Passive Muscle Force Research Articles

Hierarchical Extensibility in the PEVK Domain of Skeletal-Muscle Titin

Titin is the main determinant of passive muscle force. Physiological extension of titin derives largely from its PEVK (Pro-Glu-Val-Lys) domain, which has a different length in different muscle types. Here we characterized the elasticity of the full-length, human soleus PEVK domain by mechanically manipulating its contiguous, recombinant subdomain segments: an N-terminal (PEVKI), a middle (PEVKII), and a C-terminal (PEVKIII) one third. Measurement of the apparent persistence lengths revealed a hierarchical arrangement according to local flexibility: the N-terminal PEVKI is the most rigid and the C-terminal PEVKIII is the most flexible segment within the domain. Immunoelectron microscopy supported the hierarchical extensibility within the PEVK domain. The effective persistence lengths decreased as a function of ionic strength, as predicted by the Odijk-Skolnick-Fixman model of polyelectrolyte chains. The ionic strength dependence of persistence length was similar in all segments, indicating that the residual differences in the elasticity of the segments derive from nonelectrostatic mechanisms.

Spinal muscle forces, internal loads and stability in standing under various postures and loads--application of kinematics-based algorithm.

This work aimed to evaluate trunk muscle forces, internal loads and stability margin under some simulated standing postures, with and without external loads, using a nonlinear finite element model of the T1-S1 spine with realistic nonlinear load-displacement properties. A novel kinematics-based algorithm was applied that exploited a set of spinal sagittal rotations, initially calculated to minimize balancing moments, to solve the redundant active-passive system. The loads consisted of upper body gravity distributed along the spine with or without 200 N held in the hands, either in the front of the body or on the sides. Nonlinear and linear stability/perturbation analyses at deformed, stressed configurations with a linear stiffness-force relationship for muscles identified the system stability and critical muscle stiffness coefficient. Predictions were in good agreement with reported measurements of posture, muscle EMG and intradiscal pressure. Minimal changes in posture (posterior pelvic tilt and lumbar flattening) substantially influenced muscle forces, internal loads and stability margin. Addition of 200 N load in front of the body markedly increased the system stability, global muscle forces, and internal loads, which reached anterior shear and compression forces of approximately 500 N and approximately 1,200 N, respectively, at lower lumbar levels. Co-activation in abdominal muscles (up to 3% maximum force) substantially increased extensor muscle forces, internal loads and stability margin, allowing a smaller critical muscle coefficient. A tradeoff existed between lower internal loads in passive tissues and higher stability margins, as both increased with greater muscle activation. The strength of the proposed model is in accounting for the synergy by simultaneous consideration of passive structure and muscle forces under applied postures and loads.

Molecular Basis of Passive Stress Relaxation in Human Soleus Fibers: Assessment of the Role of Immunoglobulin-Like Domain Unfolding

Titin (also known as connectin) is the main determinant of physiological levels of passive muscle force. This force is generated by the extensible I-band region of the molecule, which is constructed of the PEVK domain and tandem-immunoglobulin segments comprising serially linked immunoglobulin (Ig)-like domains. It is unresolved whether under physiological conditions Ig domains remain folded and act as “spacers” that set the sarcomere length at which the PEVK extends or whether they contribute to titin's extensibility by unfolding. Here we focused on whether Ig unfolding plays a prominent role in stress relaxation (decay of force at constant length after stretch) using mechanical and immunolabeling studies on relaxed human soleus muscle fibers and Monte Carlo simulations. Simulation experiments using Ig-domain unfolding parameters obtained in earlier single-molecule atomic force microscopy experiments recover the phenomenology of stress relaxation and predict large-scale unfolding in titin during an extended period (>∼20 min) of relaxation. By contrast, immunolabeling experiments failed to demonstrate large-scale unfolding. Thus, under physiological conditions in relaxed human soleus fibers, Ig domains are more stable than predicted by atomic force microscopy experiments. Ig-domain unfolding did not become more pronounced after gelsolin treatment, suggesting that the thin filament is unlikely to significantly contribute to the mechanical stability of the domains. We conclude that in human soleus fibers, Ig unfolding cannot solely explain stress relaxation.

424 能動的・受動的筋力を考慮した人体下肢運動解析

424 能動的・受動的筋力を考慮した人体下肢運動解析

Which Angle for which Surgical Strategy in Comitant Strabismus?

