Relation between location of a motor unit in the human biceps brachii and its critical firing levels for different tasks

Abstract

Critical firing levels (CFLs) of single motor units (MU?.) in the long head of the human biceps brachii muscle were determined in combinations of two isometric tasks flexion of the elbow, supination of the lower arm, and exorotation of the humerus, as well as the corresponding antagonistic tasks. The MU activity was recorded by 25-pm bipolar wire electrodes. Four main patterns of MU recruitment, related to the recording location in the muscle, were found: (i) MUs active only when flexing the elbow were located mostly laterally. (ii) MUs active only when supinating were all located medially. (iii) MUs whose CFL depended on a linear combination of flexion and supination forces were all located medially. Some of these MUs could not be recruited during pronation. (iv) Nonlinearly behaving MUs, located centrally. The relative weights of tlexion and supination input were constant for all units, whose CFL depended on a linear sum of flexion and supination forces, as well as for the nonlinearly behaving units. Supination and exorotation showed equivalent CFL changes when they were combined with the flexion task. Extension did not change the CFL for supination-or exorotation tasks. No clear difference was found between the ratios of the peak twitch forces in flexion and supination direction for laterally and medially located small muscle areas or single MUs. A simple model of the motoneuron pool organization is proposed to explain our findings.Abbreviations: CFL-critical firing level, MU-motor unit, BLH-human biceps brachii long head, MVC-maximum voluntary contraction, S-supination, E-exorotation, F-flexion.

Brachialis muscle activity can be assessed with surface electromyography

“…1) did not show more similar values than the coefficients to the more distant electrodes. When regarding the activity of the four sites within the BB no significant difference between these four sites in the association to elbow flexion or forearm supination was apparent (see , although previous studies using intramuscular or multichannel EMG recordings indicated an inhomogeneous activity during the same type of contraction as in the current study (Perot et al, 1996;Staudenmann et al, 2013;ter Haar Romeny et al, 1984;van Zuylen et al, 1988). This discrepancy may be related to the limitation of conventional bipolar EMG which cannot extract muscular activity with the same precision as multichannel EMG (Staudenmann et al, 2010(Staudenmann et al, , 2013.…”

“…Recordings of the BB were obtained from the proximal and distal portion of the two muscle heads, as distinct EMG activities of the two muscle parts were observed for specific contractions (Perot et al, 1996;Staudenmann et al, 2013;ter Haar Romeny et al, 1984;van Zuylen et al, 1988). Furthermore, the activity of the BR was recorded over the medial and lateral sides of the distal upper arm where the muscle becomes superficial, and the electrodes over the BRR were attached as displayed in Fig.…”

Motor unit firing behavior in human arm flexor muscles during sinusoidal isometric contractions and movements

“…Assuming that the force twitch of a MU can be de scribed by a second-order linear low-pass system, former studies (ter Haar Romeny et al 1984) have obtained val ues for the time constant of the force twitch of MUs in the biceps brachii near 50 ms. However, a MU contributes to sinusoidal contractions with a sequence of APs, not with a single AP.…”

Redundancy or heterogeneity in the electric activity of the biceps brachii muscle? Added value of PCA-processed multi-channel EMG muscle activation estimates in a parallel-fibered muscle

“…Measuring propagating waves on both the proximal and the distal side of the innervation zone would in such ideal cases be redundant and would not add information on muscle activity and muscle force. However, heterogeneity in muscle activity in the medio-lateral, but also some in longitudinal direction within BB has been reported during voluntary contractions (ter Haar Romeny et al, 1984;van Zuylen et al, 1988).…”