Additional Supporting Information may be found in the online version of this article.

MDS_23505_sm_suppinfofig1.tif32726KSupporting Information Figure 1: Samples of high frequent discharges (HFD) A: Repetitive short trains (up to 5) each comprising about 10 discharges with a frequency of 300 Hz at rest. B: HFD occurred with frequencies of 250-300 Hz and a variable duration during spontaneous EMG activity (50-100 ms; upper line) and at rest (5-6 s; bottom line). C: Testing of F waves following right peroneal and tibial nerve electrical stimulation. HFD occurred at the expected F wave latency (of about 54ms) (Figure 1C, left) or shortly thereafter (Figure 1C, right). During the F wave latency HFD never occurred. From left to right, the three vertical bars of the figure indicate the following: M wave latency, split-screen with amplification of 10 mV/div, F wave latency.
MDS_23505_sm_suppinfovid1.wmv26760KSupporting Information Video 1. 87-year-old patient with generalized stiffness and myotonic-like slow muscle relaxation involving all muscle groups, excluding the oculomotor and oropharyngeal muscles. Gluteal and hamstring muscles are hypertrophied. Spontaneously and upon percussion there is muscle twitching of the brachial and lower leg muscles. There is also dimpling of the thenar eminence following percussion. Repetitive exercise and walking for a few meters improves symptoms (“warm up”).
MDS_23505_sm_suppinfovid2.wmv14291KSupporting Information Video 2. EMG of the gastrocnemius muscle showed different types of spontaneous and after-muscle-contraction high-frequency discharges, i) monomorphic high frequency discharges (HFD) (approximetaly 10 potentials at 250-300 Hz), ii) crescendo / decrescendo bursts with short (200-300ms) and longer (5-6 seconds) duration, at 250-300 Hz.
MDS_23505_sm_suppinfovid3.wmv411KSupporting Information Video 3. Ultrasound of the right gastrocnemius and soleus muscle. M-Mode demonstrates involuntary movements of the gastrocnemius muscle. The movements in the muscle lasted about 300 ms with a range from 115- 600 ms.

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