In osteoblast cells, cells change their intracellular Ca²⁺ concentration ([Ca²⁺]_i) as the response to mechanical stimuli. Although it has been reported that osteoblast cells responded to many kinds of mechanical stimuli including stretch of substrate, shear stress in fluid flow, direct indentation of glass microneedle and hydrostatic pressure etc., the detail of the characteristics of intracellular calcium signaling response to substrate stretch still remains unclear because motion artifact during stretch application causes out of focus plane and observation area, and complicates the in situ time lapse observation of change in [Ca²⁺]_i in a single cell level. In this study, we combined our originally developed cell stretching MEMS device with the ratiometric microscopy method with two kinds of visible wavelength calcium indicator dyes. The cell stretching micro device and the ratiometric method reduce the influence of motion artifact during stretch application, and enable us to quantitatively evaluate the characteristics of cellular calcium signaling response to stretch. MC3T3-E1 osteoblastic cells were plated onto the cell stretching micro device and fluorescently labeled by Ca²⁺ indicator Fluo 3 and Fura Red. A uniaxial stretch with three magnitudes of strain 5%, 10% and 15% with constant strain rate were applied to the cells, and in situ time lapse observation of cellular calcium signaling response to stretch was conducted with high temporal and spatial resolution. We succeeded in obtaining time lapse fluorescent image sequences during stretch application without excessive out of focus and blank time. The results revealed that MC3T3-E1 cells change the intensity of calcium signaling response to stretch according to the stretch strain magnitude. As stretch strain magnitude was increased, the amount of change in fluorescent ratio value of Ca²⁺ indicators in stretched cells also increased. This result suggests the possibility that osteoblastic cells can sense the magnitude of mechanical stimuli at upstream of mechanotransduction pathway such as influx of extracellular Ca²⁺.

View full abstract

The authors proposed an electromyography computed tomography (EMG-CT) method to measure the distribution of muscle activity in the forearm using surface EMG signals from multiple surface electrodes. The present study is to develop a method to estimate muscle stress, i.e., force generated during contraction per unit area in the whole cross-section of the forearm based on EMG-CT. While three subjects performed hand gripping trials using three hand grip devices, EMG signals around the forearm were measured using EMG-CT. An EMG conduction model of the forearm was constructed using an outline geometry of the subjects' forearm which was measured with a handy 3D scanner. The stress of muscle was calculated from the relationship between gripping force and total muscle activity. As a result, the distribution of muscle stress in the forearm during hand gripping was visualized in a tomographic image. It was clear that the stress was concentrated in the flexor digitorum superficialis, flexor digitorum profundus, flexor carpi radialis, and extensor digitorum communis region. The maximum stress in the forearm muscles increased from 0.08 ± 0.01 to 0.18 ± 0.02 MPa when gripping force increased from 77 to 242 N. This study provides a novel method of measuring muscle stress in forearm.

View full abstract

Human fingernails play an important role in daily life and while playing instruments. We previously showed that the strain generated at the radial and ulnar parts of the nail was asymmetric when the thumb pad was pressed down on a flat surface. The purpose of the present study was to investigate whether nail strain is asymmetrical while pressing or plucking a string with the thumb. Biaxial strain gauges were attached to the surface of the thumbnail of 15 subjects; the gauges were attached to the proximal-radial, distal-central, and proximal-ulnar parts of the thumbnail. We measured the maximum strain generated while pressing or plucking a string with the thumb pad. The strain values obtained were then compared among the measurement locations. We also measured the effects of contact angles and contact points on the thumbnail strain. Upon investigating the effect of the measurement locations, we found that during string-pressing motion, the maximum strain was significantly lower in the proximal-radial-axial direction than in the distal-central-axial and proximal-ulnar-axial directions. This indicated that an asymmetric strain generated at the proximal-radial and ulnar sides. We also investigated the effect of the contact angle and found that the maximum strain varied with changes in contact angle. The investigation of the effect of the contact point revealed that the maximum strain during string-pressing motion varied with changes in the contact point, although no change was observed during string plucking. Asymmetrical thumbnail strain may have been caused by the asymmetrical nature of the flexor tendon insertion and the distal phalangeal tuberosity of the thumb. Uniform pressure applied to the thumb pad became asymmetric in the radioulnar direction because of the interaction between the thumb pulp, distal phalanx, and nail. This asymmetry may then have been transmitted to the nail.

View full abstract

The objective of this study was to investigate the effect of knee joint motion on swimming performance for a transfemoral prosthesis in swimming. A flat spring at the knee joint was introduced into a prosthesis which had already been developed, in order to enable the knee joint to move according to the fluid force acting on the lower limb. A swimming experiment, in which a subject using the prosthesis swam in a pool, was conducted. Five types of flat spring for the knee joint were prepared for the experiment. From the experiment, it was found that there was a proper stiffness of the flat spring from which the subject felt comfortable during swimming. Next, simulations for the flutter kick and crawl stroke were conducted. The validity of the simulation method was confirmed since the simulated joint angles in the simulation of the flutter kick were sufficiently consistent with the experimental ones in respect to the amplitudes and phases for both the knee and ankle. During the simulation of the crawl stroke, it was found that the amplitude of the hip joint torque on the prosthetic leg compared to the healthy leg increased according to the increase in the stiffness of the knee joint. Since this amount was considered to directly correspond with the ‘reaction during kick' in the VAS evaluation of the experiment, in which the subject evaluated the stiffer knee flat spring as ‘heavier', the consistency of the tendencies between the simulation and experiment was confirmed. From the simulation results, it was also suggested that the subject in the experiment preferred stiffness at the knee joint which minimized the difference between the amplitude of the hip joint torque on the healthy leg and that on the prosthetic leg.

View full abstract