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Jocelyn F. Hafer, Mark S. Miller, Jane A. Kent and Katherine A. Boyer

-specific recommendations are needed for adults to maintain mobility and joint health throughout the life span. The aim of this study was to examine sex-specific effects of both PA and age on gait mechanics and knee extensor muscle torque, power, and fatigability. We hypothesized that there would be differences by sex in

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Sarah A. Wyszomierski, April J. Chambers and Rakié Cham

Slips and falls are a serious public health concern in older populations. Reduced muscle strength is associated with increased age and fall incidence. Understanding the relationships between specific joint muscle strength characteristics and propensity to slip is important to identify biomechanical factors responsible for slip-initiated falls and to improve slip/fall prevention programs. Knee corrective moments generated during slipping assist in balance recovery. Therefore, the study goal was to investigate the relationship between knee flexion/extension strength and slip severity. Isometric knee flexion/extension peak torque and rate of torque development (RTD) of the slipping leg were measured in 29 young and 28 older healthy subjects. Motion data were collected for an unexpected slip during self-paced walking. Peak slip velocity (PSV) of the slipping heel served as a slip severity measure. Within-sex and age group regressions relating gait speed-controlled PSV to strength of the slipping leg revealed significant inverse PSV-knee extension peak torque and PSV-knee flexion/extension RTD relationships in young males only. Differences in PSV-strength relationships between sex and age groups may be caused by greater ranges of strength capabilities in young males. In conclusion, the ability to generate higher, more rapid knee flexion/extension muscle moments (greater peak torque/RTD) may assist in recovery from severe slips.

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Jocelyn F. Hafer and Katherine A. Boyer

Aging is associated with declining strength, 1 , 2 a downward shift in muscle torque–velocity characteristics, 3 slowing gait speed, 4 , 5 and changes in gait mechanics. 6 In particular, strength or power of the knee extensor muscles correlates with walking speed 4 , 7 , 8 and mobility

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Blair Crewther, Christian Cook, John Fitzgerald, Michal Starczewski, Michal Gorski and Joanna Orysiak

separately, in the order flexion–extension. The positioning of subjects in each assessment has been described ( 16 ). Verbal encouragement was given to ensure maximal contractions in each exercise. Joint muscle torques were analyzed as maximal values, before summing values in the upper body (UBMT), inclusive

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Rebecca L. Krupenevich, Nick Murray, Patrick M. Rider, Zachary J. Domire and Paul DeVita

Since vision is used in studies of muscle force control, reduced muscle force control might be related to reduced visual ability. We investigated relationships between steadiness in eye movements and quadriceps muscle torque (a surrogate for force) during isometric contractions of constant and varying torques. Nineteen young adults with an average age of 20.7 years and 18 old adults with an average age of 71.6 years performed three vision tasks, three vision and torque tasks at 40% maximal voluntary contraction (MVC), and three vision and torque tasks at 54 nm. Age groups had identical torque steadiness (CV) in 40%-MVC and 54-nm conditions (p > .05). Old had similar vertical (p > .05) but decreased horizontal visual steadiness (SD) (p < .05) compared with young. Correlations between visual steadiness and muscle torque steadiness failed to show a significant relationship (p > .05). We were unable to identify a substantial relationship between muscle torque steadiness and eye movement, as a component of visual steadiness, and conclude that reduced visual steadiness does not contribute to reduced muscle torque steadiness.

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Caroline J. Ketcham, Natalia V. Dounskaia and George E. Stelmach

The present study investigates whether regulation of interactive torque during multijoint movements decays with advanced age as a result of declines in the motor system. Young and elderly adults repeatedly drew a circle and ovals oriented in different directions using shoulder and elbow joint movements. Each template was traced at three levels of cycling frequency with and without vision. Although vision did not affect performance, increases in cycling frequency caused distortions of movement trajectories in both groups. The pattern of distortions differed, however, between the groups. These differences were accounted for by differences in elbow control. Young adults provided regulation of elbow amplitude and timing by matching muscle torque magnitude with increased interactive torque. In contrast, elderly adults did not increase muscle torque magnitude and modulated torque timing for elbow motion regulation. This strategy is discussed as adaptation to decrements in the aging motor system.

