Individuals with cerebellar ataxia often face significant challenges in controlling reaching, especially when multi-joint movements are involved. This study investigated the effects of kinematic and dynamic demands on reaching movements by individuals with cerebellar ataxia and controls using a virtual reality task. Participants reached target locations designed to elicit a range of coordination strategies between shoulder and elbow joint movements. The hand paths of the cerebellar group exhibited greater initial reaching direction error, larger trajectory curvature, and more inter-subject variability compared to controls. Kinematic simulations indicated that early hand movement errors were sensitive to the required onset times and rates of joint movements and were most impaired when opposite direction joint movements were required (e.g., elbow extension with shoulder flexion). This highlights significant disruptions in multi-joint motion planning and/or feedforward control in the cerebellar group. Dynamic analysis showed that cerebellar participants' movements were more impaired in reaching directions where interaction torques would normally assist the desired elbow and shoulder movements, which required them to rely more on muscle torques to move. These reach directions were also those that required opposite direction joint movements. Overall, our data suggest that reaching deficits in cerebellar ataxia result from 1) the early-phase motion planning deficits that are exacerbated by stringent timing coordination requirements and 2) the inability to compensate for interaction torques, particularly when they assist the intended movement.