**1. Introduction**

Spinal cord injuries (SCIs) can cause motor dysfunction including loss of maximal strength and impaired force control that is partially explained by altered muscle activation [1]. In patients with incomplete SCIs, there is a reduction in nervous system activation of skeletal muscle below the lesion. Hence, some motor units (motor neurons) of a muscle may not be recruited despite maximal effort, due to denervation or loss of central neural activation, whereas others may discharge at lower than normal rates [2,3]. Recording of muscle activity by electromyography (EMG) has proved to be useful for evaluating central and peripheral determinants of motor dysfunction [4,5]. In contrast to clinical measures of motor function, EMG is sensitive enough to detect muscle activity after SCI in the absence of palpable muscle contraction and joint movement [6]. Abnormal EMG findings from impaired muscles after SCI include long-lasting involuntary motor activation [7,8], loss of functioning motor units [9–15], impaired motor unit voluntary control [16–18], and muscle fiber denervation and reinnervation [3,19,20]. EMG has demonstrated to be a valuable tool for assessment of paralyzed muscle changes in persons with SCI.

The relation between surface EMG amplitude and voluntary isometric muscle force has been explored in people with motor disorders such as stroke [21–24]. Alterations in EMG–force relation compared with matched healthy control subjects have been observed,

**Citation:** Li, L.; Hu, H.; Yao, B.; Huang, C.; Lu, Z.; Klein, C.S.; Zhou, P. Electromyography–Force Relation and Muscle Fiber Conduction Velocity Affected by Spinal Cord Injury. *Bioengineering* **2023**, *10*, 217. https://doi.org/10.3390/ bioengineering10020217

Academic Editor: Aurélien Courvoisier

Received: 12 January 2023 Accepted: 30 January 2023 Published: 6 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

that may be related to altered motor control and motor unit properties. In contrast with stroke, few have examined the EMG–force relation after SCI. Thomas and colleagues reported linear (or curvilinear) EMG–force relations in the triceps brachii of persons with chronic SCI that were similar to healthy controls [25].

The EMG–force relation has mainly been examined with conventional single channel surface electrodes. High density surface EMG (HD-sEMG) arrays provide advantages over conventional single channel EMG [26]. For example, Jordanic et al. compared the performance of HD-sEMG and single channel EMG in the upper limb of SCI subjects and found that spatial activation of motor units was dependent on the contraction intensities and the type of exercise, and the related spatial features can improve the identification of specific co-activation patterns during motor performance [27]. Among various HD-sEMG array designs, a one-dimensional linear electrode array is convenient to use [28]. The linear electrode array can simultaneously measure EMG from different locations of the muscle fibers and thus may detect activity in severely paralyzed muscles that may be undetected using conventional single channel EMG. Linear electrode arrays have other useful applications including estimation of locations of innervation zones (IZ) and muscle fiber conduction velocity (MFCV) [29–32].

In this study, we completed an analysis of surface EMG from linear electrode array attached on the biceps brachii (BB) muscle in persons with SCI and a matched group of healthy controls. The purpose of the study was to characterize the BB MFCV and EMG–force relationship, and whether they are affected by SCI.

#### **2. Materials and Methods**

#### *2.1. Subjects*

The participants of this study included 15 SCI survivors (3 female and 12 male, 44.6 ± 16.1 years,) with injury duration 1–36 years, injury level from C2–C8 and American Spinal Injury Association (ASIA) impairment scale A to D. More information on injury level and ASIA impairment scale can be found in [33]. All SCI survivors were recruited from the outpatient clinic of TIRR Memorial Hermann Hospital (Houston, TX, USA). Their clinical characteristics are summarized in Table 1. In addition, 14 able-bodied subjects (3 female and 11 male, 39.7 ± 12.4 years) with no known history of neuromuscular disorder were recruited as the control group. There was no age difference between the two groups (*p* = 0.37). This study was approved by the Institutional Review Board of the University of Texas Health Science Center at Houston and TIRR Memorial Hermann Hospital and performed in accordance with the Declaration of Helsinki. All subjects gave written consents (or had a witnessed verbal consent if unable to write) before participating in the experiment.

**Table 1.** SCI subject information.