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Andrea Biscarini

We have developed a 2-D analytical biomechanical model for monoarticular open kinetic-chain exercises with lever selectorized equipment, and different relative placement between the joint center of rotation (J) and the center of rotation (C) of the resistance input lever (“off-center” exercises). All the relevant geometrical aspects of such exercises have been characterized: the change with the joint angle of the distance between the resistance pad (P) and J, and of the angle between CP and JP (i.e., the angle between the resistance input lever and the exercising limb). These changes may strongly affect the joint load and the muscle torque in inverse dynamic problems, given the joint kinematics and the mass of the selected weight stack. Therefore, the muscle torque, the shear and axial components of the joint load have been calculated analytically as a function of the relative positioning of C and J, and the length CP, in addition to the parameters that define the joint kinematics, the equipment mechanics, and the external load. From these results we have derived the optimal cam profiles for “off-center” exercises, as well as the geometrical “off-center” setting that minimizes the shear component of the tibiofemoral joint load in leg extension equipment.

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Alan J. Sokoloff, Timothy C. Cope, T. Richard Nichols and Arthur W. English

Differences in the directions of torque produced by motor units might be used by the nervous system to coordinate posture and movement. Here we report plantar flexion and abduction isometric torques exerted at the ankle by 158 motor units in the cat medial gastrocnemius (MG) muscle. In five cats, motor unit torque direction differed by an average of 10°. Torque direction was weakly correlated with unit contraction time in 3 of 5 experiments, with tetanic force in 3 of 5 experiments, and with conduction velocity in 1 of 5 experiments. The direction of whole muscle torque, however, was constant at all levels of MG activation produced in sural and crossed extension reflexes. Thus, although there is a range in the direction of torque produced by motor units in the cat MG, we find no evidence for the ordering of motor units according to torque direction during MG activation.

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Andrew S. Cole, Megan E. Woodruff, Mary P. Horn and Anthony D. Mahon

Relationships between physiological parameters and 5-km running performance were examined in 15 male runners (17.3 ± 0.9 years). Running economy (RE) and blood lactate concentration ([BLa]) at 241.2 m/min, VO2max, velocity at VO2max (vVO2max), vertical jump height and muscle power, and isokinetic knee extension strength at 60°/sec and 240°/sec were measured. The participants’ best 5-km race time over the last month of the cross-country season (16.98 ± 0.76 min) was used in the analysis. The data were analyzed using Pearson correlation coefficients. Significant relationships to run time were observed for VO2max (r = -.53), RE (r = .55), and vVO2max (r = -.66), but not [BLa], isokinetic muscle torque, or vertical jump. Identifying the unique strength and power characteristics related to running performance in this age group is warranted.

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Natalia V. Dounskaia, Caroline J. Ketcham and George E. Stelmach

Influence of mechanical interactions between the shoulder and elbow on production of different coordination patterns during horizontal arm movements is investigated in the present study. Subjects performed cyclical movements along a circle and along lines of 4 different orientations. Cycling frequency was manipulated to highlight control features responsible for interactive torque regulation. When the shoulder was involved in motion, torque analysis revealed that this joint was controlled similarly during all movement types. At the elbow, however, each movement type required a specific pattern of regulation of interactive torque with muscle torque. When interactive torque acted in the direction of the required elbow rotation, the demands for active control were lower than when the interactive torque resisted elbow motion and had to be actively suppressed. Kinematic analysis demonstrated that increases in cycling frequency systematically deformed the fingertip path. The amount of these deformations differed across movement types, being more pronounced for movements where the interactive torque resisted joint motion. It appears that interactive torque can assist or resist movement at the joints, making control of some movement types more difficult than others.