Biomedical subjects

Marcelo Epstein

Publications and source records attributed to Marcelo Epstein.

4 recordsLinked to original sources

PubMed2005

Should tendon and aponeurosis be considered in series?

Fibres, aponeuroses, and tendons are often considered mechanically "in series" in skeletal muscles. This notion has led to oversimplified calculations of fibre forces from tendon forces, to incorrect derivations of constitutive laws for aponeuroses, and to misinterpretations of the recovery of elastic energy in stretch-shortening cycles of muscles. Here, we demonstrate theoretically, using examples of increasing complexity, that tendon and aponeurosis are not in series in a muscle fibre-aponeurosis-tendon complex. We then demonstrate that assuming the tendon and aponeurosis to be in series can lead to the appearance of mechanical work creation in these passive viscoelastic structures, a result that is mechanically impossible. Finally, we explain the mechanical role of the incompressible muscle matrix in force transmission from fibres to aponeuroses and tendon, and emphasize that incompressibility necessitates the introduction of extra forces necessary to maintain this constraint. Unfortunately, this requirement eliminates, for all but the simplest cases, a theoretical approach of muscle modeling based on intuitive free-body diagrams.

Animals↗

PubMed2005

Vibrations of Euler's disk.

A model of a partially deformable Euler disk is presented that allows transverse vibrations to be treated with the techniques of classical analytical mechanics. The model clearly shows that the increasing audible frequency produced during motion can be directly related to the forcing effect of the reaction and the angular velocity of the contact point. The material of the disk seems to play a role in affecting the intensity and quality of the sound, but not its pitch. Moreover, the friction force grows rapidly with the decline of the disk, thus causing the slipping that is partially responsible for the abrupt end of the motion. The model also supports the conjecture [P. Kessler and O. M. O'Reilly, Regul. Chaotic Dyn. 7, 49 (2002)] that the vibrations themselves contribute to this phenomenon by causing a loss of contact with the surface at small angles of inclination.

Journal Article↗

PubMed2005

An articular cartilage contact model based on real surface geometry.

Abnormal, excessive stresses acting on articular joint surfaces are speculated to be one of the causes for joint degeneration. However, articular surface stresses have not been studied systematically, since it is technically difficult to measure in vivo contact areas and pressures in dynamic situations. Therefore, we implemented a numerical model of articular surface contact using accurate surface geometries. The model was developed for the cat patellofemoral joint. We demonstrated that small misalignments of the patella relative to the femur change the joint contact mechanics substantially for a given external load. These results suggest that misalignment might be studied as one of the factors causing articular cartilage disorder and joint degeneration.

Animals↗

PubMed2003

Aspects of skeletal muscle modelling.

The modelling of skeletal muscle raises a number of philosophical questions, particularly in the realm of the relationship between different possible levels of representation and explanation. After a brief incursion into this area, a list of desiderata is proposed as a guiding principle for the construction of a viable model, including: comprehensiveness, soundness, experimental consistency, predictive ability and refinability. Each of these principles is illustrated by means of simple examples. The presence of internal constraints, such as incompressibility, may lead to counterintuitive results. A one-panel example is exploited to advocate the use of the principle of virtual work as the ideal tool to deal with these situations. The question of stability in the descending limb of the force-length relation is addressed and a purely mechanical analogue is suggested. New experimental results confirm the assumption that fibre stiffness is positive even in the descending limb. The indeterminacy of the force-sharing problem is traditionally resolved by optimizing a, presumably, physically meaningful target function. After presenting some new results in this area, based on a separation theorem, it is suggested that a more fundamental approach to the problem is the abandoning of optimization criteria in favour of an explicit implementation of activation criteria.

Animals↗