Mechanical output following muscle stretch in forearm supination against inertial loads

Muscle stretch enhances force produced in both single fibers and voluntarily activated human muscle. This study determined how initial conditions of muscle stretch (and associated eccentric work), muscle length, and load inertia contributed to human concentric muscular output during maximal voluntary forearm supination. Outputs of angular velocity and concentric work over specific displacements and times of motion were calculated. Multiple regression analysis was performed using these outputs and initial conditions as dependent and independent variables, respectively. Initial conditions were shown to be significant and systematic determinants of muscle output in concentric contraction. Evidence of a temporary shift in the force-velocity curve was found and discussed regarding its beneficial contribution to load movement. Greater benefit was considered to be due to the fact that muscle stretch allows time for achievement of maximal muscular recruitment prior to concentric contraction. This produces large forces at the onset of the concentric phase, in comparison with contractions starting from rest. These findings were discussed with regard to both single- and multi-segment movement patterns.     

Changes in muscle geometry during forearm pronation and supination and their relationships to EMG cross-correlation measures

Geometric artifact may alter the amplitude and frequency of the electromyography (EMG) signal. Artifacts include the changing geometry of muscles with respect to electrodes and potential crosstalk from adjacent muscles. This study addresses: (1) the geometrical relationships between common electrode placement sites for six forearm muscles, (2) the geometrical change of forearm muscles in pronation and supination, and (3) the relationships between EMG cross-correlation and muscle geometry. EMG and ultrasonography images were recorded during pronation, supination, and neutral forearm postures while exerting 20% maximum grip strength. Proportions of anatomical structures were then calculated for 15 mm, 20 mm, and 25 mm radial pick-up zone distances, representing greater than 90% of observed myoelectrical signal energy. We found that guidelines for electrode placements were supported and no single posture maximized the proportion of the target muscle detected. Secondly, other muscles were present in the most conservative 15 mm radius pick up zone; it is unlikely that surface EMG can completely differentiate between forearm muscle activities. Thirdly, forearm orientation did not appear to be an important factor in changing the geometrical relationships between surface electrodes and the muscles studied, and fourthly, certain muscles (e.g., FDS) may be more vulnerable to EMG crosstalk.     

A new kinematic model of pro- and supination of the human forearm

We introduce a new kinematic model describing the motion of the human forearm bones, ulna and radius, during forearm rotation. During this motion between the two forearm extrem-positions, referred to as supination (palm up) and pronation (palm down), effects occur, that cannot be explained by the the established kinematic model of R. Fick from 1904. Especially, the motion of the ulna is not properly reproduced by Fick's model. During forearm rotation an evasive motion of the ulna is observed by various authors, using magnetic resonance imaging MRI) technology. Our new kinematic model also simulates this evasive motion. Furthermore, the model is enlarged to include angulations of the forearm bones. Using these results the influence of forearm fractures on the range of forearm motion can be predicted. This knowledge can be used by surgeons to choose the optimal therapy in re-establishing free forearm mobility.     

Mechanical properties of single hyaluronan molecules

Hyaluronan (HA) is a major component of the extracellular matrix. It plays an important role in the mechanical functions of the extracellular matrix and stabilization of cells. Currently, its mechanical properties have been investigated only at the gross level. In this study, the mechanical properties of single HA molecules were directly measured with an optical tweezer technique, yielding a persistence length of 4.5 +/- 1.2 nm. This information may help us to understand the mechanical roles in the extracellular matrix infrastructure, cell attachment, and to design tissue engineering and drug delivery systems where the mechanical functions of HA are essential.     

Mechanical properties of a slow muscle in the cockroach

The mechanical properties of the metacoxal muscle, 177d, in the cockroach, Periplaneta americana, was investigated. The muscle exhibited a mean resting tension of 2.6 ± 1.3g SD. Neurally evoked tension summed with the resting tension and the relaxation phase of the evoked tension varied from less than 1 s to several minutes. This residual tension varied not only in duration but also in amplitude. Stimulation of inhibitory axons increased the rate of relaxation and thereby abolished the residual tension. However, inhibitory stimulation never reduced the resting tension. Stimulation of the main leg nerve at several times the threshold of the inhibitory axons could evoke residual tension. Recording of synaptic potentials from the two histochemically different fiber types (dorsal and ventral groups) revealed large hyperpolarizations in the ventral fibers and decreased duration and amplitude of excitatory potentials in the dorsal fibers. These results suggest that there are a variety of ways in which tension can be evoked, maintained, and controlled in these muscles.     

Mechanical properties of denervated amphibian muscle

Denervated amphibian muscle does not show the prolongation of action potential found in mammalian denervated muscle. It was, therefore, predicted that denervated amphibian muscle would not show prolongation of the mechanical twitch. The sartorius muscles in one leg of toads--Xenopus borealis--were denervated for 140-268 days. Isometric twitch time to peak, time to half relaxation and twitch/tetanus ratio were not changed following denervation, confirming our prediction. Twitch tension decreased to 68% and tetanic tension decreased to 75% of control values. The maximum velocity of unloaded shortening (muscle length/s) was also unchanged.     

