Recently, side-by-side Recreational Off-Highway Vehicles (ROVs) have brought elements of the on-road vehicle occupant environment to the off-road trail-riding world. In general, ROV occupant protection during normal operation and in accident scenarios is provided predominately by a roll cage, seatbelts, contoured seats with seat backs, handholds, and other components. Typical occupant responses include both passive (inertial) and active (muscular) components. The objective of the current study was to evaluate and quantify these passive and active occupant responses during belted operation of an ROV on a closed course, as well as during 90-degree tip-over events. Passive occupant responses were evaluated using anthropomorphic test devices (ATDs) in 90-degree tip-overs simulated on a deceleration sled. Active occupant responses were evaluated using instrumented vehicles and volunteer occupants, wherein vehicle dynamics and gross occupant kinematics, muscle activity, occupant-to-vehicle pressure distributions and forces were quantified during riding on a closed course and during 90-degree tip-overs in a roll-spit fixture. For comparison, each test subject also performed a series of common physical activities while instrumented. Results indicated that the seatbelt was a critical component to the occupant protection system and that belted occupants were passively maintained within the occupant compartment during 90-degree tip-over events. Results also demonstrated that the active responses of the occupants further contributed to occupant stability and correlated significantly with the vehicle's lateral acceleration. Changes in patterns of muscle activation occurred approximate with the vehicle's change in turn direction and indicated a clear push-pull strategy of the occupant during left and right turns, respectively. Roll-spit testing demonstrated that the majority of lateral restraint was provided by the bucket seat, wherein modest lap belt forces constrained the pelvis within the seat contour and facilitated the generation of lateral contact forces. Forces exerted by occupants' extremities on the vehicle during a 90-degree tip-over were comparable to, or less than, the forces resulting from common physical activities.