Introduction to HD-EMG

High-Density Electromyography (HD-EMG, also called high-density surface EMG or HDsEMG) is a non-invasive method that measures muscle activity's spatial and temporal patterns through EMG signals recorded on the skin's surface. The EMG signals are acquired using at least four closely spaced electrodes from different locations over the muscle(s) of interest [1].

HD-EMG signals offer valuable insights into muscular activation, making it useful for neuromuscular research. The spatiotemporal signals provide information about regional activation of the muscles, including the magnitude and the size of the active region(s) of the muscle. The spatiotemporal recording characterizes how the action potential propagates along the muscle fiber, enabling estimation of muscle fiber properties such as average muscle fiber conduction velocity and innervation zone, the region where the alpha motor neuron attaches to the muscle fiber. Using decomposition methods, single motor unit activity can be extracted to represent the neural drive to the muscle. HD-EMG has various research applications, including rehabilitation, neuromuscular control, biomechanics, and signal processing.

What is the origin of HD-EMG signals?

HD-EMG signals originate from the electrical activity of motor units in skeletal muscles, which are composed of a motor neuron and the muscle fibers it innervates. The greater the motor neuron stimulation (either through higher frequency firing or greater motor unit recruitment), the larger the electrical signal. These signals are detected by electrodes placed on the skin surface. At the neuromuscular junction (motor end plate), the motor neuron connects with each muscle fiber, and the action potential propagates through the muscle from the innervation zone.

A motor unit (Figure 1) is the smallest functional unit of a muscle, comprising a motor neuron and its innervated muscle fibers. Motor units are recruited in a specific order as described by Henneman’s size principle, starting with small units for low force and progressing to larger ones as the required force increases [2].

The HD-EMG signals are composed of motor unit action potentials (MUAPs), which are the cumulative action potentials produced by all muscle fibers following the discharge of the innervating motor neuron. As the action potential travels through a muscle fiber, the 2D arrangement of the electrodes on the grid provides characteristics of muscle activity, such as single motor unit activity [3].

Figure 1: The motor unit structure, including the motor neuron, muscle fibers and motor endplates (photo adapted from Konrad).

Bipolar EMG and HD-EMG Differences

Bipolar EMG is the conventional EMG method, measuring the difference in voltage between two electrodes on one muscle, representing muscle activity of that muscle. However, for some researchers, this provides insufficient information about muscle activity. The spatial resolution of HD-EMG is higher, with multiple channels evenly spaced in either one direction (array) or two directions (grid), as shown in Figure 3a. The topographical distribution of electrodes allows for measurement of the activation of a large area of the muscle [4].

Other than the number of electrodes, the electrode diameter of HD-EMG is smaller than conventional bipolar EMG. This reduction in electrode size aims to minimize the spatial low-pass filtering effect on the distribution of electric potentials on the skin [1]. The low-pass filtering effects result from averaging the measured voltage beneath the surface of the electrode, which reduces the high-frequency content of the signal [5].

Lastly, the small inter-electrode distance (IED) of the HD-EMG grids increases the spatial resolution, facilitating signal interpolation for EMG images. The small IED guarantees the inclusion of all necessary muscle activation information needed for interpolation, preventing spatial aliasing [1, 3]. Using an IED of max 10 mm gives an accurate image reconstruction (EMG image) [1]. An example of EMG image reconstruction using an HD-EMG grid is included in Figure 4.

Figure 3: a) Textile HD-EMG grid (4x8L) b) Bipolar EMG measurement of muscle activation, with 2 electrodes on the muscle and ground electrode on wristband.

Figure 4 a) 8x4 electrode grid, b) Application of HD-EMG on each side of spine, c) Single Differential Root-Mean-Square Value, d) interpolated EMG image. From [5]