BackgroundThe fusion-free angle measured in the awake state was adopted as the angle of reference and the basis for conventional muscle surgery. The surgical effect is amount dependent.Further analysisThe basic angle does not, however, correspond to the fusion-free angle in all types of comitant strabismus. In infantile esotropia it corresponds to the minimum angle. The difference between the maximum and the minimum angle represents the angle of variability. Weakening-reinforcing techniques (conventional surgery) to correct the minimum angle should therefore be combined with a progressive lessening technique (posterior fixation suture) to reduce the variability. In normosensorial strabismus the prism adaptation test, or prismotherapy, allows accurate determination of the basic angle to be corrected. The imbalance of the passive forces in an antagonistic muscle pair can be assessed during surgery under general anaesthesia in two ways, the eye position and the measurement of muscle extensibility. These two parameters are partially independent from the minimum angle measured in the awake state. The intraoperative data can, therefore, be considered as valuable parameters for adjusting the amount of conventional surgery.Patients, method and resultsA series of 80 children with infantile esotropia were operated on between 11 and 100 months of age. The surgical procedure consisted of bilateral posterior fixation sutures and uni- or bilateral medial rectus recessions. The intraoperative data were recorded in all cases. The amount of global recession was calculated on the minimum angle and adjusted if necessary during surgery according to the intraoperative data. The long-term results (follow-up of two years or more) show that the failures arose more often in the cases in which no adjustment was made.Discussion and conclusionAn intraoperative adjustment of the amount of surgery is indicated according to the imbalance of passive muscle forces. Our long-term results, as yet uncompleted, suggest that the variance in results can be reduced by this strategy, along with taking into account the different angles of a deviation.

The Effect of Typing Posture on Wrist Extensor Muscle Loading

High static loading of the forearm extensor musculature has been observed during keying tasks. To reduce the level of loading, one must first understand the contributing factors. A simulation of the human finger was used to determine muscle force contributions during a static index finger key press at several wrist postures. The planar model included active and passive muscle forces of the intrinsic and extrinsic finger muscles. The model was expanded to include the passive forces from the other fingers as well as the weight of the hand to determine the exertion required of the wrist extensor muscles to maintain the given wrist and finger postures. Model results indicated that greater than 25% of maximal exertion is required of the wrist extensors when the wrist is extended to 30. The increased moment contribution from passive forces of the extrinsic finger flexor muscles was responsible for the majority of the increased wrist extensor contribution as the wrist was extended. These findings are in relative agreement with previous electromyographic studies and may indicate a mechanism for forearm extensor pain in office workers. Potential applications of this research include ergonomic modeling of the upper limb to determine internal loads that may lead to work-related disorders.

Passive force characteristics of an architecturally complex muscle

In architecturally complex muscles with large attachment areas, it can be expected that during movement different muscle regions undergo different amounts of length excursions. As a consequence, the amount of passive force produced by the regions will differ. Therefore, we tested the hypothesis that during movement the vector of the passive force of such a muscle, which defines the magnitude, position and orientation of the resultant force of the various regions, has no fixed position, between the muscle's center of origin and insertion. As a model for an architecturally complex muscle we used the masseter muscle. It was expected that during jaw opening anterior muscle regions are more stretched than posterior regions, leading to an anterior shift of the passive force vector. A three-component force transducer was used to measure both the position and magnitude of passive force in the masseter muscle of 9 rabbits. Forces were recorded during repeated cycles of stepwise opening and closure of the jaw. The muscle exhibited a clear hysteresis: passive force measured during jaw opening was larger than that during jaw closing. With an increase of the jaw gape there was an approximately exponential increase of the magnitude of the passive muscle force, while simultaneously the passive force vector shifted anteriorly. Moment arm length of passive force increased by about 100%. This anterior shift contributed substantially to the increase of the passive muscle moment generated during jaw opening. It can be concluded that in architecturally complex muscles the increase of the passive resistance moment which is associated with muscle lengthening might not only be due to an increase of the magnitude of passive muscle force but also to an increase of the moment arm of this force.

