Spatial variation and inconsistency between estimates of onset of muscle activation from EMG and ultrasound

Study abstract: Delayed onset of muscle activation is a descriptor of impaired motor control. Activation
onset can be estimated from electromyography (EMG)-registered muscle excitation and
from ultrasound-registered muscle motion, which enables non-invasive measurements in
deep muscles. However, in voluntary activation, EMG- and ultrasound-detected activation
onsets may not correspond. To evaluate this, ten healthy men performed isometric elbow
flexion at 20% to 70% of their maximal force. Utilising a multi-channel electrode
transparent to ultrasound, EMG and M(otion)-mode ultrasound were recorded
simultaneously over the biceps brachii muscle. The time intervals between automated and
visually estimated activation onsets were correlated with the regional variation of EMG
and muscle motion onset, contraction level and speed. Automated and visual onsets
indicated variable time intervals between EMG- and motion onset, median (interquartile
range) 96 (121) ms and 48 (72) ms, respectively.  In 17% of trials (computed analysis) or
23% (visual analysis), motion onset was detected before local EMG onset. Multi-channel
EMG and M-mode ultrasound revealed regional differences in activation onset, which
decreased with higher contraction speed (Spearman ρ≥0.45, P<0.001). In voluntary
activation the heterogeneous motor unit recruitment together with immediate motion
transmission may explain the high variation of the time intervals between local EMG- and
ultrasound-detected activation onset.

Data description:

EMG data: folder includes the EMG, torque and synchronization signal data as .otb files. The respective program can be downloaded without costs from http://www.otbioelettronica.it/index.php?lang=en. Data consist of two series (Misome2 and Misome3) of isometric trials at different force levels. The first three digits refer to the subject number.

M-mode ultrasound data: folder includes the M-mode clips of all recorded trials in .tvd format. The respective program can be downloaded for free at http://www.telemedultrasound.com/download/software-downloads/?lang=en. In addition, images of activation onset in DICOM format are provided. The data are sorted for subjects and series (Misome2 and Misome3). The following explanation refers to the filenames of the DICOM images. Usually, the M-mode trace started at the left side synchronously with the synch signal. In this case the rightmost frame of the trace is missing to indicate that the left edge represents the start of the trace. If the file name includes “ons50”, the frame includes the 50. frame = the rightmost frame, still starting with the synch signal. If the filename includes on100, the rightmost frame is the 100. frame and the duration of 50 frames must be added to the visible onset time. Filenames that include “basel” refer to frames that represent a proper delineation of the baseline.

Excel data sheet: Data sheet that includes the computed and visual EMG, M-mode ultrasound and torque onsets, the rate of torque development and the differences between the different types of onsets. The column headers explain the data type, most comprehensively in the first computed data sheet. The colors facilitate the orientation with blue columns referring to torque onsets and green columns referring to ultrasound onsets. The violet EMG channels are those in vicinity to the ultrasound beam. In the second version of each sheet the 5% slowest trials are separated.