Most of the archival literature on the biomechanical responses during load carriage is focused on posterior load carriage and is limited to understand the stress during carrying work activities. Anderson et al. [3
] did a study on a biomechanical analysis of anterior load carriage but the focus of that study was to explore the low back biomechanics during these load activities and specially to examine the effects of load height and walking speed on trunk muscle height and trunk posture but the study had not examined the effect on muscle activity of the neck muscle, cervical spine biomechanics and proprioception. As frequent material activities found in construction work environment were found to put substantial stress on the neck, shoulder and lower back resulting pain and discomfort [19
]. There was also found the changes in spinal curvature and proprioception of school boys carrying different weights of backpack [12
]. Therefore, the present study was designed to provide some quantitative data to explore the cervical spine biomechanics during the anterior load carriage and specially to examine the effects of one carrying task design parameter that is load height on neck muscle activity, proprioception and neck posture.
The contribution of muscles of cervical spine was evaluated during anterior load carriage in this study as Swedish construction workers studied by Holm storm et al. [20
] among whom 41% had neck and shoulder disorders. Ashish D Nimbarte et al. [19
] also studied role of neck muscles during lifting and holding tasks at shoulder height.
Neck and shoulder disorders affect 23.6% of male and 32.1% of female among Taiwanese construction workers by Guo11 et al. which supported the selection criteria for female subjects in this study. In girls the prevalence of neck pain increase with age [21
In this study normal healthy individuals were selected. The individuals with history of cervical and low back pain
were excluded because the differences in proprioception in individuals with back pain and those free from back pain were found [22
]. Also there was found relationship between head posture and severity and disability of patients with neck pain [7
]. Also a positive and significant relationship between pain intensity and superficial muscle activity was shown [23
Chris Ho Ting et al. [7
] measured forward head posture through craniovertebral angle to find the relationship between head posture and severity and disability of patients with neck pain. Shivananda et al. [24
] also analyzed cervical and shoulder posture in school children using back pack experiments study in which craniovertebral angle, shoulder saggital posture angles, craniovertebral angles were calculated. June Quek et al. [25
] assessed forward head posture through craniovertebral angle to explore the mediating effects of forward head posture on relationship between thoracic kyphosis and cervical mobility in older adults with cervical dysfunction.
In this study anterior load carriage at three different levels had a significant effect on the craniovertebral angle that the craniovertebral angle was increasing with the load height. This can be explained by the fact that it occurred in order to compensate to the posterior shift of the trunk. The posterior shifting of the trunk to shift the centre of the new loaded system to a more balance location was noted by Anderson et al. [3
] who found a significant effect of load height on the saggital angles at T9 and T12 levels. Shoulder height load carriage resulted in larger postural deviation than carriage at the lower heights. It has also been shown that there is an increasing trunk inclination and increasing head on trunk extension with increasing back pack load Chow et al. [12
The upper trapezius muscle, especially along the C7 level, has been widely studied in occupational investigations to evaluate the neck disorders. To our knowledge, no previous study evaluating occupational tasks involving anterior load carriage has reported the activity of upper trapezius in the cervical region while evaluating biomechanical analysis. Understanding the activation of this muscle is vital, as it is bigger (surface) muscle in the neck region
and anatomically connects the shoulder to the skull. Such an anatomical orientation may require this muscle to support the shoulder during anterior load carriage. The results of this study clearly show that upper trapezius was sensitive to lifting weight and there is increase in activity of upper trapezius muscle. The posture had a significant impact on the activation level of the neck muscles and the upper trapezius muscle was most active at the flexed neck posture [19
]. As an increase in craniovertebral angle with an increase of load height was found in our study, this explained the increase in activation of upper trapezius with increase in load height. It has also been shown that under dynamic conditions, placing the load in the C1-C7 region created significantly higher level of muscle activity for both trapezius as compared to those captured at T1-T6 region [15
]. Over all, the result of this study indicated that trapezius muscle play an important role during lifting and holding tasks.
It has been shown that there is a decrease in repositioning consistency of the lumbar spine with the increasing back pack load [12
]. The results of this study showed that there is maximum decrease in proprioception of the cervical spine at shoulder level but there is slight increase in proprioception at elbow level as compared to the knuckle level. Study by James E carpenter et al. [26
] showed that proprioception is diminished in the presence of muscle fatigue, also dorsal neck muscle fatigue alters cervical position sense [27
]. This alteration in the fatigue muscle was due to the metabolites and / or inflammatory substances as in the fatigued muscle the nociceptors are activated by the end metabolic products (including bracykinin, arachidonic acid, prostaglandin E2, potassium, and lactic acid), which were produced during the previous muscular contractions. These metabolites and/or inflammatory substances within the muscle during fatiguing exercise modify the proprioceptive input by increasing the threshold for muscle spindle discharge [28
]. Another study done by Anderson et al. [3
] found increased activity of anterior deltoid and erector spine when the load was lifted to the level of shoulder height. The results of this study showed increased activity of trapezius muscle when the load was lifted to the level of shoulder height. As there was more of muscular activity at the shoulder level when the load was lifted up to this level, this had resulted in more production of metabolic products. This explained the decrease in proprioception of cervical spine at shoulder level.
Posture affects muscle activation. Ashish D Nimbarte et al. [19
] found that during neck flexion trapezius is active and during neck extension sternocleidomastoid is active. As in this study anterior load carriage caused flexion of neck, so at that time trapezius was most active but not the sternocleidomastoid because no natural extension of neck was there. This explained that why the results of neck proprioception in extension were varied in this study.
This study had several limitations that need to be considered. In actual, occupational sites have harsh outdoor environments due to noise, vibration, space and time constraints that could impose psychological stress which will further effects the muscle activation. Secondly, for standardization purposes, the subjects in this study lifted certain percentage of their body weight. In actual working conditions, regardless of body size or strength the workers lift weight of different sizes and dimensions. Thirdly, relatively younger people with less experience in physical demanding work were tested in this study. Considering the relatively awkward lifting posture tested in this study, the ability of the participants at three different levels might have been less than actual load carrying workers. Moreover, it is possible that the muscle activation pattern in experienced workers may be different than the relatively in experienced subjects. The results obtained in this study solely on female participants may not equally to males. Diet, hydration status of the subjects was not considered in this study.
This study considered the relatively under-explored area of anterior load carriage. Workers in the agriculture industry perform a signiï¬cant amount of this type of exertion, and understanding the eï¬ects of load height may help ergonomists develop appropriate ergonomic interventions for the prevention of neck injury and fatigue
The future study can be recommended on symptomatic subjects to compare and to see the effects. The study can be recommended to evaluate effect of walking speed on muscle activation patterns, proprioception and kinematic response of cervical spine.