Selective activation of neuromuscular compartments within the human trapezius muscle

Abstract

Task-dependent differences in relative activity between “functional” subdivisions within human muscles are well documented. Contrary, independent voluntary control of anatomical subdivisions, termed neuromuscular compartments is not observed in human muscles. Therefore, the main aim of this study was to investigate whether subdivisions within the human trapezius can be independently activated by voluntary command using biofeedback guidance. Bipolar electromyographical electrodes were situated on four subdivisions of the trapezius muscle. The threshold for “active” and “rest” for each subdivision was set to >12% and <1.5% of the maximal electromyographical amplitude recorded during a maximal voluntary contraction. After 1 h with biofeedback from each of the four trapezius subdivisions, 11 of 15 subjects learned selective activation of at least one of the four anatomical subdivisions of the trapezius muscle. All subjects managed to voluntarily activate the lower subdivisions independently from the upper subdivisions. Half of the subjects succeeded to voluntarily activate both upper subdivisions independently from the two lower subdivisions. These findings show that anatomical subdivisions of the human trapezius muscle can be independently activated by voluntary command, indicating neuromuscular compartmentalization of the trapezius muscle. The independent activation of the upper and lower subdivisions of the trapezius is in accordance with the selective innervation by the fine cranial and main branch of the accessory nerve to the upper and lower subdivisions. These findings provide new insight into motor control characteristics, learning possibilities, and function of the clinically relevant human trapezius muscle.

Introduction

The motor control and function of the human trapezius muscle is of considerable interest due to the high prevalence of trapezius myalgia (Coté et al., 2008). The trapezius muscle is the largest and most superficial of the upper back and neck muscles. Based on the different fiber directions and mechanical actions of the upper, middle, and lower regions of the trapezius muscle (Johnson et al., 1994, Lindman et al., 1990), it is regarded to consist of different anatomical as well as functional subdivisions.

Already during the 2nd world war, Inman et al. (1944) demonstrated task-dependent differences in relative activity between the trapezius subdivisions, supporting a functional subdivisioning of the human trapezius muscle. Later, task-dependent relative differences in activity between the different subdivisions of the human trapezius muscle have been observed during different experimental conditions (Falla et al., 2007, Jensen and Westgaard, 1997, Johnson and Pandyan, 2005, Johnson and Pandyan, 2005). Functional subdivisioning is also observed in several other human muscles, e.g. biceps brachii, triceps brachii, deltoid, masseter, first dorsal interosseus and transversus abdominis (Brown et al., 2007, McMillan and Hannam, 1992, Ter Haar Romeny et al., 1984, Urquhart and Hodges, 2005, van Groeningen and Erkelens, 1994, Zijdewind et al., 1995).

Intra-muscular subdivisions (i.e. subpopulations of motor units located in close proximity to each other) that can be activated selectively such that one subdivision is active while another remains passive are termed neuromuscular compartments (English et al., 1993). Selective activation of intra-muscular subdivisions is observed in animal muscles (e.g. cat sartorius and lateral gastrocnemius) during locomotion (English, 1984, Hoffer et al., 1987). Obviously, it is unknown whether the observed selective activation of intra-muscular subdivisions in animals is generated by voluntary command. In human muscles, single motor units are observed to be individually controlled by voluntary command through selective inhibition (Basmajian, 1963, Smith et al., 1974), but neuromuscular compartments have not been shown. It is therefore unknown whether the anatomical subdivisions of the trapezius muscle can be independently activated by voluntary command and thereby regarded as neuromuscular compartments.

A neurophysiologic basis for selective control of each subdivision of the trapezius muscle is the selective innervation of the fine cranial and main branch of the spinal accessory nerve of the upper and lower subdivisions of the human trapezius muscle (Kierner et al., 2002). Moreover, large individual differences in motor control of the trapezius subdivisions are expected due to the large inter-individual differences in branching of the accessory nerve innervating the different subdivisions of the human trapezius muscle (Shiozaki et al., 2007).

The activity level of the different subdivisions of the trapezius muscle is regarded to be important for both development and rehabilitation of various shoulder and neck pathologies (Cools et al., 2007a, Cools et al., 2007b, Ludewig and Cook, 2000, Ludewig et al., 2004). Therefore, insight into motor control characteristics and learning possibilities of the trapezius subdivisions may be of relevance in rehabilitation of shoulder and neck pathologies.

Biofeedback of muscle activity by electromyography is well suited to study principles of motor control and learning (Basmajian, 1963, Hardyck et al., 1966, Smith et al., 1974). In the current study, on-line biofeedback from the different anatomical subdivisions of the trapezius was used to investigate whether selective and independent activation of the trapezius subdivisions could be attained by voluntary command.