Biomedical subjects

Kotaro Sasaki

Publications and source records attributed to Kotaro Sasaki.

7 recordsLinked to original sources

PubMed2006

Mechanical energetic contributions from individual muscles and elastic prosthetic feet during symmetric unilateral transtibial amputee walking: a theoretical study.

Energy storage and return (ESAR) foot-ankle prostheses have been developed in an effort to improve gait performance in lower-limb amputees. However, little is known about their effectiveness in providing the body segment mechanical energetics normally provided by the ankle muscles. The objective of this theoretical study was to use muscle-actuated forward dynamics simulations of unilateral transtibial amputee and non-amputee walking to identify the contributions of ESAR prostheses to trunk support, forward propulsion and leg swing initiation and how individual muscles must compensate in order to produce a normal, symmetric gait pattern. The simulation analysis revealed the ESAR prosthesis provided the necessary trunk support, but it could not provide the net trunk forward propulsion normally provided by the plantar flexors and leg swing initiation normally provided by the biarticular gastrocnemius. To compensate, the residual leg gluteus maximus and rectus femoris delivered increased energy to the trunk for forward propulsion in early stance and late stance into pre-swing, respectively, while the residual iliopsoas delivered increased energy to the leg in pre- and early swing to help initiate swing. In the intact leg, the soleus, gluteus maximus and rectus femoris delivered increased energy to the trunk for forward propulsion in the first half of stance, while the iliopsoas increased the leg energy it delivered in pre- and early swing. Thus, the energy stored and released by the ESAR prosthesis combined with these muscle compensations was able to produce a normal, symmetric gait pattern, although various neuromuscular and musculoskeletal constraints may make such a pattern non-optimal.

Amputation, Surgical↗

PubMed2006

Pd-Fe nanoparticles as electrocatalysts for oxygen reduction.

We have synthesized new electrocatalysts for the O2 reduction reaction that does not contain Pt. They consist of carbon-supported Pd-Fe alloys and have very high oxygen reduction. The nanoparticles with a Pd:Fe molar ratio of 3:1 (Pd3Fe/C) show a higher mass activity than that of commercial Pt/C. The surface-specific activity of the Pd-Fe alloys is related to the Pd-Pd bond distance: the shorter the bond distance, the higher the activity. This new class of electrocatalysts promises to alleviate some major problems of existing fuel cell technology by simultaneously decreasing materials cost and enhancing performance.

Journal Article↗

PubMed2005

Mixed-metal pt monolayer electrocatalysts for enhanced oxygen reduction kinetics.

We have synthesized a new class of electrocatalysts for the O2 reduction reaction, consisting of a mixed monolayer of Pt and another late transition metal (Ir, Ru, Rh, Re, or Os) deposited on a Pd(111) single crystal or on carbon-supported Pd nanoparticles. Several of these mixed monolayer electrocatalysts exhibited very high activity and increased stability of Pt against oxidation, as well as a 20-fold increase in a Pt mass-specific activity, compared with state-of-the-art all-Pt electrocatalysts. Their superior activity and stability reflect a low OH coverage on Pt, caused by the lateral repulsion between the OH adsorbed on Pt and the OH or O adsorbed on neighboring, other than Pt, late transition metal atoms. The origin of this effect was identified through a combination of experimental and theoretical methods, employing electrochemical techniques, in situ X-ray absorption spectroscopy, and periodic, self-consistent density functional theory calculations. This new class of electrocatalysts promises to alleviate some major problems of existing fuel cell technology by simultaneously decreasing materials cost and enhancing performance. Our studies suggest a new way of synthesizing improved ORR catalysts through the modification and control of the surface reactivity of Pt-based mixed monolayers supported on transition metals other than Pt. In addition to improving the ORR catalysts, co-depositing oxophilic metals may be a promising possibility for improving a variety of other catalysts.

Catalysis↗

PubMed2005

Differences in muscle function during walking and running at the same speed.

Individual muscle contributions to body segment mechanical energetics and the functional tasks of body support and forward propulsion in walking and running at the same speed were quantified using forward dynamical simulations to elucidate differences in muscle function between the two different gait modes. Simulations that emulated experimentally measured kinesiological data of young adults walking and running at the preferred walk-to-run transition speed revealed that muscles use similar biomechanical mechanisms to provide support and forward propulsion during the two tasks. The primary exception was a decreased contribution of the soleus to forward propulsion in running, which was previously found to be significant in walking. In addition, the soleus distributed its mechanical power differently to individual body segments between the two gait modes from mid- to late stance. In walking, the soleus transferred mechanical energy from the leg to the trunk to provide support, but in running it delivered energy to both the leg and trunk. In running, earlier soleus excitation resulted in it working in synergy with the hip and knee extensors near mid-stance to provide the vertical acceleration for the subsequent flight phase in running. In addition, greater power output was produced by the soleus and hip and knee extensors in running. All other muscle groups distributed mechanical power among the body segments and provided support and forward propulsion in a qualitatively similar manner in both walking and running.

