Distribution of Head Acceleration Events Varies by Position and Play Type in North American Football.

OBJECTIVE: The goal of this pilot study was to evaluate the number of head acceleration events (HAEs) based on position, play type, and starting stance. DESIGN: Prospective cohort study. SETTING: Postcollegiate skill development camp during practice sessions and 1 exhibition game. PARTICIPANTS: Seventy-eight male adult North American football athletes. INDEPENDENT VARIABLES: A position was assigned to each participant, and plays in the exhibition game were separated by play type for analysis. During the exhibition game, video data were used to determine the effects of the starting position ("up" in a 2-point stance or "down" in a 3- or 4-point stance) on the HAEs experienced by players on the offensive line. MAIN OUTCOME MEASURES: Peak linear acceleration and number of HAEs greater than 20 g (g = 9.81 m/s2) were measured using an xPatch (X2 Biosystems, Seattle, WA). RESULTS: Four hundred thirty-seven HAEs were recorded during practices and 272 recorded during the exhibition game; 98 and 52 HAEs, the greatest number of HAEs by position in the game, were experienced by the offensive and defensive linemen, respectively. Linebackers and tight ends experienced high percentages of HAEs above 60 g. Offensive line players in a down stance had a higher likelihood of sustaining a HAE than players in an up stance regardless of the type of play (run vs pass). CONCLUSIONS: Changing the stance of players on the offensive line and reducing the number of full-contact practices will lower HAEs.

Neurophysiological features of tactile versus visual guidance of ongoing movement.

Although studies have investigated tactile and visual processing for perception, sensory processing for ongoing action remains poorly understood. The purpose of this study was to explore modality-specific patterns of cortical activation and functional connectivity in a practiced trajectory tracking task. Participants traced irregular shapes with their index finger using either touch or vision for guidance. In 60 tactile-motor (TM) trials, movement was guided only by tactile feedback of semicircular bumps on a plastic tile. In 60 visuo-motor (VM) trials, movement was guided only by vision of dots on a screen seen through a small window at the finger tip. The order of TM and VM trials was counterbalanced across 16 participants. Electroencephalography (EEG) was used to estimate cortical activation (task-related spectral power) and functional connectivity (task-related magnitude-squared coherence) in the alpha (8-12 Hz) and beta (13-30 Hz) bands during the last 12 movement trials in each sensorimotor task. TM vs. VM tasks exhibited a larger drop in global beta cortical activation, and greater alpha coherence between central, parietal, and occipital regions. VM vs. TM tasks were characterized by low global alpha coherence. Trace time and cortical activation of the last 12 VM trials were reduced in the group performing the VM task after the TM task compared to those performing the VM task first. Beta connectivity initiated by the first task was maintained on the subsequent second task, regardless of the task order. Identification of modality- and order-specific EEG characteristics provides insight into the sensory control of movement.

Effects of visual noise on binocular summation in patients with strabismus without amblyopia.

PURPOSE: Patients with strabismus often complain of difficulty navigating through visually stimulating environments without clear explanation for this symptom. Binocular summation (BiS), defined as the superiority of binocular over monocular viewing on visual threshold tasks, is decreased in conditions that cause large interocular differences in visual acuity, but is not well studied in strabismic populations without amblyopia. The authors hypothesized that strabismus may lead to decreased BiS for tasks related to discrimination within increased background complexity. The goal of this study was to test the extent of BiS in patients with strabismus during discrimination of a luminance target disk embedded in visual noise. METHODS: Participants included 10 exotropic, 10 esotropic, and 13 age-matched control patients. Performance of a task detecting a luminance-target was measured at 0, 10, and 20 µdeg(2) of visual noise for binocular and monocular conditions. BiS was calculated as the ratio of binocular contrast sensitivity to monocular contrast sensitivity for the target embedded in noise. RESULTS: Patients with strabismus had lower BiS values than controls, with a significant decrease on linear regression in patients with strabismus at 20 µdeg(2) of noise (P = .05), with a trend toward significance at 10 µdeg(2) of noise (P = .07). Patients with strabismus showed a mean binocular inhibition (summation ratio < 1) at both noise levels. CONCLUSIONS: These findings support the hypothesis that strabismus can lead to decreased BiS and even binocular inhibition. Despite literature showing enhanced BiS in visually demanding situations such as high levels of visual noise or low contrast, BiS was not significantly affected by visual noise in either group.

