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Jill L. McNitt-Gray

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Jill L. McNitt-Gray

The target article, thoughtfully constructed by Dr. Prilutsky, effectively synthesizes available data on multijoint movements regarding coordination patterns of major two- and one-joint muscles, provides evidence for an optimization criterion that predicts critical features of muscle activation patterns, and explores the functional consequences of muscle coordination. This work also provides a clear set of definitions and an organizational framework that is currently needed for a productive interdisciplinary discussion regarding the underlying control mechanisms used during realistic multijoint movements. Although identification of an optimization criterion that predicts muscle recruitment strategies would greatly simplify control logic required for rehabilitation and musculoskeletal modeling, our experimental data during landings indicate more than one criterion may exist. Preliminary review of our experimental landing data suggests the rules identified by Prilutsky apparently hold for some subjects during portions of the landing movements. The presence of more than one muscle activation pattern used to achieve the same NJMs demonstrates there may be more than one optimization criterion that predicts critical features of muscle activation patterns. The functional consequences of more than one control criterion may also prove to be an asset, particularly when adapting to different environmental constraints.

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Jill L. McNitt-Gray

During a landing impact, the human body is exposed to large forces and moments that create the potential for injury. To determine the effect of impact velocity and landing experience on the strategy selected, the preferred landing strategies used by male collegiate gymnasts and recreational athletes from three drop heights were characterized using mechanical descriptors. Kinematic and reaction force data were acquired simultaneously using highspeed film and a force plate. Reaction forces and lower extremity joint motion were used to characterize the strategies. Results indicated that statistically significant increases in joint flexion (with the exception of ankle joint flexion), angular velocity, and impact force resulted as impact velocity increased. Gymnasts and recreational athletes demonstrated similar adjustment patterns to increases in landing impact velocities; however, significant differences in degree of joint flexion, total landing phase time, and relative adjustments over impact velocity conditions were found.

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Jill L. McNitt-Gray

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Travis J. Peterson and Jill L. McNitt-Gray

Golf shots off uneven terrain often require modifications in address position to complete the swing successfully. This study aimed to determine how golf players coordinate the legs to regulate linear and angular impulse (about an axis passing vertically through the center of mass) while modifying the lower-extremity address position during the swing. Nine highly skilled golf players performed swings with a 6-iron under the Normal, Rear Leg Up, and Target Leg Up conditions. Components of linear and angular impulse generated by the rear and target legs (resultant horizontal reaction force, resultant horizontal reaction force angle, and moment arm) were quantified and compared across the group and within a player (α = .05). Net angular impulse did not change between conditions. Target leg angular impulse was greater in the Target Leg Up condition than Rear Leg Up condition. Regulation of linear and angular impulse generation occurred while increasing stance width and redirecting resultant horizontal reaction forces to be more parallel to the target line under modified address positions. Net linear impulse perpendicular to the target was near 0 or slightly posterior. Net linear impulse parallel to the target was less toward the target in the Target Leg Up condition compared with Normal and Rear Leg Up conditions. These results indicate individuals utilized player-specific mechanisms to coordinate the legs and regulate impulse generation during the golf swing under modified address positions.

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Witaya Mathiyakom, Jill L. McNitt-Gray, and Rand R. Wilcox

Angular impulse generation is dependent on the position of the total body center of mass (CoM) relative to the ground reaction force (GRF) vector during contact with the environment. The purpose of this study was to determine how backward angular impulse was regulated during two forward translating tasks. Control of the relative angle between the CoM and the GRF was hypothesized to be mediated by altering trunk–leg coordination. Eight highly skilled athletes performed a series of standing reverse somersaults and reverse timers. Sagittal plane kinematics, GRF, and electromyograms of lower extremity muscles were acquired during the take-off phase of both tasks. The magnitude of the backward angular impulse generated during the push interval of both tasks was mediated by redirecting the GRF relative to the CoM. During the reverse timer, backward angular impulse generated during the early part of the take-off phase was negated by limiting backward trunk rotation and redirecting the GRF during the push interval. Biarticular muscles crossing the knee and hip coordinated the control of GRF direction and CoM trajectory via modulation of trunk–leg coordination.

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Rand Wilcox, Travis J. Peterson, and Jill L. McNitt-Gray

The paper reviews advances and insights relevant to comparing groups when the sample sizes are small. There are conditions under which conventional, routinely used techniques are satisfactory. But major insights regarding outliers, skewed distributions, and unequal variances (heteroscedasticity) make it clear that under general conditions they provide poor control over the type I error probability and can have relatively poor power. In practical terms, important differences among groups can be missed and poorly characterized. Many new and improved methods have been derived that are aimed at dealing with the shortcomings of classic methods. To provide a conceptual basis for understanding the practical importance of modern methods, the paper reviews some modern insights related to why methods based on means can perform poorly. Then some strategies for dealing with nonnormal distributions and unequal variances are described. For brevity, the focus is on comparing 2 independent groups or 2 dependent groups based on the usual difference scores. The paper concludes with comments on issues to consider when choosing from among the methods reviewed in the paper.

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Witaya Mathiyakom, Rand Wilcox, and Jill L. McNitt-Gray

Studying how elite athletes satisfy multiple mechanical objectives when initiating well-practiced, goal-directed tasks provides insights into the control and dynamics of whole-body movements. This study investigated the coordination of multiple body segments and the reaction force (RF) generated during foot contact when regulating forward angular impulse in backward translating tasks. Six highly skilled divers performed inward somersaults (upward and backward jump with forward rotation) and inward timers (upward and backward jump without rotation) from a stationary platform. Sagittal plane kinematics and RFs were recorded simultaneously during the takeoff phase. Regulation of the forward angular impulse was achieved by redirecting the RF about the total body center of mass. Significantly more backward-directed RF was observed during the first and second peak horizontal RF of the inward somersaults than the inward timers. Modulation of the horizontal RF altered the RF direction about the center of mass and the lower-extremity segments. Backward leg and forward trunk orientation and a set of relatively large knee extensor and small hip flexor net joint moments were required for forward angular impulse generation. Understanding how the forward angular impulse is regulated in trained individuals provides insights for clinicians to consider when exploring interventions related to fall prevention.

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Antonia M. Zaferiou, Rand R. Wilcox, and Jill L. McNitt-Gray

This study determined how dancers regulated angular and linear impulse during the initiation of pirouettes of increased rotation. Skilled dancers (n = 11) performed single and double pirouette turns with each foot supported by a force plate. Linear and angular impulses generated by each leg were quantified and compared between turn types using probability-based statistical methods. As rotational demands increased, dancers increased the net angular impulse generated. The contribution of each leg to net angular impulse in both single and double pirouettes was influenced by stance configuration strategies. Dancers who generated more angular impulse with the push leg than with the turn leg initiated the turn with the center of mass positioned closer to the turn leg than did other dancers. As rotational demands increased, dancers tended to increase the horizontal reaction force magnitude at one or both feet; however, they used subject-specific mechanisms. By coordinating the generation of reaction forces between legs, changes in net horizontal impulse remained minimal, despite impulse regulation at each leg used to achieve more rotations. Knowledge gained regarding how an individual coordinates the generation of linear and angular impulse between both legs as rotational demand increased can help design tools to improve that individual’s performance.