Examining a countermovement jump (CMJ) force-time curve related to net impulse might be useful in monitoring athletes' performance. This study aimed to investigate the reliability of alternative net impulse calculation and net impulse characteristics (height, width, rate of force development, shape factor, and proportion) and validate against the traditional calculation in the CMJ. Twelve participants performed the CMJ in two sessions (48 hours apart) for test–retest reliability. Twenty participants were involved for the validity assessment. Results indicated intra-class correlation coefficient (ICC) of ≥ 0.89 and coefficient of variation (CV) of ≤ 5.1% for all of the variables except for rate of force development (ICC = 0.78 and CV = 22.3%). The relationship between the criterion and alternative calculations was r = 1.00. While the difference between them was statistically significant (245.96 ± 63.83 vs. 247.14 ± 64.08 N s, p < 0.0001), the effect size was trivial and deemed practically minimal (d = 0.02). In conclusion, variability of rate of force development will pose a greater challenge in detecting performance changes. Also, the alternative calculation can be used practically in place of the traditional calculation to identify net impulse characteristics and monitor and study athletes' performance in greater depth.

During landing, the human body is required to absorb impact forces throughout its tissues. Muscle and connective tissue is able to dissipate much of this force, however, a portion of the impact is delivered to the bones. Forces acting on the human skeleton can cause microscopic fractures which may lead to stress fracture. The present study seeks to calculate changes in the magnitude of strain using noninvasive methods. A musculoskeletal model representing a healthy male subject (22 years, 78.6 kg, 1.85 m) was created. A flexible tibia, created from a computed tomography scan of the subject’s right tibia, was included in the model. Motion capture data were collected while the subject performed drop landings from three separate heights (26, 39, and 52 cm) and used to compute simulations in LifeMOD. Surface electromyography and joint angle data were compared to their simulated counterparts using a cross correlation. Maximum magnitudes of principal and maximum shear strain were computed. The model had reasonable agreement between joint angle curves. A large Cohen’s d effect size showed that our subject had increased tibial strain and strain rate as the drop height increased. This study demonstrates a valid method of simulating tibial strain during landing movements. Future studies should focus on recruiting a larger sample and applying this method. / School of Physical Education, Sport, and Exercise Science

Although people can typically maintain balance on moving trains, or press the appropriate button on an elevator with little conscious effort, the apparent ease of these sensorimotor tasks is courtesy of neural mechanisms that continuously interpret many sensory input signals to activate muscles throughout the body. The overall hypothesis of this work is that motor behaviors emerge from the interacting constraints and features of the nervous and musculoskeletal systems. The nervous system may simplify the control problem by recruiting muscles in groups called muscle synergies rather than individually. Because muscles cannot be recruited individually, muscle synergies may represent a neural constraint on behavior. However, the constraints of the musculoskeletal system and environment may also contribute to determining motor behaviors, and so must be considered in order to identify and interpret muscle synergies. Here, I integrated techniques from musculoskeletal modeling, control systems engineering, and data analysis to identify neural and biomechanical constraints that determine the muscle activity and ground reaction forces during the automatic postural response (APR) in cats. First, I quantified the musculoskeletal constraints on force production during postural tasks in a detailed, 3D musculoskeletal model of the cat hindlimb. I demonstrated that biomechanical constraints on force production in the isolated hindlimb do not uniquely determine the characteristic patterns of force activity observed during the APR. However, when I constrained the muscles in the model to activate in a few muscle synergies based on experimental data, the force production capability drastically changed, exhibiting a characteristic rotation with the limb axis as the limb posture was varied that closely matched experimental data. Finally, after extending the musculoskeletal model to be quadrupedal, I simulated the optimal feedforward control of individual muscles or muscle synergies to regulate the center of mass (CoM) during the postural task. I demonstrated that both muscle synergy control and optimal muscle control reproduced the characteristic force patterns observed during postural tasks. These results are consistent with the hypothesis that the nervous system may use a low-dimension control scheme based on muscle synergies to approximate the optimal motor solution for the postural task given the constraints of the musculoskeletal system. One primary contribution of this work was to demonstrate that the influences of biomechanical mechanisms in determining motor behaviors may be unclear in reduced models, a factor that may need to be considered in other studies of motor control. The biomechanical constraints on force production in the isolated hindlimb did not predict the stereotypical forces observed during the APR unless a muscle synergy organization was imposed, suggesting that neural constraints were critical in resolving musculoskeletal redundancy during the postural task. However, when the model was extended to represent the quadrupedal system in the context of the task, the optimal control of the musculoskeletal system predicted experimental force patterns in the absence of neural constraints. A second primary contribution of this work was to test predictions concerning muscle synergies developed in theoretical neuromechanical models in the context of a natural behavior, suggesting that these concepts may be generally useful for understanding motor control. It has previously been shown in abstract neuromechanical models that low-dimension motor solutions such as muscle synergies can emerge from the optimal control of individual muscles. This work demonstrates for the first time that low-dimension motor solutions can emerge from optimal muscle control in the context of a natural behavior and a realistic musculoskeletal model. This work also represents the first explicit comparison of muscle synergy control and optimal muscle control during a natural behavior. It demonstrates that an explicit low-dimension control scheme based on muscle synergies is competent for performance of the postural task across biomechanical conditions, and in fact, may approximate the motor solution predicted by optimal muscle control. This work advances our understanding how the constraints and features of the nervous and musculoskeletal systems interact to produce motor behaviors. In the future, this understanding may inform improved clinical interventions, prosthetic applications, and the general design of distributed, hierarchal systems.