Mechanical properties of muscle spindles in Xenopus laevis

Mechanical properties of isolated living muscle spindles from Xenopus laevis were examined in order to determine their role in sensory transduction. The reticular zone of the intrafusal muscle fibers was identified microscopically by: (1) its position beneath the sensory endings, (2) its length, 50–100 μm, (3) its extension during intrafusal muscle contraction, and (4) its coarse striations with a period of about 1.5 times the normal sarcomere length. The reticular zone in the passive muscle spindle did not extend until the spindle was stretched to about 1.05–1.1 its maximal length in the animal (L _m ). Evidence was obtained that the absence of extension of the reticular zone at normal muscle lengths was due to the presence of the spindle capsule which acted as a stiff element in parallel with the sensory region. At those lengths at which the reticular zone did extend (> L _m ), no rate — sensitive mechanical properties were detected in response to step and ramp extensions. The sensory discharge of the spindle showed no dynamic transient in response to ramp extensions if the reticular zone were not extended. During extension of the reticular zone a dynamic sensory transient appeared. It is concluded that current notions on the mechanical origin of the rate — sensitive properties of the sensory discharge of the muscle spindle do not apply to Xenopus laevis. In addition, it is not likely that the passive spindle in this animal is a sensitive stretch receptor.     

Effects of acetylcholine and nitric oxide on forearm blood flow at rest and after a single muscle contraction

We tested thehypothesis that ACh or nitric oxide (NO) might be involved in thevasodilation that accompanies a single contraction of the forearm.Eight adults (3 women and 5 men) completed single 1-s-durationcontractions of the forearm to raise and lower a weight equivalent to~20% maximal voluntary contraction through a distance of 5 cm. In asecond protocol, each subject had a cuff, placed completely about theforearm, inflated to 120 mmHg for a 1-s period, then released as asimulation of the mechanical effect of muscle contraction. Threeconditions were studied, always in this order:1) control, with intra-arterialinfusion of saline; 2) after muscarinic blockade withatropine; and 3) after NO synthase inhibitionwith N^G-monomethyl-L-arginine(L-NMMA) plus atropine. Forearm blood flow (FBF),measured by combined pulsed and echo Doppler ultrasound, was reduced atrest with L-NMMA-atropinecompared with the other two conditions. After the single contraction,there were no effects of atropine, butL-NMMA reduced the peak FBF andthe total postcontraction hyperemia. After the single cuff inflation,atropine had no effects, whereasL-NMMA caused changes similar tothose seen after contraction, reducing the peak FBF and the totalhyperemia. The observation thatL-NMMA reduced FBF in responseto both cuff inflation and a brief contraction indicates that NO fromthe vascular endothelium might modulate the basal level of vasculartone and the mechanical component of the hyperemia with exercise. It isunlikely that ACh and NO from the endothelium are involved in thedilator response to a single muscle contraction.

     

Mechanical properties of tropomyosin and implications for muscle regulation

Tropomyosin is a protein that controls the interactions of actin and myosin as a part of the regulation of muscle contraction. The 420 Å long α-helical coiled-coil molecules form long filaments, both in muscle and in crystals. The x-ray diffraction data from tropomyosin crystals have indicated large scale motions of the filaments that can be related to the inherent mechanical properties of the molecule, and by extension, to the role of tropomyosin in the cooperative activation of the thin filaments of muscle. Diffuse scattering analysis has provided information about the amplitudes of the motions that has been used to calculate the intrinsic flexibility of the molecule. It can then be shown that each tropomyosin molecule by itself can only mediate interactions of the nearest-neighboring tropomyosin molecules along the filament. The repeating nature of the thin filament, however, allows the entire filament to activate cooperatively. © 1996 John Wiley & Sons, Inc.     

Mechanical plasticity and contractile properties of airway smooth muscle

Smooth muscle has the unique ability to adapt easily and quickly to length changes without compromising its ability to generate force. This ability is referred to as mechanical plasticity and is now considered to be an important aspect of smooth muscle that affects both its contractile and relaxation behaviour. It is therefore important to incorporate knowledge of plasticity into further studies of smooth muscle behaviour. It is also important that future studies be focused on deciphering the mechanism of smooth muscle length adaptation and plasticity. This review outlines some of the proposed mechanisms determining plasticity. However, it should be said that there are other proposed mechanisms not touched upon here, which may be equally as important. This review also focuses on the relevance of smooth muscle plasticity in asthma, but it is important to remember that there are other places where smooth muscle plasticity may play an equally important role.     