Preliminary studies on respiratory activity in speech

Work at Edinburgh University in the 1950s, summarized by Ladefoged [Three Area of Experimental Phonetics (Oxford University Press, Oxford, 1967)] claimed that (1) the mean power for speech was provided by actions of the respiratory muscles. The external intercostals check the outgoing airflow when speaking with high lung volumes, and the internal intercostals, rectus abdominis, and other muscles become more active as lung volume decreases. (2) Stressed syllables are produced by increases of respiratory power above the mean, usually by further activity of the internal intercostals. Hixon and Weismer (1995) pointed out technical limitations of these early studies, but wrongly indicated that they were of little value. These studies have been replicated with modern methods to measure intramuscular EMG and airflow and pressure. The two subjects studied to date follow the pattern previously described. In addition the improved techniques indicate that speakers exercise further control by bringing in stabilizing antagonistic muscles. These results are consistent with the general hypothesis that the central nervous system is able to control subglottal pressure by using various combinations of passive elastic and active muscle forces, thereby achieving motor equivalence and acoustic invariance over a wide range of lung volumes.

Selected Contribution: Improved anoxic tolerance in rat diaphragm following intermittent hypoxia.

Intermittent hypoxia (IH), associated with obstructive sleep apnea, initiates adaptive physiological responses in a variety of organs. Little is known about its influence on diaphragm. IH was simulated by exposing rats to alternating 15-s cycles of 5% O2 and 21% O2 for 5 min, 9 sets/h, 8 h/day, for 10 days. Controls did not experience IH. Diaphragms were excised 20-36 h after IH. Diaphragm bundles were studied in vitro or analyzed for myosin heavy chain isoform composition. No differences in maximum tetanic stress were observed between groups. However, peak twitch stress (P < 0.005), twitch half-relaxation time (P < 0.02), and tetanic stress at 20 or 30 Hz (P < 0.05) were elevated in IH. No differences in expression of myosin heavy chain isoforms or susceptibility to fatigue were seen. Contractile function after 30 min of anoxia (95% N2-5% CO2) was markedly preserved at all stimulation frequencies during IH and at low frequencies after 15 min of reoxygenation. Anoxia-induced increases in passive muscle force were eliminated in the IH animals (P < 0.01). These results demonstrate that IH induces adaptive responses in the diaphragm that preserve its function in anoxia.

Mechanical manipulation of single titin molecules with laser tweezers.

Titin (also known as connectin) is a giant filamentous polypeptide of multi-domain construction spanning between the Z- and M-lines of the vertebrate muscle sarcomere. The molecule is significant in maintaining sarcomeric structural integrity and generating passive muscle force via its elastic properties. Here we summarize our efforts to characterize titin's elastic properties by manipulating single molecules with force-measuring laser tweezers. The titin molecules can be described as an entropic spring in which domain unfolding occurs at high forces during stretch and refolding at low forces during release. Statistical analysis of a large number (> 500) of stretch-release experiments and comparison of experimental data with the predictions of the wormlike chain theory permit the estimation of unfolded titin's mean persistence length as 16.86 A (+/- 0.11 SD). The slow rates of unfolding and refolding compared with the rates of stretch and release, respectively, result in a state of non-equilibrium and the display of force hysteresis. Folding kinetics as the source of non-equilibrium is directly demonstrated here by the abolishment of force hysteresis in the presence of chemical denaturant. Experimental observations were well simulated by superimposing a simple domain folding kinetics model on the wormlike chain behavior of titin and considering the characteristics of the compliant laser trap. The original video presentation of this paper may be viewed on the web at http:¿www.pote.hu/mm/prezentacio/mkpres/++ +mkpres.htm.

Passive characteristics of conditioned skeletal muscle for ventricular assistance.

Systems to drive a ventricular assist device (VAD) with power from skeletal muscle have been proposed and are under development. During VAD filling, these systems must counter passive muscle force to control the precontraction length and optimize power output. To determine how muscle conditioning with electrical stimulation alters basic biomechanical characteristics and influences available power, goat latissimus dorsi were evaluated in vivo after an 8 week training protocol with an implanted myostimulator. Conditioned muscles displayed increased passive stiffness. After conditioning, the slope of the exponential passive force-length relation, at a passive force of 10 N, significantly increased from 5.1 to 7.6 N/cm (p = 0.003). Similarly, for a passive force of 10 N, the length relative to the zero developed force length decreased from 5.5 to 4.2 cm (p < 0.014). The linear relationship between slope (dF/dL) and force (F) also demonstrated a significant intercept shift. The latter relationship is independent of absolute length. Consistent with other studies, conditioning also resulted in fatigue resistance, fiber type transformation, and reductions in maximum developed force and shortening velocity. In the context of available power for cardiac assist, the results demonstrate that the influence of passive characteristics is accentuated after conditioning and has a substantial effect on available power.

In vivo finger flexor tendon force while tapping on a keyswitch.

