Objective Patients with Unverricht‐Lundborg disease/EPM1 develop increasing locomotory disability or ataxia in the course of their disease. To test our hypothesis that negative myoclonus is the reason for this increasing ataxia, we investigated a possible correlation over time. Methods In 15 patients with EPM1who were confirmed to have a mutation in the CSTB gene, polygraphic video‐EEG–EMG recordings were performed in freely moving or standing patients. The criterion for the duration of the negative myoclonus was the measured length of the silent periods on the EMG. Results All 15 patients had documented negative myoclonus when standing and walking. The mean duration of silent periods significantly increased from 100 (SD: 19.1) ms at time point T1 to 128 (SD: 26.6) ms at T2 in seven of eight patients, based on two recordings and a mean interval of 12.8 (SD: 4.9) years. Using a cross‐sectional approach, all 15 patients were classified based on whether they were ambulatory, could walk with aid, or needed a wheelchair. Ambulatory patients had a mean duration of 97.3 (SD: 16.5) ms, patients who could walk with aid had a mean duration of 106.7 (SD: 16) ms, and patients who were wheelchair‐bound had a mean duration of 138 (SD: 23.6) ms. In addition to the prolongation of the silent periods, there was an observed increase in frequency of the negative myoclonus, becoming more continuous and tremulous. Significance Using simultaneous EEG/EMG recordings in freely moving or standing patients, we have shown that the locomotor disability or ataxia is due to negative myoclonus in voluntary innervated muscles. The reason for the progression is the prolongation of the silent periods as measured by the duration of the negative myoclonus and their increase in frequency.