AN EXAMINATION OF THE SLOW AND FAST STRETCH SHORTENING CYCLE IN CROSS COUNTRY RUNNERS AND SKIERS

Performance in fast and slow stretch shortening cycle (SSC) activity was examined. 13 NCAA Div. I cross country skiers and runners performed a countermovement jump (CMJ) and a drop jump (DJ) on a force platform. These jumping actions were classified as slow and fast SSC activities respectively based on ground contact times. In the slow SSC subjects achieved significantly greater jump heights while in the fast SSC subjects produced greater peak ground reaction force and measured higher on the reactive strength index. A weak correlation was found between slow SSC and fast SSC ability suggesting that training in slow SSC tasks might not accrue benefit in fast SSC ability and vice versa. Consideration to ground contact duration and the principle of specificity should be given when using the CMJ or the DJ as a testing tool or as a training exercis

ORAL CREATINE SUPPLEMENTATION AND SHORT-TERM DYNAMIC POWER PRODUCTION IN HEALTHY YOUNG MEN

This experiment examined the effect of creatine monohydrate supplementation on corrected peak power output and peak acceleration during repeated, high intensity sprint cycling. The investigation was randomised, placebo-controlled, double-blind and adopted a crossover design. Eight male, adult volunteers participated in this study. Subjects were administered with creatine monohydrate (0.28g.kg-1 per day) or a glucose placebo. Following experimental treatments, subjects underwent 10 maximal effort, 6-s sprints on a cycle ergometer with a work to rest ratio of 5:1. The exercise protocol was fatiguing in nature with peak power output and peak acceleration decreasing significantly from the first to last sprint. Creatine supplementation significantly attenuated the decline in peak power output in the latter stages of the intermittent sprint cycling

Dynamical Evolution of the Extra Dimension in Brane Cosmology

The evolution of the extra dimension is investigated in the context of brane world cosmology. New cosmological solutions are found. In particular, solutions in the form of waves travelling along the extra dimension are identified.Comment: Latex file, 10 page

Adiabatic instability in coupled dark energy-dark matter models

We consider theories in which there exists a nontrivial coupling between the dark matter sector and the sector responsible for the acceleration of the universe. Such theories can possess an adiabatic regime in which the quintessence field always sits at the minimum of its effective potential, which is set by the local dark matter density. We show that if the coupling strength is much larger than gravitational, then the adiabatic regime is always subject to an instability. The instability, which can also be thought of as a type of Jeans instability, is characterized by a negative sound speed squared of an effective coupled dark matter/dark energy fluid, and results in the exponential growth of small scale modes. We discuss the role of the instability in specific coupled CDM and Mass Varying Neutrino (MaVaN) models of dark energy, and clarify for these theories the regimes in which the instability can be evaded due to non-adiabaticity or weak coupling.Comment: 20 pages, 2 figures; final published versio

LOWER LIMB PERFORMANCE IN ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTED INDIVIDUALS

This experiment examined lower limb performance in individuals with previous history of anterior cruciate ligament rupture and reconstruction. 10 volunteers took part in the study. Lower limb performance was assessed with maximal effort countermovement, drop and rebound jumps on a force sledge apparatus. The subjects’ contra-lateral, uninjured leg was used as an internal control. No significant differences were observed in any of the measured variables, in any of the jumping procedures between the involved and uninvolved leg. This suggests that current reconstructive practices and post-surgery rehabilitation techniques may have the ability to restore ACL deficient legs to a similar level of performance to an uninvolved control leg in dynamic, non-fatiguing, maximal effort jumping activity

THE EFFECT OF lWO FORCE SLEDGE APPARATUS JUMPING PROTOCOLS ON TRIAL TO TRIAL RELIABILITY AND MUSCLE PERFORMANCE

This experiment examined the effect of differing jump protocols performed on a specially constructed force sledge apparatus on trial to trial reliability and overall lower extremity muscular performance. Eight adult volunteers, of varying activity profiles, participated in this study. On the force sledge apparatus, subjects performed bouts of drop jumps and rebound jumps. Measurements of flight time, ground contact time, reactive strength index, peak ground reaction force, displacement of spring mass and vertical stiffness were obtained for three drop jumps and three rebound jumps on the sUbjects' preferred jumping leg. The results indicated that the rebound jump protocol provided a tighter control of ground contact times and displacement of spring mass, indicatin9' a more consistent jumping strategy. Additionally, a significantly higher peak ground reaction force was produced in the rebound jump protocol