Force may be a risk factor for musculoskeletal disorders of the upper extremity associated with typing and keying. However, the internal finger flexor tendon forces and their relationship to fingertip forces during rapid tapping on a keyswitch have not yet been measured in vivo. During the open carpal tunnel release surgery of five human subjects, a tendon-force transducer was inserted on the flexor digitorum superficialis of the long finger. During surgery, subjects tapped with the long finger on a computer keyswitch, instrumented with a keycap load cell. The average tendon maximum forces during a keystroke ranged from 8.3 to 16.6 N (mean = 12.9 N, SD = 3.3 N) for the subjects, four to seven times larger than the maximum forces observed at the fingertip. Tendon forces estimated from an isometric tendon-force model were only one to two times larger than tip force, significantly less than the observed tendon forces (p = 0.001). The force histories of the tendon during a keystroke were not proportional to fingertip force. First, the tendon-force histories did not contain the high-frequency fingertip force components observed as the tip impacts with the end of key travel. Instead, tendon tension during a keystroke continued to increase throughout the impact. Second, following the maximum keycap force, tendon tension during a keystroke decreased more slowly than fingertip force, remaining elevated approximately twice as long as the fingertip force. The prolonged elevation of tendon forces may be the result of residual eccentric muscle contraction or passive muscle forces, or both, which are additive to increasing extensor activity during the release phase of the keystroke.

Failure of stimulated skeletal muscle mainly contributed by passive force: an in vivo rabbit model

Objective. To evaluate the role of active and passive muscle forces in the failure mechanism of stimulated muscle. Design. An in vivo rabbit model. Background. Eccentric contractions can result in a greater incidence of muscle injury. However, the relative role of the active and passive muscle force in the failure mechanism of the activated muscle is not well elucidated. Methods. After anaesthesia, New Zealand white rabbits were fixed in a frame on a materials testing machine. The triceps surae muscle-tendon units were passively stretched to rupture with our without continuous nerve stimulation. The force and muscle length were simultaneously recorded. Active muscle force, passive muscle force, and ratio of the active to passive muscle were calculated and depicted against strain. Results. The results showed that the mean maximal passive force of triceps surae muscle was 293.1 N at a strain of 38%. The mean peak active muscle force was 21.5 N at a strain of 21%. The ratio of active to passive muscle force reached its peak first, followed by the active muscle force, and then the passive muscle force. The ratio of active to passive muscle force at the peak total force was only 3.3%. Conclusions. The stimulated muscle can exert its maximal response at extreme physiological extension. Injury of the stimulated muscle is caused mainly by passive muscle force.

Superior displacement of the hip in total joint replacement: Effects of prosthetic neck length, neck‐stem angle, and anteversion angle on the moment‐generating capacity of the muscles

The purpose of this study was to determine the effects of superior displacement of the hip center and changes in three prosthetic parameters (neck length, neck-stem angle, and anteversion angle) on the capacity of muscles to generate force and moment about the hip. A three-dimensional model that calculates the maximum isometric forces and moments generated by 25 muscles crossing the hip over a wide range of body positions was used to evaluate the effects of a 2 cm elevation of the hip center and changes in the prosthetic parameters. After superior displacement of the hip center, the neck length was increased from 0 to 3 cm, the neck-stem angle was varied between 110 and 150 degrees, and the anteversion angle was varied between 0 and 40 degrees. Our analysis showed that a 2 cm superior displacement of the hip center would decrease the moment-generating capacity of the four muscle groups studied (abductors, adductors, flexors, and extensors) if neck length were not increased to compensate for decreased muscle length. In the computer model of an adult man that we used, a 2 cm increase in neck length restored the moment-generating capacity of the muscles by increasing muscle length and force-generating capacity. However, a 3 cm increase in neck length increased passive muscle forces substantially, which potentially could limit joint motion. An increased neck-stem angle (i.e. a valgus neck) decreased the abduction moment arm but increased the moment-generating capacity of the other muscle groups. A change in the anteversion angle from 0 to 40 degrees had a relatively small effect on the isometric moment-generating capacity of the muscles studied.

