The effects of sway velocity on the triceps surae H-reflex during quiet standing

We have previously observed a change in the magnitude of the soleus (SOL) and medial gastrocnemius (MG) H-reflexes during different sway positions of quiet standing. The purpose of the present study was to extend the earlier finding by examining whether the SOL and MG H-reflexes are additionally influenced by the velocity of sway, i.e., whether the body is swaying in either the forward or backward direction. Five healthy subjects participated in the study. The mean position of the centre of pressure (COP) in the antero-posterior direction was determined while the subject stood quietly on a force plate for 60 s. In contrast to the earlier study, where the H-reflex was tested at the outermost positions of sway (±6 mm from the baseline mean), the current study elicited a SOL and MG H-reflex as the COP passed through the mean position of sway. This resulted in two sway conditions, where the position of the COP was the same but the sway velocity was different (10 mm s-1 forward and 10 mm s-1 backward). During the forward as compared to the backward velocity condition, there was a 20% and 25% increase in the amplitude of the H-reflex for the SOL and MG muscles, respectively, while the size of their respective background activities were the same. SOL and MG M-waves, as well as the level of background activity from the antagonist (tibialis anterior), were not different between the two sway conditions and thus cannot account for the observed changes to the amplitude of the H-reflexes. It can be concluded from these results that the direction (velocity) of sway has the ability to influence the size of the SOL and MG H-reflexes. The facilitation of the SOL and MG H-reflexes observed while swaying forward may be due to a reduction in presynaptic inhibition or an improvement in Ia synaptic efficacy brought about by changes in muscle length.

Inflatable rescue boat-related injuries in Queensland surf lifesavers: The epidemiology - biomechanics interface

The objective was to describe the relationship between epidemiological and biomechanical factors in the causal pathway of inflatable rescue boat (IRB)-related injuries in Australian surf lifesavers; to develop epidemiological and biomechanical methodologies and measurement instruments that identify and measure the risk factors, for use in future epidemiological studies. Epidemiological and biomechanical models of injury causation were combined. Host, agent and environmental factors that influenced total available force for transfer to host were specified. Measurement instruments for each of the specified risk factors were developed. Instruments were piloted in a volunteer sample of surf lifesavers. Participant characteristics were recorded using demographic questionnaires; IRB operating techniques were recorded using a custom-made on-board camera (Grand RF-Guard) and images of operating techniques were coded by two independent observers. Ground reaction forces transmitted to the host through the lifesaver's feet at the time of wave impact were measured using a custom-built piezoelectric force platform. The demographic questionnaire was found practical; the on-board camera functioned successfully within the target environment. Agreement between independent coders of IRB operating technique images was significant (p < 0.001) with Kappa values ranging from 0.5 to 0.7. Biomechanical instruments performed successfully in the target environment. Peak biomechanical forces were 415.6N (left foot) and 252.9N (right foot). This study defines the relationship between epidemiological and biomechanical factors in modifying the risk of IRB-related injury in a population of surf lifesavers. Preliminary feasibility of combining epidemiological and biomechanical information has been demonstrated. Further testing of the proposed model and measurement instruments is required.