Biomechanical Phenomena↗

PubMed2005

Muscle mechanical work and elastic energy utilization during walking and running near the preferred gait transition speed.

Mechanical and metabolic energy conservation is considered to be a defining characteristic in many common motor tasks. During human gait, the storage and return of elastic energy in compliant structures is an important energy saving mechanism that may reduce the necessary muscle fiber work and be an important determinant of the preferred gait mode (i.e., walk or run) at a given speed. In the present study, the mechanical work done by individual muscle fibers and series-elastic elements (SEE) was quantified using a musculoskeletal model and forward dynamical simulations that emulated a group of young healthy adults walking and running above and below the preferred walk-run transition speed (PTS), and potential advantages associated with the muscle fiber-SEE interactions during these gait modes at each speed were assessed. The simulations revealed that: (1) running below the PTS required more muscle fiber work than walking, and inversely, walking above the PTS required more muscle fiber work than running, and (2) SEE utilization in running was greater above than below the PTS. These results support previous suggestions that muscle mechanical energy expenditure is an important determinant for the preferred gait mode at a given speed.

Adult↗

PubMed2005

Ankle plantar flexor force production is an important determinant of the preferred walk-to-run transition speed.

The mechanisms that govern the voluntary transition from walking to running as walking speed increases in human gait are not well understood. The objective of this study was to examine the hypothesis that plantar flexor muscle force production is greatly impaired at the preferred transition speed (PTS) due to intrinsic muscle properties and, thus, serves as a determinant for the walk-to-run transition. The plantar flexors have been shown to be important contributors to satisfying the mechanical energetic demands of walking and are the primary contributors to the observed ground reaction forces (GRFs) during the propulsion phase. Thus, if the plantar flexor force production begins to diminish near the PTS despite an increase in muscle activation, then a corresponding decrease in the GRFs during the propulsion phase would be expected. This expectation was supported. Both the peak anterior/posterior and vertical GRFs decreased during the propulsion phase at walking speeds near the PTS. A similar decrease was not observed during the braking phase. Further analysis using forward dynamics simulations of walking at increasing speeds and running at the PTS revealed that all lower extremity muscle forces increased with walking speed, except the ankle plantar flexors. Despite an increase in muscle activation with walking speed, the gastrocnemius muscle force decreased with increasing speed, and the soleus force decreased for walking speeds exceeding 80% PTS. These decreases in force production were attributed to the intrinsic force-length-velocity properties of muscle. In addition, the running simulation analysis revealed that the plantar flexor forces nearly doubled for similar activation levels when the gait switched to a run at the PTS due to improved contractile conditions. These results suggest the plantar flexors may serve as an important determinant for the walk-to-run transition and highlight the important role intrinsic muscle properties play in determining the specific neuromotor strategies used in human locomotion.

Adult↗

PubMed2003

Differential regulation of VLA-2 expression on Th1 and Th2 cells: a novel marker for the classification of Th subsets.

We found that T(h)1 cells derived from ovalbumin (OVA)-specific TCR transgenic (DO11.10) mice showed significantly higher levels of VLA-2 (CD49b/CD29) expression than T(h)2 cells. In the early days (until 6 days) during induction of T(h)1 or T(h)2 cells, the expression of VLA-2 was gradually increased on both T(h) subsets. Thereafter, VLA-2 expression was further up-regulated on T(h)1 cells until 13 days, while a significant decrease of VLA-2 was observed in T(h)2 cells, resulting in a marked difference of expression at day 13. Up-regulation of VLA-2 on T(h)1 cells was not impaired in IFN-gamma(-/-) T(h) cells nor blocked by anti-IL-12 mAb treatment on wild-type T(h) cells, suggesting that up-regulation of VLA-2 on T(h)1 cells occurs in an IFN-gamma- and IL-12-independent manner. In contrast, T(h) cells cultured under IL-4-depleted T(h)2 conditions abrogated the down-regulation of VLA-2 expression, suggesting that down-regulation of VLA-2 expression on T(h)2 cells was dependent on IL-4. The finding that STAT6(-/-) T(h)2 cells did not show any down-regulation of VLA-2 expression and expressed the same levels of VLA-2 as T(h)1 cells indicated a critical role for the IL-4 receptor/STAT6 signaling pathway in IL-4-dependent down-regulation of VLA-2 on T(h)2 cells. Stimulation of T(h)1 cells by VLA-2 ligands such as collagen type I or agonistic mAb provided co-stimulation for anti-CD3 mAb-induced IFN-gamma production. However, these ligations had little effect on the IL-4 production of T(h)2 cells. Together, these results indicate that VLA-2 is a novel functional marker that dissociates T(h)1 from T(h)2 cells, and thus might be useful for therapeutic monitoring of T(h)1-dependent immune diseases such as rheumatoid arthritis or Crohn's disease.

Animals↗