Neck injuries account for a significant portion of accidents and compensation in the general population. The incidence of such injuries is coincidently quite high in both rugby league and rugby union players. It has been reported that in rugby union, the majority of injuries appear to be the result of buckling of the cervical spine (Kuster et al., 2012). Unfortunately, very little is known about the exact mechanism of injury and the resultant functional disturbances experienced by players (King et al., 2011). Even though our understanding of the relationship between neck injury, neck pain and muscle fatigue is improving, clinically practical methods of assessment disability due to injuries to the cervical spine are few in number (MacDermid et al., 2009; Humphreys, 2008).
Researchers have reported long lasting altered balance due to cervical extensor muscle fatigue (Duclos et al., 2004; Gosselin et al., 2004; Schieppati et al., 2003; Stapley et al., 2006) or after participants were subjected to cervical mechanical stress (Field et al., 2008; Gosselin and Blouin, 2000). Duclos (2004) showed the effects to be present after 30 seconds of either lateral flexion or posterior oblique isometric contraction. No information to date is available on the postural effects of neck flexion or anterior oblique isometric contraction.
The main cause of altered balance following neck muscle isometric contraction appears to be proprioceptive conflicts and possibly central fatigue which in turn increases the sway velocity during quiet stance (Gandevia, 2001; Gosselin et al., 2004). Afferences from small sized muscles have been shown to be modified by central fatigue (Pettorossi et al., 1999). The density of muscle spindles is higher in the small intrinsic, deep dorsal and sub-occipital cervical muscles than in other cervical muscle groups which accounts for their important role in proprioception (Djupsjobacka et al., 1995; Peck et al., 1984; Rix and Bagust, 2001) therefore suggested that fatigue of sub-occipital muscles could alter balance due to the activation of tonic gamma motor neurons in response to a build-up of muscle contraction metabolites. The consequence of such an accumulation (of [K.sup.+], arachidonic and lactic acids) is the promotion of group III and IV afferent signals, leading to positive feedback and further excitation of muscle spindles and gamma motor system hyperactivity (Knutson, 2000; Thunberg et al., 2001). At this point it is unknown if altered function of other cervical muscle groups will influence balance.
Surface electromyography (SEMG) has been used extensively to study muscle fatigue. If muscle contractions are maintained over a certain period, the SEMG power density spectrum shifts towards the lower frequencies, with the change in SEMG being proportional to the build-up of [H.sup.+] in the muscle as metabolites are produced and broken down (Oddsson et al., 1997). The median frequency, as measured by SEMG during muscle contraction, is therefore a good indicator of muscle fatigue as it is directly proportional to the build-up of muscle metabolites during a prolonged voluntary muscle contraction (Beck et al., 2013; Gonzalez-Izal et al., 2010; Merletti et al., 1992; Solomonow et al., 1990).
Increased postural sway is a common symptom of neck trauma as seen in rugby injuries and it is generally attributed to injury involving peripheral and/or central components of the cervical somatosensory and/or vestibular system (Loudon et al., 1997; Madeleine et al., 2004; Treleaven et al., 2005). Although computerized static posturography is widely used to measure balance impair ments, its main limitation is its high intrinsic inter-individual variability (Di Berardino et al., 2009). Investigators have attempted to decrease what is considered unacceptable static posturography variation by introducing either dynamic disturbances during quiet stance (Cyr et al., 1988; Koozekanani et al., 1980) or by changing the subject's position into a more unstable stance. Unfortunately the high costs of computerized dynamic posturography systems places this type of assessment out of the reach of most clinicians. Investigators have thus turned to measuring quiet stance with subjects standing on foam pads which is thought to amplify postural sway by decreasing the reliability of somatosensory information from cutaneous mechanoreceptors on the plantar surface (Di Berardino et al., 2009).
The purpose of the present study was thus to measure the effect of sustained isometric cervical muscle contraction in eight different directions on balance and perceived stability.
Forty four (n = 44) healthy male players from the National Conference League premiership (age = 24 [+ or -] 2 years; weight = 91[+ or -] 5.5kg, height = 1.84 [+ or -] 0.03m) volunteered for inclusion in this cross-over design study. Exclusion criteria applied included cervical trauma, neck or lower limb pain and visual disturbances during the last three months. Ethical approval was obtained from University's Ethics Committee, and all participants read the information sheet and signed a consent form. Participants were randomized into two equal groups of 22 individuals according to their surnames.
Cervical isometric contraction
Neck muscle fatigue was induced through isometric contractions (Schieppati et al., 2003, Gosselin et al., 2004). Participants were asked to stand comfortably on a predetermined target, with the feet touching each other on a force platform with their arms to their side and leaning slightly towards a support pad. This was provided to help stabilize body movement during the experiment, and was part of a custom built supporting structure (Figure 1). A head weight training harness was placed on the participant's head, from which a cable extended horizontally (standardized between participants) and was attached via a pulley system to an adjustable mass. No significant contraction of the thoracic or lumbar muscle chain below the level of the support was assumed required in order to maintain a steady stance. A marker was attached to the pulley and the experimenter observed if the pulley remained co-planar with a reference point fixed to the supporting structure. During the isometric contraction, the experimenter would give a verbal cue in order for the participant to either increase or decrease the cervical muscle force against the weight thereby maintaining a static head/neck position during 15 minutes. Eight different effort orientations were used each at 45[degrees] offset from the previous one (Figure 1). In order to decrease bias due to a participant's tiredness, the experiment was conducted over two days. On day one group 1 was tested in four randomized orthogonal positions (viz. E, F, RLF & LLF). On day two the remaining effort orientations were tested (viz. RPO, LPO, LAO & RAO). The order was reversed for Group 2.
The load set on the cable was approximated to 35% of the maximum isometric voluntary contraction (Table 1), and was calculated individually for each participant by adapting the isokinetic neck strength profile of elite rugby players data (Olivier and Du Toit, 2008). The isokinetic values were less than other reported valued but permitted us nonetheless to standardize the loads used (Geary et al., 2013). Neck length was measured from the spinous process of the vertebral prominence (C7) to the occipital notch at the base of the skull, while the head was held in the Frankfort plane (Olivier and Du Toit, 2008).
Due to the absence of normative data for the oblique contractions, we averaged the torque from either the extension and lateral flexion or the lateral flexion and flexion. For example, the RPO load was determined by averaging the E and RLF torques.
Surface electromyography was used to assess changes in muscles involved in isometric contraction. A two-channel EMG system was used to record the SEMG signal during the isometric contraction (iWorks system Model 214, Dover, USA). Standard settings were selected for the assessment, with the low pass filter set to 500Hz, the high pass filter to 10Hz, and the gain to 500 with the common mode rejection ratio set to 110dB. The inter-electrodes distance was 2 cm. Due to the difficulty in accessing specific muscles with surface electrodes, the electrode placement was location-specific rather than muscle-specific (Strimpakos et al., 2005). Electrode placements for the different directions of contraction are presented in Table 2.
The leads were linked to the skin and connected to the iWorks unit. Sampling started 15 seconds after the onset of the muscle contraction and lasted for 4.096 seconds at a rate of 2 KHz and once more after 14 minutes of isometric contraction. The real time wave form was displayed through LabScribe V2.0 software (iWorks, Dover, USA). The raw EMG was processed by performing a linear magnitude fast Fourier transform (FFT) followed by an integration and normalization of the spectrum (values from 0 to 1 volt), with normalization achieved through division by the maximum recorded value. The median frequency of the EMG...