Walking in orthostatic tremor modulates tremor features and is characterized by impaired gait stability

Primary orthostatic tremor (OT) is characterized by high-frequency lower-limb muscle contractions and a disabling sense of unsteadiness while standing. Patients consistently report a relief of symptoms when starting to ambulate. Here, we systematically examined and linked tremor and gait characteristics in patients with OT. Tremor and gait features were examined in nine OT patients and controls on a pressure-sensitive treadmill for one minute of walking framed by two one-minute periods of standing. Tremor characteristics were assessed by time-frequency analysis of surface EMG-recordings from four leg muscles. High-frequency tremor during standing (15.29 ± 0.17 Hz) persisted while walking but was consistently reset to higher frequencies (16.34 ± 0.25 Hz; p < 0.001). Tremor intensity was phase-dependently modulated, being predominantly observable during stance phases (p < 0.001). Tremor intensity scaled with the force applied during stepping (p < 0.001) and was linked to specific gait alterations, i.e., wide base walking (p = 0.019) and increased stride-to-stride fluctuations (p = 0.002). OT during walking persists but is reset to higher frequencies, indicating the involvement of supraspinal locomotor centers in the generation of OT rhythm. Tremor intensity is modulated during the gait cycle, pointing at specific pathways mediating the peripheral manifestation of OT. Finally, OT during walking is linked to gait alterations resembling a cerebellar and/or sensory ataxic gait disorder.