Hand load contributions to cervical spine compressive forces

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Title: Hand load contributions to cervical spine compressive forces
Author: Pickens, Adam W.
Abstract: Musculoskeletal injuries to the cervical region of the spine have historically been linked to many different injury mechanisms . These injury mechanisms range from acceleration injuries (whiplash ) to injuries associated with lifting heavy loads (Hagberg , 1987 ; Herberts et al . , 1981 ) . Aaras and Ro (1997 ) found that frequently repeated lifts , as low as 1 % of the MVC , are correlated to musculoskeletal injuries of the neck and shoulder . In epidemiologic studies by NIOSH (1997 ) , it has been found that repetition , forceful exertion , and extreme postures contribute to musculoskeletal disorders of the cervical spine and shoulder . These musculoskeletal disorders have led to an increase in the number of permanent disability cases due to degeneration of cervical spine discs (Nygren et al . , 1995 ) . It has been demonstrated (Woldstad & Nicolalde , 2001 ) that EMG levels increase in the musculature of the neck as hand loads are increased . From their findings , Woldstad and Nicolalde (2001 ) theorized that this increase in muscle activity corresponds to an increase in the compressive forces acting on the cervical spine . Currently , while there are predictive shoulder models and predictive neck models , the need for a combined neck and shoulder model exists . That is the aim of this study , to test the validity of using outputs from existing an existing shoulder model as inputs for a current neck model in an attempt to quantify the compressive forces exerted on the spine due to hand loads . For the purposes of this study , the shoulder model that will be the focus is the Chalmers Computerized Shoulder Model . This model is based on the shoulder modeling work done by Hogfors et al . (1987 ) and a series of papers by various authors following that original publication by Hogfors et al . The Chalmers shoulder model uses anthropometric , static posture , and muscle parameter inputs to estimate muscle forces in shoulder muscles and the resultant forces on the shoulder . Output for this model is given in the form of force as a percentage of estimated maximum force for each muscle . The muscles in question are muscles shared by the cervical spine and the shoulder girdle : sternocleidomastoid , trapezius , and levator scapulae . To test the model outputs , ten (5 male , 5 female ) subjects were used for data collection . Surface EMG electrodes were attached to the specified muscles and each subject performed a series of maximum voluntary isometric tasks (MVC ) designed to isolate the muscles . Upon completion of the MVC tasks , subjects performed a series of lifting tasks modeled after the luggage -lifting task performed by Transportation Security Administration baggage screeners at airports , a task historically associated with high shoulder and neck injury rates . Subjects performed four (4 ) randomized trials each of nine (9 ) different luggage size /weight combinations . Three -dimensional motion capture data was recorded for each trial for position input into the model . Peak EMG values during the lifting task for each muscle for each trial were compared to the values of the MVC peak EMG values . In an attempt to replicate the data output of the Chalmers model , the peak trial values were calculated as percentages of the maximum values collected during the MVC collection period . Results indicated significant differences between the observed EMG values and model predictions . Results indicated that overall , there were significant interactions between the height and weight factors as well as between the weight and gender factors . These interactions indicated : (a ) overall as height of the bag increased , EMG values were greater than the model predictions , (b ) across all weights , EMG values were higher than the model predictions with the greatest difference being at the heaviest weight , (c ) for male subjects , as hand load increased , the model was less accurate in its prediction of muscle activity , (d ) for females , this indicates that overall , the model under predicts muscle activity for the 24 -pound and 36 -pound loads and is fairly accurate for the 48 -pound load .
URI: http : / /hdl .handle .net /2346 /10655
Date: 2008-08


Pickens, Adam W. Hand load contributions to cervical spine compressive forces. Doctoral dissertation, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /10655 .

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