The purpose of this study was to examine differences in drop landing ground reaction forces between athletes with and without patellar tendinitis. Subjects included 30 recreational athletes, 15 with patellar tendinitis and 15 without. Subjects with patellar tendinitis were tested twice, before (PTI) and after (PTF) rehabilitation. The non-patellar tendinitis (NPT) group was tested once. Subjects performed three trials of a drop landing from a height of 40 cm onto the force plate. Video data was collected to determine the deepest angle of knee flexion during landing. Statistical analysis using ANOVA revealed significant differences in maximum vertical force for the initial peak, post hoc analysis revealed differences between PTI and NPT groups aswell as between PTF and NPT groups. Results indicate athletes with patellar tendinitis exhibit higher initial peak 1 VGRF than athletes without patellar tendinitis. Results indicate that athletes who consistently land with elevated peak 1 ground reaction force are more likely to develop patellar tendinitis. / School of Physical Education

In gymnastics, females are often afflicted with lower extremity injuries during the landing phase of a backward rotating skill. The purpose of this study was to assess the efficacy of using a drop landing and backward somersault landing to compare and contrast the kinetic and kinematic differences between the two tasks in order to determine if a drop landing is a suitable representative task to analyze when examining landing injury mechanisms. Eleven female NCAA Division I gymnasts (age 19.3 ± 0.9 yrs; body height 1.66 ± 0.05 m; body mass 61.36 ± 6.02 kg) were recruited to perform drop landings and backward somersaults. Two force plates along with a 3D movement analysis system were used to collect kinetic and kinematic data. A repeated measures ANOVA was used to examine the differences in the variables with the significance level set at 0.05. There were mechanical differences and significance found between the peak vertical ground reaction forces, loading rate, kinetic and kinematic variables in the sagittal and frontal planes during the two tasks. It is evident that results may underestimate the effect of gymnastics landing impacts on risk of lower extremity injury because of the mechanical differences and significance found between the two tasks. / Access to thesis permanently restricted to Ball State community only.

Thesis (M.S.)--University of Texas at El Paso, 2007. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (M.A.)--University of North Carolina at Chapel Hill, 2005. / Includes bibliographical references (leaves 73-77). Also available online (PDF file) by a subscription to the set or by purchasing the individual file.

Thesis (M.A.)--University of North Carolina at Chapel Hill, 2005. / Includes bibliographical references (leaves 73-77).

Thesis (M.A.)--University of North Carolina at Chapel Hill, 2005. / Includes bibliographical references (leaves 109-114). Also available online (PDF file) by a subscription to the set or by purchasing the individual file.

Thesis (M.A.)--University of North Carolina at Chapel Hill, 2005. / Includes bibliographical references (leaves 109-114).