Background
Humans deal frequently with changes in path direction (Segal et al. 2008), yet few studies evaluate gait along curved trajectories. Given the importance of neck sensorimotor control during walking and turning [1], it is clinically relevant to consider the impact of the neck pain which may can impair the muscular ability to absorbing the ground reaction forces propagating from the legs to the head during gait [2] and can altered the coactivation to avoid the pain [3].


Aims
This study examined the shock attenuation function at neck, trunk and leg level and the time-varying multi-muscle coactivation function on different muscles at neck and back level during walking along rectilinear and curvilinear trajectories in people with chronic neck pain (CNP) versus asymptomatic control (AC). 

Methods
Twenty-six AC (age: 26±10.6 years) and twenty-one subjects with CNP (age: 28.5±12.4 years, average pain intensity: 4.5±1.9/10) performed three gait repetitions at natural speed, along a rectilinear path (10m) and a curvilinear path clockwise and counterclockwise (1m radius) and simultaneously,  a motion analysis system was used for the acquisition of  kinematics and muscle activity. 

During the gait subjects wore six passive reflective positioned above the anatomical reference points and a system of acquisition sEMG (FreeEMG, BTS SpA, Milan, Italy) with probes places on splenius capitis sternocleidomastoid and erector spinae right and left side muscles.

From the processed EMG signals, we calculate the simultaneous activation of the muscles using the time-varying multi-muscle coactivation function (TMCf) proposed by Ranavolo [4]. Mean shock attenuation index (MSA) at neck, trunk and leg was calculated from the power spectral density of the respective segments during the right gait cycle [5]. The MSA and TMCf were compared between groups by using parametric tests (two-tailed) and repeated measure analysis of variance (ANOVA) according to the data distribution.

Results
The mean MSA at the neck level  was significantly higher (less shock absorption) in CNP compared to AC during external direction (P=0.020, see Figure 1) and the TMCf in the sternocleidomastoid was significantly higher in CNP during the same direction (P=0.021, see Figure 2) as well, while no significant differences were found for the other gait directions for MSA indexes or TMCf mean.

Conclusion
The reduced shock absorption at the neck level in people with CNP can be due to impaired control of the neck muscles. The higher speed of the outer (external to the curvilinear direction) part of the body motivates the higher MSA neck for CNP during the counterclockwise condition (outer right gait cycle). Challenging the motor control, with curvilinear gait, highlights impairments in the shock absorbing function and in the coactivation of the neck in people with CNP.

IMPAIRED SHOCK ATTENUATION IN PEOPLE WITH CHRONIC NECK PAIN DURING CURVILINEAR GAIT