Quantitative analysis of the “warm‐up” phenomenon in myotonic dystrophy type 1

Abstract

To quantitate improvement in hand‐grip myotonia and muscle strength (i.e., the “warm‐up” phenomenon) in myotonic dystrophy type 1 (DM1), six successive, standardized maximum voluntary isometric contractions (MVICs) were recorded on 2 separate days using a computerized isometric hand‐grip myometer in 25 genetically confirmed DM1 patients and in 17 normal controls. An automated computer program placed cursors along the declining (relaxation) phase of the MVICs at 90%, 50%, and 5% of peak force (PF) and calculated relaxation times (RTs) between these points. Mean 90% to 5% RT (a measure of myotonia) rapidly declined from 2.5 s in MVIC 1 to 0.8 s in MVIC 6 (warm‐up = 1.7 s) in DM1; in controls, it remained 0.4 s for all six MVICs (warm‐up = 0). In DM1, 70% of warm‐up occurred between MVIC 1 and 2, almost exclusively in the terminal 50% to 5% phase of muscle relaxation. Day 1 warm‐up was highly correlated with the severity of myotonia, and with day 2 warm‐up. Improvement in myotonia was not accompanied by either transient paresis or improvement in PF. We conclude that, with this testing paradigm: warm‐up of myotonia in DM1 can be reliably measured; is proportional to severity of myotonia; occurs rapidly, being most prominent between the first and second grips; mainly results from shortening of the terminal phase of muscle relaxation; and is not accompanied by significant warm‐up in force output. Muscle Nerve, 2005