Cumulative jerk as an outcome measure in nonambulatory Duchenne muscular dystrophy

Quantitative or semiquantitative outcome measures for patients with Duchenne muscular dystrophy (DMD) are important, as they can be objective indicators of the natural history of DMD; these measures also aid in the evaluation of the efficacy of various treatments. However, the most widely used standard outcome measures in patients with DMD, such as the North Star Ambulatory Assessment and the 6-min walk test, cannot be applied after patients have become nonambulatory. We evaluated the utility and reliability of accelerometric analysis of motor activity in nonambulatory patients with DMD.

We measured the motor activity of the upper extremity in 7 nonambulatory patients with DMD, by using an accelerometer attached at the wrist of the dominant arm. To eliminate gravitational acceleration, we measured the changes in acceleration between measurements. The root of the sum of squared values of the changes per unit time in the 3 axes of the accelerometer was defined as a jerk. The total sum of the jerk values obtained at a measurement frequency of 15.625 Hz for 8 h was defined as the cumulative sum of jerks (Cj).

The Cj values had significant and very strong or strong correlations with the Brooke Upper Extremity Scale (r_s = -0.973; p = 0.00023) and the arm function scores for the DMD Functional Ability Self-Assessment Tool (r_s = 0.810, p = 0.027). The values also had a very strong or strong correlation with the elbow flexion strength (nondominant arm: r = 0.931, p = 0.002; dominant arm: r = 0.750, p = 0.052).

Cj assessment is a useful method to evaluate motor activities in nonambulatory patients with DMD.