These data support the premise that longer lower limb tendons (especially Achilles tendon) and less flexible lower limb joints are associated with improved running economy.

The velocities of individual dislocations of edge and mixed types in pure aluminum single crystals were determined as a function of applied-resolved shear stress and temperature. The dislocation velocities were determined from measurements of the displacements of individual dislocations produced by stress pulses of known duration. The Berg-Barrett x-ray technique was employed to observe the dislocations, and stress pulses of 15 to 108 !'sec duration were applied by propagating torsional waves along the axes of [111} oriented cylindrical crystals. Resolved shear stresses up to 16X 10 6 dynes/em• were applied at temperatures ranging from -150° to +70°C, and dislocation velocities were found to vary from 10 to 2800 em/sec over these ranges of stress and temperature. The experimental conditions were such that the dislocation velocities were not significantly influenced by impurities, dislocation curvature, dislocation-dislocation interactions, or long-range internal stress fields in the crystals. The velocity of dislocations is found to be linearly proportional to the applied-resolved shear stress, and to decrease with increasing temperature, Qualitative comparison of these results with existing theories leads to the conclusion that the mobility of individual dislocations in pure aluminum is governed by dislocation-phonon interactions. The phonon-viscosity theory of dislocation mobility can be brought into agreement with the experimental results by reasonable choices ot the values of certain constants appearing in the theory.

The purpose of this study was to determine where stretch-shortening cycle (SSC) potentiation of force, power, velocity, and acceleration occurs across the concentric phase of ballistic leg presses. Second, we examined the influence of late eccentric phase force and length of the amortization phase on potentiated concentric phase performance variables. Twenty-one male runners (age: 31.9 ± 4.7 years) performed SSC and concentric-only (CO) ballistic leg press throws. Potentiations of concentric actions were calculated as the difference between SSC and CO contractions. An analysis splitting the concentric range of motion (ROM) into 6 equal time intervals determined force and acceleration were potentiated (p < 0.05) only during the first one-sixth time interval of concentric motion, whereas velocity and power were potentiated (p < 0.05) at all time intervals over the entire concentric motion with the exception of power over the last one-sixth time interval. A more precise analysis examining 20-millisecond time intervals across the first 200 milliseconds of concentric motion determined force was potentiated only over the first 140 milliseconds and acceleration only over the first 160 milliseconds. Eccentric force measured during the last 100 milliseconds of eccentric motion was related to potentiated force during the initial 200 milliseconds of concentric motion (r = 0.44, p < 0.05) and potentiated mean power across the full concentric ROM (r = 0.62, p < 0.01). Results indicate that in contrast to power and velocity, potentiation of force and acceleration occurs only early during the concentric phase of SSC ballistic leg presses. Correlational findings imply late eccentric phase force is important for generating force and power during the concentric phase of the SSC and thus training focusing on enhancing late phase eccentric force appears important for developing explosive force and power during SSC movements.

The velocity of dislocations of mixed edge-screw type in copper crystals of 99.999% purity has been measured as a function of stress at room temperature. Dislocation displacements produced by torsion stress pulses of microsecond duration were detected by etch pitting {100} surfaces. A nearly linear relationship between dislocation velocity and resolved shear stress was found. Stresses from 2.8×106 to 23.1×106 dyn/cm2 produced velocities from 160 to 710 cm/sec. These data give a value of the damping constant for high-velocity dislocations of 7×10−4 dyn·sec/cm2, in good agreement with the values deduced from internalfriction measurements. The results also agree, within experimental and theoretical uncertainties, with the phonon viscosity model for the mobility of dislocations.