Bryan K Slinker | PubMed Health

PubMed2005

Relationships between hoof-acceleration patterns of galloping horses and dynamic properties of the track.

OBJECTIVE: To define relationships between hoof-acceleration patterns of galloping horses and dynamic properties of the track. ANIMALS: 8 Thoroughbred horses without lameness. PROCEDURE: Acceleration-time curves were recorded by use of accelerometers attached to each hoof as each horse galloped over the track straightaway. Four sessions were conducted for each horse, with the track surface modified by sequentially adding water before each session. These acceleration-time curves were analyzed to determine peak accelerations during the support phase of the stride. Track dynamic properties (hardness, rebound, deceleration rate, rebound rate, and penetration) were recorded with a track-testing device. Moisture content and dry density were measured from soil samples. Stepwise multiple regression was used to identify relationships between hoof-acceleration variables and track dynamic properties. RESULTS: Track rebound rate was most consistently related to hoof variables, especially through an inverse relationship with negative acceleration peaks for all hooves. Also, rebound rate was related to initial acceleration peak during propulsion of the hooves of the forelimb and the nonlead hind limb as well as to the second acceleration peak during propulsion of the lead hooves of the hind limb and nonlead forelimb. CONCLUSIONS AND CLINICAL RELEVANCE: The inverse relationship between track rebound rate and negative acceleration peaks for all hooves reflects the most important dynamic property of a track. Any factor that reduces negative acceleration of the hooves will increase stride efficiency by allowing smoother transition from retardation to propulsion and therefore may be important in determining the safety of racing surfaces.

Acceleration↗

PubMed2004

Interpreting cardiac muscle force-length dynamics using a novel functional model.

To describe the dynamics of constantly activated cardiac muscle, we propose that length affects force via both recruitment and distortion of myosin cross bridges. This hypothesis was quantitatively tested for descriptive and explanative validity. Skinned cardiac muscle fibers from animals expressing primarily alpha-myosin heavy chain (MHC) (mouse, rat) or beta-MHC (rabbit, ferret) were activated with solutions from pCa 6.1 to 4.3. Activated fibers were subjected to small-amplitude length perturbations [deltaL(t)] rich in frequency content between 0.1 and 40 Hz. In descriptive validation tests, the model was fit to the ensuing force response [deltaF(t)] in the time domain. In fits to 118 records, the model successfully accounted for most of the measured variation in deltaF(t) (R(2) range, 0.997-0.736; median, 0.981). When some residual variations in deltaF(t) were not accounted for by the model (as at low activation), there was very little coherence (<0.5) between these residual force variations and the applied deltaL(t) input function, indicating that something other than deltaL(t) was causing the measured variation in deltaF(t). With one exception, model parameters were estimated with standard errors on the order of 1% or less. Thus parameters of the recruitment component of the model could be uniquely separated from parameters of the distortion component of the model and parameters estimated from any given fiber could be considered unique to that fiber. In explanative validation tests, we found that recruitment and distortion parameters were positively correlated with independent assessments of the physiological entity they were assumed to represent. The recruitment distortion model was judged to be valid from both descriptive and explanative perspectives and is, therefore, a useful construct for describing and explaining dynamic force-length relationships in constantly activated cardiac muscle.

Actin Cytoskeleton↗