RELIABILITY OF THE REACTIVE STRENGTH INDEX AND TIME TO STABILIZATION DURING DEPTH JUMPS

Reliability of reactive strength index (RSI) and time to stabilization (TTS) was examined during three maximal effort depth jumps from 30cm (N=22). Measures of jump height (JH), ground contact time (CT), RSI and TTS were obtained and analyzed for reliability. The JH, CT and RSI were shown to be highly reliable from trial-to-trial (ICCsingle > 0.9). Time to stabilization was not reliable from trial-to-trial (ICCsingle < 0.5). The RSI can be used to monitor performance or to optimize the height of depth jumps. Results suggest that coaches utilizing these procedures with large numbers of athletes may be able to use a single measure of RSI rather than repeated trials. Time to stabilization reliability must be improved before attempting to use it to quantify the landing phase of plyometric exercises

RATE OF FORCE DEVELOPMENT AND TIME TO PEAK FORCE DURING PLYOMETRIC EXERCISES

Rate of force development (RFD) during the first 100 and 250 msec of the positive acceleration phase of plyometric exercises and time to peak force were determined in 23 NCAA Div. I athletes. Subjects performed a countermovement jump (CMJ), cone hop (CH), tuck jump (TJ), single leg CMJ (SLJ), and squat jump with 30% 1 RM squat (SJ30) on a force platform. Results showed SLJ and SJ30 had lower RFD100 and higher time to peak force, while CH and TJ had higher RFD100 and shorter time to peak force. These findings are in agreement with previous research that shows that quick movement exercises have high RFD. However, RFD250 may be an inappropriate measure to classify very quick jumps, such as the CH, because RFD values approach zero or become negative when subjects are close to or already leaving the ground

The Constraints in Spherically Symmetric General Relativity II --- Identifying the Configuration Space: A Moment of Time Symmetry

We continue our investigation of the configuration space of general relativity begun in I (gr-qc/9411009). Here we examine the Hamiltonian constraint when the spatial geometry is momentarily static (MS). We show that MS configurations satisfy both the positive quasi-local mass (QLM) theorem and its converse. We derive an analytical expression for the spatial metric in the neighborhood of a generic singularity. The corresponding curvature singularity shows up in the traceless component of the Ricci tensor. We show that if the energy density of matter is monotonically decreasing, the geometry cannot be singular. A supermetric on the configuration space which distinguishes between singular geometries and non-singular ones is constructed explicitly. Global necessary and sufficient criteria for the formation of trapped surfaces and singularities are framed in terms of inequalities which relate appropriate measures of the material energy content on a given support to a measure of its volume. The strength of these inequalities is gauged by exploiting the exactly solvable piece-wise constant density star as a template.Comment: 50 pages, Plain Tex, 1 figure available from the authors

Predicting the Starquakes in PSR J0537-6910

We report on more than 7 years of monitoring of PSR J0537-6910, the 16 ms pulsar in the Large Magellanic Cloud, using data acquired with the RXTE. During this campaign the pulsar experienced 23 sudden increases in frequency (``glitches'') amounting to a total gain of over six ppm of rotation frequency superposed on its gradual spindown of d(nu)/d(t) = -2e-10 Hz/s. The time interval from one glitch to the next obeys a strong linear correlation to the amplitude of the first glitch, with a mean slope of about 400 days ppm (6.5 days per uHz), such that these intervals can be predicted to within a few days, an accuracy which has never before been seen in any other pulsar. There appears to be an upper limit of ~40 uHz for the size of glitches in_all_ pulsars, with the 1999 April glitch of J0537 as the largest so far. The change in the spindown of J0537 across the glitches, Delta(d(nu)/d(t)), appears to have the same hard lower limit of -1.5e-13 Hz/s, as, again, that observed in all other pulsars. The spindown continues to increase in the long term, d(d(nu)/d(t))/d(t) = -1e-21 Hz/s/s, and thus the timing age of J0537 (-0.5 nu d(nu)/d(t)) continues to decrease at a rate of nearly one year every year, consistent with movement of its magnetic moment away from its rotational axis by one radian every 10,000 years, or about one meter per year. J0537 was likely to have been born as a nearly-aligned rotator spinning at 75-80 Hz, with a |d(nu)/d(t)| considerably smaller than its current value of 2e-10 Hz/s. The pulse profile of J0537 consists of a single pulse which is found to be flat at its peak for at least 0.02 cycles.Comment: 54 pages, 12 figures. Accepted for publication in The Astrophysical Journal. Cleaner figure 2. V4 -- in line with version accepted by Ap