Leveraging Shared Control to Empower People with Tetraplegia to Participate in Extreme Sports
Ahmad Alsaleem, Ross Imburgia, Mateo Godinez, Andrew Merryweather, Roger Altizer, Tamara Denning, Jeffery Rosenbluth, Stephen Trapp, Jason Wiese · 2019 · Proceedings of the 21st International ACM SIGACCESS Conference on Computers and Accessibility (ASSETS 2019) · doi:10.1145/3308561.3353775
Summary
This paper describes the iterative design and field evaluation of Tetra-Ski, a power-assisted adaptive ski chair that enables people with tetraplegia to participate in alpine skiing — a high-risk, high-independence activity previously inaccessible to this population. Existing adaptive skiing equipment like the Dualski requires a tethered skier to control all turns and direction, making the experience entirely dependent. Tetra-Ski uses electric actuators mounted below the chair to move the skis, controlled via either a joystick or sip-and-puff (SipNPuff) controller. The system was developed over five years of iterative design beginning in 2014, involving inductive ideation sessions with physicians, psychologists, occupational and physical therapists, individuals with SCI/D, adaptive recreation camp participants, ski trainers, and a gold medal Paralympic skier. A key innovation is the shared-control scheme: rather than requiring fully independent piloting, Tetra-Ski distributes control responsibilities between the chair occupant (who controls turning direction) and a tethered skier (who manages ski wedge angle and speed via a wireless remote). Users can operate in basic mode (shared control) or advanced mode (full independent control), and can switch between modes dynamically during a ski run based on fatigue, terrain, or confidence. A Unity3D training simulator was also developed to prepare users before real-world skiing, using real-world elevation data and matching hardware response delays.
Key findings
A field study with eight participants (ages 20-67, all with tetraplegia or SCI) and interviews with three trainers at other sites demonstrated that Tetra-Ski is usable, enjoyable, and has positive psychosocial effects. All eight participants safely completed multiple runs on ski slopes. Self-reported anxiety either stayed the same or decreased from pre- to post-skiing. Participants’ SUS usability scores ranged from 65 to 97.5, with most falling in the "good" to "excellent" range. The PIADS (Psychosocial Impact of Assistive Devices Scale) showed positive scores across all three subscales — competence, adaptability, and self-esteem — for every participant. Participants expressed strong empowerment: "it gave me a sense of empowerment over my quadriplegia and control over my environment." The shared-control scheme proved more valuable than initially anticipated. It was originally conceived as a workaround for SipNPuff limitations, but ultimately enabled the full range of participants to have safe, satisfying experiences regardless of their physical abilities. P3, whom medical staff predicted could not safely control Tetra-Ski independently, was able to independently manage turns on shallow slopes in basic mode. The simulation improved participants’ performance between first and final trials, though SipNPuff users needed more simulation time than joystick users. A key usability challenge was the turning delay — the skis turn gradually to prevent tipping, but this non-instantaneous response frustrated joystick users who expected immediate feedback and led them to release and re-push the joystick rather than holding continuously.
Relevance
This paper makes important contributions to both adaptive sports technology and the broader field of shared-control assistive systems. The shared-control framework — where control responsibilities are dynamically distributed between user and assistant based on ability, fatigue, and conditions — has applications far beyond skiing, potentially extending to other outdoor recreation (kayaking, sailing, cycling) and daily activities. The finding that participants valued enjoyment and the feeling of control over complete independence challenges a common assumption in assistive technology design that full autonomy should always be the goal. The iterative evolution of the SipNPuff control scheme through three versions documents valuable lessons about designing binary input systems for physically demanding, real-time activities — including the critical insight that respiratory fatigue from over-sipping is invisible to trainers, unlike visible hand fatigue with joysticks. For accessibility practitioners, the case of P3 — who exceeded medical expectations on the slopes after the team designed for flexible interdependence rather than rigid ability thresholds — illustrates the importance of ability-based design over diagnosis-based exclusion.
Tags: tetraplegia · spinal cord injury · adaptive sports · shared control · sip-and-puff · joystick · physical accessibility · assistive technology · recreation · simulation