The ratio of thoracic to lumbar compression force is posture dependent.

Despite the evidence suggesting that between 8% and 55% of manual labourers experience thoracic pain, research on spinal loading during occupational tasks has been almost invariably limited to the lumbar spine. In this study, we determined the ratio of thoracic to lumbar compression force and the relative risk of injury to each region in various postures. Compressive forces on the spine were calculated based on previously reported thoracic and lumbar intradiscal pressures and disc cross-sectional areas. Flexion postures were associated with an approximate doubling in lumbar compression force but only small increases (or even decreases) in thoracic compression. The ratio of thoracic to lumbar compression was above the tolerance ratio (i.e. the ratio of thoracic to lumbar compressive strength) during upright postures and below the tolerance ratio during flexion postures, indicating that upright postures may pose a greater relative risk of injury to the thoracic spine than to the lumbar spine. Practitioner summary: Previously reported thoracic and lumbar in vivo disc pressures during various postures were compared. The ratio of thoracic and lumbar compression increased during upright postures and decreased in flexed postures, indicating that upright postures may pose a greater risk of injury to the thoracic spine than to the lumbar spine.

Active trunk stiffness during voluntary isometric flexion and extension exertions.

OBJECTIVE: Compare muscle activity and trunk stiffness during isometric trunk flexion and extension exertions. BACKGROUND: Elastic stiffness of the torso musculature is considered the primary stabilizing mechanism of the spine. Therefore, stiffness of the trunk during voluntary exertions provides insight into the stabilizing control of pushing and pulling tasks. METHODS: Twelve participants maintained an upright posture against external flexion and extension loads applied to the trunk. Trunk stiffness, damping, and mass were determined from the dynamic relation between pseudorandom force disturbances and subsequent small-amplitude trunk movements recorded during the voluntary exertions. Muscle activity was recorded from rectus abdominus, external oblique, lumbar paraspinal, and internal oblique muscle groups. RESULTS: Normalized electromyographic activity indicated greater antagonistic muscle recruitment during flexion exertions than during extension. Trunk stiffness was significantly greater during flexion exertions than during extension exertions despite similar levels of applied force. Trunk stiffness increased with exertion effort. CONCLUSION: Theoretical and empirical analyses reveal that greater antagonistic cocontraction is required to maintain spinal stability during trunk flexion exertions than during extension exertions. Measured differences in active trunk stiffness were attributed to antagonistic activity during flexion exertions with possible contributions from spinal kinematics and muscle lines of action. APPLICATION: When compared with trunk extension exertions, trunk flexion exertions such as pushing tasks require unique neuromuscular control that is not simply explained by differences in exertion direction. Biomechanical analyses of flexion tasks must consider the stabilizing muscle recruitment patterns when evaluating spinal compression and shear loads.

Active trunk stiffness increases with co-contraction.

Trunk dynamics, including stiffness, mass and damping were quantified during trunk extension exertions with and without voluntary recruitment of antagonistic co-contraction. The objective of this study was to empirically evaluate the influence of co-activation on trunk stiffness. Muscle activity associated with voluntary co-contraction has been shown to increase joint stiffness in the ankle and elbow. Although biomechanical models assume co-active recruitment causes increase trunk stiffness it has never been empirically demonstrated. Small trunk displacements invoked by pseudorandom force disturbances during trunk extension exertions were recorded from 17 subjects at two co-contraction conditions (minimal and maximal voluntary co-contraction recruitment). EMG data were recorded from eight trunk muscles as a baseline measure of co-activation. Increased EMG activity confirms that muscle recruitment patterns were different between the two co-contraction conditions. Trunk stiffness was determined from analyses of impulse response functions (IRFs) of trunk dynamics wherein the kinematics were represented as a second-order behavior. Trunk stiffness increased 37.8% (p < 0.004) from minimal to maximal co-activation. Results support the assumption used in published models of spine biomechanics that recruitment of trunk muscle co-contraction increases trunk stiffness thereby supporting conclusions from those models that co-contraction may contribute to spinal stability.