Relevant shoulder biomechanics

The glenohumeral joint has a vast three-dimensional mobility that is balanced by the need to maintain joint stability. Motion with stability is made possible by the intricate interactions of the articular, capsuloligamentous, and musculotendinous stabilizers. Because the articular architecture has been found to contribute little native joint stability, the joint must depend on capsuloligamentous and musculotendinous structures for stability. Knowledge of the location and orientation of the glenohumeral ligaments is essential to understanding the biomechanics of the joint. In addition, two principal muscle layers, the outer deltoid complex and the inner rotator cuff musculature, have particularly essential functions in glenohumeral joint stability. These dynamic stabilizers impart stability through several mechanisms: passive muscle forces, joint-compressive forces, positioning allowing secondary ligamentous tightening, and bulk-effect forces. The inferior glenohumeral ligament complex with its anterior and posterior portions is probably the most essential capsulogigamentous contributor to stability. It should be emphasized that stability depends on contributions from all components to appropriately distribute applied loads. This concept of load sharing illustrates that a small deficiency in any single component may lead to significant cumulative damage. Clinical interest in glenohumeral biomechanics will continue to take on greater significance as the general population continues to become more active. Current knowledge of biomechanics has shown that both the rotator cuff and the capsuloligamentous complex are essential to shoulder stability. Further study with dynamic glenohumeral models should allow better definition of the important interaction between the static and dynamic stabilizers.

EMG-force relations of a single skeletal muscle acting across a joint: Dependence on joint angle

The electromyogram (EMG)-force relations of a single skeletal muscle (soleus) acting on a joint set isometrically at various flexion angles was studied using electrical nerve stimulation as the experimental method. The EMG-force relationships were linear at extremes of joint extension and became progressively nonlinear as the flexion angle increased. Joint angles at extreme extension showed the least passive and active force whereas their corresponding EMG versus force relations were linear. At extremes of joint flexion large passive force was accompanied by minimal active force. The largest active forces were recorded at midrange of the joints' excursion (90°). The source of the increasing nonlinearity was traced to large variability in the active and passive muscle forces as a function of joint angle, excluding all but minor variability in the EMG with joint position. We concluded that length-dependent variations of active and passive forces of the muscle (length-tension relations) compounded with the variation of its moment arm when acting across the joint require major consideration in biomechanical studies in which EMG is used to represent muscle force indirectly.

Optimization of skeletal configuration: Studies of scoliosis correction biomechanics

A scheme for optimizing configurations in models of skeletal structures is presented. Use of the scheme is illustrated through determination of biomechanically optimal correction of a right-thoracic scoliosis by passive brace and active muscle forces. The locations and magnitudes of the passive brace forces, and the trunk muscle groups and their corresponding contraction intensity magnitudes that would optimally correct the geometric deformities of the spine were determined. The results suggest that, from a biomechanical viewpoint, both brace and muscle forces are capable of substantial correction of a model thoracic scoliosis. However, comparison of model results with long-term clinical results suggests that even under optimal conditions it is unlikely that scoliosis can be fully corrected by passive brace forces or active muscle contractions.

Mechanical properties of passive rat muscle during sinusoidal stretching.

The dynamic passive response of the left gastrocnemius medialis muscle of thirty male Wistar rats was studied as a function of muscle dimensions and absolute and relative amount of connective tissue. Values of the absolute active and passive length-force curves (active force, passive force, active working range) correlated well (coefficients of correlation in a range of 0.62-0.92) with morphological variables (such as muscle optimum length, mean muscle fibre optimum length, physiological cross section, muscle weight and amount of intramuscular connective tissue). To eliminate dimensional effects the active and passive length-force curves were normalized taking maximal active twitch force and muscle optimum length as reference values (100%). The width of the normalized active length-force curve (relative active working range) was correlated negatively with muscle weight, muscle optimum length and physiological cross section. Relative amount of connective tissue and passive tension at optimum length (both independent of muscle dimensions) were positively correlated, indicating that passive muscles are stiffer when relative amount of intramuscular connective tissue is higher. Sinusoidal movements with several amplitudes and frequencies of movement were imposed on the passive gastrocnemius medialis muscle over a range of muscle lengths. In accordance with the approximately exponential increase of static passive muscle force with length, muscle length has a large influence on the shape and magnitude of the hysteresis diagrams resulting from sinusoidal movements: the value of all variables selected increases approximately exponentially with muscle length with the exception of the value of loss tangent, a factor indicating the amount of energy dissipated during each cycle relative to the amount of energy stored and released elastically. Velocity of movement has only minor influence on variables of the hysteresis diagrams as is shown by changing the frequency of movement. As loss tangent and relative amount of connective tissue did not vary with muscle dimensions in the muscles studied, it is likely that material properties of the components causing passive resistance were similar in these muscles.