Testing protocol

A custom‐built finger extension device secured the forearm and hand with Velcro straps and a torque cell (RTS‐500, Transducer Techniques; Temecula, CA) was placed in‐line with the axis of the metacarpophalangeal joints with an attached comfort bar (Fig. 1A). The bar was positioned above the fingers in near full extension so that immediate effort of finger extension registered torque production, similar to clinical performance of McS with a manual strength test. A technician, blinded to clinical diagnosis, performed the McS evaluation either: (1) clinically (by hand), and (2) with the device in which downward pressure was placed on the lever that depressed the bar positioned above the fingers. The discrete McS score was recorded (grade 0–3; 0 = no weakness; 3 = marked weakness).

McArdle test setup and sample decomposed electromyography. (A) The arm was secured in a custom‐built device for measurement of finger extensor torque production. The hand was positioned so that the axes of rotation of the metacarpophalangeal joints were in‐line with the rotation axis of the torque cell. The torque cell (blue arrow) measured torque as the subject pushed against a padded comfort bar. Two gyroscopes in the headband measured neck flexion/extension (yellow arrow). The lever was utilized by the examiner to apply a downward force that the subject was asked to resist (red arrow). The EMG electrodes were place on the muscle belly of the extensor digitorum of the arm in the McArdle device. (Adapted from Schilaty et al. Biomechanical muscle stiffness measures of extensor digitorum explain potential mechanism of McArdle Sign. Clin Biomech. 2021; 82:105277). (B) Sample dEMG data demonstrates characteristic differences observed between subjects. The black line represents force production (30% MVIC target). The colored lines represent recruitment of a particular motor unit, average firing rate, and derecruitment of the motor unit. Comparison of the two signals may suggest “MU disorganization” of one group compared to another. [Colour figure can be viewed at wileyonlinelibrary.com]

Patients were fitted with a two‐axis gyroscopic headband to measure accurate flexion/extension position of the neck (Fig. 1A). All gyroscopic and torque cell data were collected at 1 kHz via proprietary LabVIEW programmed software (v2016; National Instruments, Austin, TX).

After preparation of the electrode site with dry shaving and isopropyl alcohol, a 5‐pin dEMG electrode (Delsys; Natick, MA) was affixed over the belly of the extensor digitorum according to SENIAM standards. 16 A signal quality check was performed to ensure EMG noise level was <20 μV. Two paradigms of muscle contraction were utilized: Isoinertial (patient extended fingers against constant resistance of continuous downward movement of the bar at a constant rate by lab technician) and isometric. As the dEMG technology was not capable to track motor units during dynamic contractions, isometric contractions were utilized (Fig. 1B). For the isoinertial paradigm, five paired successive trials of neck extension followed by neck flexion were performed. The first set of measurements in extension/flexion were discarded to mitigate learning effects. Peak torque values for each trial were extracted from raw data using MATLAB software (MathWorks, Inc.; v2016a). Reduction in torque between the paired trials in neck extension and flexion position was calculated as follows: [(Torque_ext − Torque_flex)/Torque_ext*100]. This value was averaged over the last four trials (percent difference IsoTorque). For the isometric paradigm, the patients first performed three maximal voluntary isometric contractions (MVIC) of finger extension. Patients then performed three trials of isometric contractions with the neck randomized across: neutral, flexion, or extension. 3 As the neutral position did not demonstrate any statistical difference from extension, the two were combined for analysis. Each isometric contraction followed a trapezoidal waveform with a 3‐sec ramp up, 10‐sec steady phase (30% MVIC), and a 3‐sec ramp down. Rest of 40 sec was allotted between each isoinertial/isometric trial. All EMG data were oversampled at 20 kHz to avoid introduction of significant phase skew across channels.