Effects of downhill treadmill running on uncoupling protein 3 mRNA expression.

Eccentric biased exercise has been reported to elicit more muscle injury than concentric or isometric exercise and potentially generate increased oxidative stress one to two days post exercise. Increased oxidative stress has been shown to up-regulate the expression of UCP3 mRNA. The aim of this study was to investigate the effects of downhill running on skeletal muscle UCP3 mRNA expression. Twenty-four male Sprague Dawley rats were randomly assigned to run continuously for 30 minutes (30-C, n = 6), or run six 5-minute bouts separated by rest periods of 2 minutes (2-R, n = 6), 4 minutes (4-R, n = 6), and 6 minutes (6-R, n = 6) on a 16 degree declined treadmill at a speed of 16 m. min (-1). Sham control animals (n = 8) were placed in a treadmill chamber during the 30-minute run session. Semi-quantitative RT-PCR was conducted to evaluate UCP3 mRNA levels in the plantaris, a muscle used eccentrically during downhill running and tibialis anterior, a muscle which undergoes very little eccentric muscle contraction during this exercise. The level of gene expression was normalized to 18 S ribosomal mRNA expression from the same PCR product. Results are reported as mean +/- standard error. UCP3 of the plantaris muscles from 2-R animals (2.36 +/- 0.13) was significantly greater than UCP3 of the plantaris from control animals (1.72 +/- 0.13), p < 0.05. UCP3 of the tibialis anterior from the continuous group (1.51 +/- 0.17) was significantly less than the UCP3 of the tibialis anterior of the control group (2.09 +/- 1.4), p < 0.05. These data suggest that downhill treadmill running is associated with an increase in UCP3 mRNA expression in the plantaris muscle. These results indicate that exercise which is biased toward eccentric exercise may up-regulate UCP3 mRNA during the period post exercise when muscle damage and repair is elevated.

Impact of muscle length during stretch-shortening contractions on real-time and temporal muscle performance measures in rats in vivo.

The objective of the present study was to investigate the impact of muscle length during stretch-shortening cycles on static and dynamic muscle performance. Animals were randomly assigned to an isometric (control, Con, n = 12), a short-muscle-length (S-Inj, 1.22-2.09 rad, n = 12), or a long-muscle-length (L-Inj, 1.57-2.44 rad, n = 12) group. The dorsiflexor muscles were exposed in vivo to 7 sets of 10 stretch-shortening contractions (conducted at 8.72 rad/s) or 7 sets of isometric contractions of the same stimulation duration by using a custom-designed dynamometer. Performance was characterized by multipositional isometric exertions and positive, negative, and net work before exposure, 6 h after exposure, and 48 h after exposure to contractions. Real-time muscle performance during the stretch-shortening cycles was characterized by stretch-shortening parameters and negative, positive, and net work. The S-Inj group recovery (force difference) was similar to the Con group force difference at 48 h, whereas the L-Inj group force difference was statistically greater at 1.39, 1.57, and 1.74 rad than the Con group force difference (P < 0.05). Negative work (P < 0.05) and net work (P < 0.05) were statistically lower in the S-Inj and L-Inj groups than in the Con group 48 h after exposure to contractions. Of the real-time parameters, there was a difference in cyclic force with treatment during the stretch-shortening cycles (P < 0.0001), with the L-Inj group being the most affected. Thus longer ranges of motion result in a more profound isometric force decrement 48 h after exposure to contractions and in real-time changes in eccentric forces.

Dynamic force responses of skeletal muscle during stretch-shortening cycles.

Muscle damage due to stretch-shortening cycles (i.e., cyclic eccentric/concentric muscle actions) is one of the major concerns in sports and occupational related activities. Mechanical responses of whole muscle have been associated with damage in neural motor units, in connective tissues, and the force generation mechanism. The objective of this study was to introduce a new method to quantify the real-time changes in skeletal muscle forces of rats during injurious stretch-shortening cycles. Male Sprague Dawley rats ( n=24) were selected for use in this study. The dorsi flexor muscle group was exposed to either 150 stretch-shortening cycles ( n=12) or 15 isometric contractions ( n=12) in vivo using a dynamometer and electrical stimulation. Muscle damage after exposure to stretch-shortening cycles was verified by the non-recoverable force deficit at 48 h and the presence of myofiber necrosis. Variations of the dynamic forces during stretch-shortening cycles were analyzed by decomposing the dynamic force signature into peak force ( F(peak)), minimum force ( F(min)), average force ( F(mean)), and cyclic force ( F(a)). After the 15th set of stretch-shortening cycles, the decrease in the stretch-shortening parameters, F(peak), F(min), F(mean), and F(a), was 50% ( P<0.0001), 26% ( P=0.0055), 68% ( P<0.0001), and 50% ( P<0.0001), respectively. Our results showed that both isometric contractions and stretch-shortening cycles induce a reduction in the isometric force. However, the force reduction induced by isometric contractions fully recovered after a break of 48 h while that induced by stretch-shortening cycles did not. Histopathologic assessment of the tibialis anterior exposed to stretch-shortening cycles showed significant myofiber degeneration and necrosis with associated inflammation, while muscles exposed to isometric contractions showed no myofiber degeneration and necrosis, and limited inflammation. Our results suggest that muscle damage can be identified by the non-recoverable isometric force decrement and also by the variations in the dynamic force signature during stretch-shortening cycles.