Mechanical and failure properties of single attached cells under compression

Eukaryotic cells are continuously subjected to mechanical forces under normal physiological conditions. These forces and associated cellular deformations induce a variety of biological processes. The degree of deformation depends on the mechanical properties of the cell. As most cells are anchorage dependent for normal functioning, it is important to study the mechanical properties of cells in their attached configuration. The goal of the present study was to obtain the mechanical and failure properties of attached cells. Individual, attached C2C12 mouse myoblasts were subjected to unconfined compression experiments using a recently developed loading device. The device allows global compression of the cell until cell rupture and simultaneously measures the associated forces. Cell bursting was characterized by a typical reduction in the force, referred to as the bursting force. Mean bursting forces were calculated as 8.7+/-2.5 microN at an axial strain of 72+/-4%. Visualization of the cell using confocal microscopy revealed that cell bursting was preceded by the formation of bulges at the cell membrane, which eventually led to rupturing of the cell membrane. Finite element calculations were performed to simulate the obtained force-deformation curves. A finite element mesh was built for each cell to account for its specific geometrical features. Using an axisymmetric approximation of the cell geometry, and a Neo-Hookean constitutive model, excellent agreement between predicted and measured force-deformation curves was obtained, yielding an average Young's modulus of 1.14+/-0.32 kPa.     

Mechanical properties of single living cells encapsulated in polyelectrolyte matrixes

We have studied the mechanical properties of encapsulated Saccharomyces cerevisiae yeast cells by performing AFM force measurements. Single living cells have been coated through the alternate deposition of oppositely charged polyelectrolyte layers and mechanically trapped into a porous membrane. Coated and uncoated cells in presence/absence of bud scars, i.e. scars resulting from previous budding events, have been investigated. No significant differences between encapsulated and bare cells could be inferred from AFM topographs. On the other hand, investigation on the system elasticity through the acquisition and analysis of force curves allowed us to put in evidence the differences in the mechanical properties between the hybrid cell/polyelectrolyte system and the uncoated cells. Analysis of the curves contact region indicates that the polyelectrolyte coating increases the system rigidity. Quantitative evaluation of the cell rigidity through the Hertz-Sneddon model showed that coated cells are characterized by a Young's modulus higher than the value obtained for uncoated cells and similar to the value observed on the bud scar region of uncoated cells.     

Simultaneous bilateral foot reconstruction using a single radial forearm flap

The radial forearm septocutaneous flap is an excellent source of thin tissue with an anatomically consistent network of large vessels that simplify microsurgical transfer. It may be divided into multiple flaps leaving a single donor-site defect. The clinical usefulness of this concept is demonstrated in a single case in which simultaneous coverage of electrical burns of both feet was required.     

Mechanical properties of human bronchial smooth muscle in vitro.

The in vitro mechanical properties of smooth muscle strips from 10 human main stem bronchi obtained immediately after pneumonectomy were evaluated. Maximal active isometric and isotonic responses were obtained at varying lengths by use of electrical field stimulation (EFS). At the length (Lmax) producing maximal force (Pmax), resting tension was very high (60.0 +/- 8.8% Pmax). Maximal fractional muscle shortening was 25.0 +/- 9.0% at a length of 75% Lmax, whereas less shortening occurred at Lmax (12.2 +/- 2.7%). The addition of increasing elastic loads produced an exponential decrease in the shortening and velocity of shortening but increased tension generation of muscle strips stimulated by EFS. Morphometric analysis revealed that muscle accounted for 8.7 +/- 1.5% of the total cross-sectional tissue area. Evaluation of two human tracheal smooth muscle preparations revealed mechanics similar to the bronchial preparations. Passive tension at Lmax was 10-fold greater and maximal active shortening was threefold less than that previously demonstrated for porcine trachealis by us of the same apparatus. We attribute the limited shortening of human bronchial and tracheal smooth muscle to the larger load presumably provided by a connective tissue parallel elastic component within the evaluated tissues, which must be overcome for shortening to occur. We suggest that a decrease in airway wall elastance could increase smooth muscle shortening, leading to excessive responses to contractile agonists, as seen in airway hyperresponsiveness.     

Mechanical properties of the stimulated papillary muscle in quick-release experiments

The behavior of heart muscle in the stimulated state is a subject of much controversy. A brief summary of the conclusions drawn by some investigators is given to bring the conflicts to focus. In 1970 Fung formulated the general problem incorporating Hill's three-elements model and Huxley's sliding element theory. This formalism is of general validity and is adopted here to study the papillary muscle behavior. Preliminary results obtained using quick-release technique are presented.     

Mechanical properties of the heart muscle in the passive state

The viscoelastic behaviour of the heart muscle (papillary muscle) in the passive unstimulated) state is studied by such methods as stress relaxation, creep, vibration and stress-strain testing. The tests are conducted on a newly developed electromechanical muscle testing device which is suitable for conducting active and passive tests on biological materials.