Myoelectric manifestations of sternocleidomastoid and anterior scalene muscle fatigue in chronic neck pain patients
Introduction
Distinct modifications of the surface electromyographic (sEMG) signal can be identified during sustained voluntary or electrically elicited muscle contractions. The analysis of myoelectric manifestations of fatigue can provide important information about physiological changes evolving in the muscle (Merletti et al., 1999, Merletti et al., 2001, van der Hoeven et al., 1993b). Such manifestations consist of a ‘slowing’ of the signal causing a compression of the spectrum reflected by a decrease of mean and median frequency, and an increase in signal amplitude (Merletti et al., 2001). The evidence strongly suggests that there is a relationship between estimates of sEMG variables during sustained isometric contractions and muscle fibre constituency (Kupa et al., 1995, Mannion et al., 1998, Sadoyama et al., 1988).
Mannion et al. (1998) demonstrated a relationship between the median frequency and muscle fibre type distribution. In their study, the median frequency of the myoelectric signal from erector spine was analysed during isometric back extension contractions. The median frequency time course was recorded from the thoracic and lumbar erector spine and data obtained were compared with biopsy samples taken from the same sites where the sEMG was recorded. The results demonstrated that smaller the proportion of slow-twitch fibres, greater the slope of median frequency, that is, faster the fatigue process (Mannion et al., 1998).
Gogia and Sabbahi (1994) analysed spectral shifts to quantify fatigability of the sternocleidomastoid (SCM) muscle in patients with osteoarthritis of the cervical spine compared to normal matched controls. A greater slope of the median frequency of the sEMG signal and thus a greater fatigability was identified in the symptomatic subjects during sustained cervical flexion contractions at 50 and 80% of the maximum voluntary contraction (MVC). These findings are in accordance with results of muscle biopsy studies of subjects with neck pain undergoing spinal surgery, which established the occurrence of muscle fibre transformations in the neck flexors (Uhlig et al., 1995). Specifically, they demonstrated that the number of type-IIC transitional fibres was significantly increased in these patients and there was transformation of slow-twitch oxidative type-I fibres to fast-twitch glycolytic type-IIB fibres. A reduction in the proportion of slow-twitch fibres is usually associated with a diminution in the tonic holding abilities of muscles. Clinical mechanical measures have identified reduced endurance in the neck flexors in patients with neck pain (Placzek et al., 1999, Treleaven et al., 1994, Watson and Trott, 1993).
The purpose of this study was to further the understanding of neck flexor muscle dysfunction in patients with neck pain by studying the fatigue patterns of two neck flexor muscles, the SCM and anterior scalene (AS). Myoelectric manifestations of muscle fatigue were investigated during sustained sub-maximal cervical flexion contractions in subjects with and without chronic neck pain. Such knowledge would help direct and assess the nature of therapeutic exercise prescribed for patients with chronic neck pain. This work was based on two preliminary reports aimed to assess the localisation of the innervation zones and sEMG variable repeatability in the SCM and AS muscles (Falla et al., 2002a, Falla et al., 2002b). The same instruments, experimental procedure and data management were used for all three studies.
Section snippets
Subjects
Twenty volunteer subjects participated in this study after giving informed consent. Patients (one male, 9 females) were aged between 18 and 47 years (mean 28.7, SD 9.2 years) and had a history of neck pain for more than 1 year (mean 4.7, SD 3.4 years). Three of the patients had whiplash-associated disorders and the remaining 7 patients had idiopathic neck pain. Patients who had either undergone cervical spine surgery, complained of any neurological signs or had participated in a neck exercise
Results
Table 1 presents the mean and SD of the duration of symptoms for the neck pain patient group, intensity of pain rated on the VAS and the subject's perceived level of disability measured with the NDI. The MVCs recorded in the two groups were not statistically different (52.1±15.7 N in the asymptomatic subjects vs. 53.6±13.4 N recorded in the patient group, P=0.76). Despite this finding, neck pain patients demonstrated a trend towards less NME for both the SCM and AS (P=0.08 and 0.19,
Discussion
The results of this study revealed greater myoelectric manifestations of muscle fatigue of the SCM and AS muscles of neck pain patients as indicated by the significantly greater estimates of the initial value and slope of the mean frequency. This was identified for contractions at both 25 and 50% of MVC and reflects the clinical observation of reduced endurance in the cervical flexors in neck pain patients (Placzek et al., 1999, Treleaven et al., 1994, Watson and Trott, 1993).
Gogia and Sabbahi
Acknowledgements
This study was supported by the University of Queensland Small Grant, the Fondazione CRT and Compagnia di San Paolo di Torino.
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