Dynamic performance of the mechanism of an automatically deployable ROPS.

The mechanism for an automatically deployable ROPS (AutoROPS) has been designed and tested. This mechanism is part of an innovative project to provide passive protection against rollover fatality to operators of new tractors used in both low-clearance and unrestricted-clearance tasks. The device is a spring-action, telescoping structure that releases on signal to pyrotechnic squibs that actuate release pins. Upper post motion begins when the release pins clear an internal piston. The structure extends until the piston impacts an elastomeric ring and latches at the top position. In lab tests the two-post structure consistently deployed in less than 0.3 s and latched securely. Static load tests of the telescoping structure and field upset tests of the fully functional AutoROPS have been successfully completed.

Static load test performance of a telescoping structure for an automatically deployable ROPS.

The automatically deployable ROPS was developed as part of an innovative project to provide passive protection against overturn fatality to operators of new tractors used in both low-clearance and unrestricted-clearance tasks. The primary objective of this phase of the research was to build a telescoping structure that would prove that a ROPS can be built that will (1) reliably deploy on signal, (2) rise in a sufficiently short amount of time, (3) firmly latch in its deployed position, and (4) satisfy SAE J2194 testing requirements. The two-post structure had previously been found to meet deployment time criteria, and design analyses indicated that neither the slip-fit joint nor the latch pins would fail at test loading. Four directions of static loading were applied to the structure to satisfy SAE requirements. For the series of static loading tests, the raised structure was found to maintain a protective clearance zone after all loads were applied. The structure is overly stiff and should be redesigned to increase its ability to absorb ground-impact energy. Results of dynamic tests and field upset tests are reported in companion articles. The next phase of development is to optimize the structure so that it will plastically deform and absorb energy that would otherwise be transferred to the tractor chassis.

Evaluation of an instrumented walkway for measurement of the kinematic parameters of gait.

The purpose of this study was to compare kinematic gait parameters measured with an instrumented walkway system (GAITRite(R)) and a video-based system (peak performance motus 3.1(R)). Subjects walked across a GAITRite mat with embedded pressure sensors. Reflective markers were attached to subjects' shoes and video capture was simultaneously performed during each trial. Video data were then digitized manually using peak software. Correlation coefficients for all parameters measured with both systems were high (>/=0.94). Significant differences between systems were found with analysis of variance (ANOVA) for two parameters, step length and stride velocity (P=0.003, 0.0002). The results of this study indicate that the instrumented walkway gave comparable results for temporal parameters but further investigation is needed to evaluate the fidelity of its spatial performance.

Aerobic exercise as a countermeasure for microgravity-induced bone loss and muscle atrophy in a rat hindlimb suspension model.

BACKGROUND: Loss of bone and skeletal muscle atrophy resulting from non-weight-bearing are major concerns associated with microgravity environment and spaceflight deconditioning. The objective of this research was to address the fundamental issue of whether bone loss and muscle atrophy could be attenuated using weight-bearing aerobic exercise on a treadmill as a countermeasure in rats subjected to simulated weightlessness by hindlimb suspension. METHOD: Bone and muscle from control and hindlimb-suspended groups with and without exercise were evaluated by bone mineral density (BMD), mechanical tests, bone histomorphometry and muscle mass. RESULTS: Femoral BMD of hindlimb-suspended (HS) rats subjected to treadmill exercise was significantly greater than femoral BMD of HS rats without exercise and also was equivalent to that of weight-bearing controls. Muscle mass from HS rats exercised on a treadmill was significantly greater than muscle mass from HS rats that did not exercise. Exercise did not result in muscle mass equal to that of controls, however. In addition, histomorphometric analysis of the metaphysis of the proximal tibia revealed that HS rats that exercised did not maintain bone formation equivalent to controls. No other bone parameters were found to vary significantly between groups. CONCLUSIONS: It was concluded that moderate aerobic exercise on a treadmill did attenuate bone loss and muscle atrophy due to simulated weightlessness by hindlimb suspension, however its effectiveness differed by tissue, anatomical site and parameter investigated.

Dynamometer for rat plantar flexor muscles in vivo.

A dynamometer is designed and fabricated to measure the force output during static and dynamic muscle actions of the plantar flexor muscles of anaesthetised rats in vivo. The design is based on a computer-controlled DC servomotor capable of angular velocities in excess of 17.5 rad s-1. The system controls the range of motion, angular velocity and electrical stimulation of the muscles, while monitoring the force output at the plantar surface of the foot. The force output is measured by a piezo-electric load cell that is rated at 5 kg capacity. Angular velocity and position are measured by a DC tachometer and potentiometer, respectively. All measurement devices are linear (r2 = 0.9998). The design minimises inertial loading during high-speed angular motions, with a variation in force output of less than 0.2%. The dynamometer proves to be an accurate and reliable system for quantifying static and dynamic forces of rat plantar flexor muscles in vivo.