Browsing by Author "Conover, William J."
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Item Hand load contributions to cervical spine compressive forces(Texas Tech University, 2008-08) Pickens, Adam W.; Woldstad, Jeffrey C.; Smith, James L.; Patterson, Patrick E.; Kobza, John E.; Conover, William J.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.Item Maximal push/pull strengths in the vertical and horizontal directions with hands above shoulder level(2011-05) Hendrikse, Egbert J.; Smith, James L.; Patterson, Patrick E.; Millet, Barbara; Conover, William J.; Jones, Keith S.Very little information is available to set guidelines that can reduce injuries for pushing and pulling in the vertical and horizontal direction when the arms are above shoulder level. The long term goal of this study was to be able set guidelines or make recommendations that could be implemented in industry for pushing and pulling in the vertical and horizontal directions with arms above shoulder level. The null hypotheses of this study were that there were no significant differences between the four heights of the maximal voluntary force exertion (180°, 150°, 120°, and 90° arm angle), the type of force applied (push or pull) or as the distance between the left and right hand was changed. Subjects were asked to push/pull on a handle that allowed for the two distances between the hands at the four different arm angles in the vertical and horizontal direction. Therefore, data were separated based on the vertical and horizontal direction. With 25 males and 25 females, significant main effects were found in the horizontal direction for gender (F(1,48) = 8.32, p < 0.01), force type (F(1,48) = 5.825, p < 0.05) and arm and arm angle (F(3,144) = 84.763, p < 0.001). In the vertical direction, significant main effects were found for gender (F(1,48) = 113041, p < 0.01), force type (F(1,48) = 81.27, p < 0.001), arm angle (F(3,144) = 76.08, p < 0.001), and also distance between the hands (F(1,48) = 45.939, p < 0.001). Additional interactions were also found to be significantly different for the horizontal and vertical directions. In the horizontal direction, the 90° arm angle produced the greatest maximum voluntary exertions. In the vertical direction it became a bit more complicated. When pushing in the vertical direction, the 150° arm angle produced the greatest maximum voluntary exertions. When pulling in the vertical direction, the 180° arm angle produced the greatest maximum voluntary exertions. Using a distance of approximately shoulder width apart only influenced push/pull strength in the vertical direction.Item Item The Effects Of Closure-Based Multiple Testing On The Power Of P-Value Combination Tests(2011-08) Henning, Kevin; Westfall, Peter H.; Conover, William J.; Mansouri, Hossein; Surles, JamesA multiple testing situation arises whenever several statistical inferences (tests or intervals, although we focus on the former) are considered simultaneously, and the goal is to make valid conclusions about each inference in the presence of the others. Many diverse approaches to dealing with the multiple testing issue, from both the frequentist and Bayesian perspectives, have been proposed, although in this dissertation, we restrict our attention to frequentist hypothesis testing. One increasingly popular procedure for multiple testing is known as the "closure method," or simply "closure." The method allows simultaneous conclusions to be made about individual hypotheses by guaranteeing that the probability of rejecting any true null hypothesis is no greater than α. The method works by testing all subset hypotheses formed by considering the set of all non-empty intersections of the individual hypotheses. To reject an individual hypothesis H_{i} requires rejecting all intersection hypotheses that involve H_{i} . Notably, any test that controls the Type I error rate at level α can be used for these intersection hypotheses, which makes the method quite general. In this dissertation, we consider the power properties of a class of tests known as p-value combination tests (PVCTs) when these tests are used in the closure setting. We consider three types of PVCT that use p-value information differently: additive combination (AC) methods, minimum-p-value (MINP) methods, and one "hybrid" approach, the Truncated Product Method (TPM). We find through simulation studies that the power properties for PVCTs as tests of intersection hypotheses do not carry over when these tests are used in the closure setting. Specifically, the AC and TPM tests generally have higher power than MINP methods as global tests, but much less power than MINP tests in the closure setting (however, we show that the TPM can be modified to perform similarly to the MINP tests by decreasing the truncation level τ as the number of tests increases). Underlying the poor performance of AC methods in closure is the hierarchical nature of closed testing, and we give details on how these "hurdles" cause the dramatic power losses we have observed.