YAW control to a stick, rather than to your head motion, it creates an uncomfortable disconnect that Oculus' CTO John Carmack has gone as far as to call 'VR poison.'" The teleportation method has been used to effect in several virtual spaces like AltspaceVR, or Cloudhead's Blink, and the game Budget Cuts uses it as a gameplay mechanic. Flight in VR has been reported to reduce discomfort, providing the ability to simulate smooth, gaze-based forward motion without dizziness. An example of this is Ubisoft's game Eagle Flight, which attempts to provide intense action while also remaining comfortable to use. VR comfort mode makes use of a sort of micro-teleportation in which the smooth movement made with the controller's right stick is replaced with a "snap-to" turn. This allows for a more pleasant experience in virtual reality while in a seated position. There's also the possibility of using gaze-based locomotion in VR comfort mode, but it's less natural than "snap-to" turns. Finally, there's floating head, which is a VR third-person viewpoint. This method allows for good VR gameplay without the locomotion problems that first-person titles have. It's a good way for VR newcomers to play something more familiar with traditional games and ease the transition into the new format.

As a final example, the studio Huge Robot created the Freedom Locomotion System to try to resolve the problem of comfortable and immersive VR locomotion. It is a locomotion package that comes close to solving the issue of VR locomotion with the current practical limitations of VR. This system has what has been called CAOTS (Controller Assisted On the Spot) movement, which is a sort of "run-in-place" movement system. According to George Kong, Huge Robot's Director, "it lets players comfortably and immersively move while leaving their hands free for interactions with the virtual world (especially important for games where you might regularly wield a weapon like a gun or sword)." Besides CAOTS, the system has other subsystems that offer different modes of locomotion and methods of interaction with the virtual world.

The realization of these and other technologies or methods to reduce the problem of nausea induced by locomotion in VR will, in principle, shorten the time in which a user gets his VR legs.