Influence of forearm orientation on biceps brachii tendon mechanics and elbow flexor force steadiness

Achilles tendon mechanics influence plantar flexion force steadiness (FS) and balance. In the upper limb, elbow flexor FS is greater in supinated and neutral forearm orientations compared to pronated, with contributions of tendon mechanics remaining unknown in position-dependent FS. This study investigated whether distal biceps brachii (BB) tendon mechanics across supinated, neutral and pronated forearm orientations influence position-dependent FS of the elbow flexors. Eleven males (23 ± 3 years) performed submaximal isometric elbow flexion tasks at low (5, 10% maximal voluntary contraction (MVC)) and high (25, 50, 75% MVC) force levels in supinated, neutral and pronated forearm orientations. Distal BB tendon elongation and CSA were recorded on ultrasound to calculate mechanics of tendon stress, strain and stiffness. Relationships between FS, calculated as coefficient of variation (CV) of force, and tendon mechanics were evaluated with multiple regressions. Supinated and neutral were ∼50% stronger and ∼60% steadier than pronated (p < 0.05). Tendon stress was ∼52% greater in supinated and neutral compared to pronated, tendon strain was ∼36% greater in neutral than pronated (p < 0.05), while tendon stiffness (267.4 ± 78.9 N/mm) did not differ across orientations (p > 0.05). At low forces, CV of force was predicted by MVC (r²: 0.52) in supinated, and MVC and stress in neutral and pronated (r²: 0.65–0.81). At high force levels, CV of force was predicted by MVC and stress in supinated (r²: 0.49), and MVC in neutral (r²: 0.53). Absolute strength and tendon mechanics influence the ability of the BB tendon to distribute forces, and thus are key factors in position-dependent FS.     

Physiological considerations of muscle force through the elbow joint

An analytical model for the determination of muscle forces across the elbow joint during isometric loading conditions has been developed. The model incorporates the muscle length-tension relationship, while considering the muscle architecture. Sensitivity analyses were performed to study the effects of the geometric and architectural factors of the muscles on the distribution of muscle forces.     

Multivariate genetic analysis of maximal isometric muscle force at different elbow angles

Thomis, Martine A., Marc Van Leemputte, Hermine H. Maes,Cameron J. R. Blimkie, Albrecht L. Claessens, Guy Marchal, Eustachius Willems, Robert F. Vlietinck, and Gaston P. Beunen. Multivariate genetic analysis of maximal isometric muscle force at different elbowangles. J. Appl. Physiol. 82(3):959-967, 1997.The maximal isometric moment at five differentelbow joint angles was measured in 25 monozygotic and 16 dizygotic maleadult twin pairs (22.4 ± 3.7 yr). Genetic model fittingwas used to quantify the genetic and environmental contributions toindividual differences in isometric strength. Additive genetic factorsexplained 66-78% of the variance in maximal torque at170-140-110 and 80° flexion (extension = 180°). At50° flexion, common and subject-specific environmental factorscontributed equally to the variation. The contribution of uniqueenvironmental factors concurs with the level of variability in muscleactivation and (dis)-comfort of torque production in the specificangle. The relative contribution of lever arm and force-lengthrelationship in torque varies according to the angle. Because thesefactors might be genetic, this variability is reflected in the geneticcontribution at the extreme angles of 170 and 50°. Multivariateanalyses suggested a general set of genes that control muscle area andisometric strength, together with a more specific strength factor.Genetic correlations were high (0.82-0.99). Genes responsible forarm-segment lengths did not contribute to muscle area nor to isometricstrength.

     

A mathematical model for predicting relative muscle force with a perturbation analysis of selected muscle parameters

A mathematical model is presented that predicts relative muscle forces using a minimum of experimentally derived input data. Tests of this model against literature values for maximum muscle force of four cat hindlimb muscles show a maximum error of only 5%. A perturbation analysis using this model demonstrates its sensitivity and applicability, as well as the congruence between this model and previous theoretical discussions of muscle function.     

Voluntary control of muscle tension in lengthened and intact limbs in different ranges of force load

Data confirming the hypothesis that there is a range of isometric loads where subjects organize voluntary control of muscle tension with the maximum precision and minimum specific energy expenditure are presented, the energy expenditure being estimated indirectly by the ratio of the area (integral) of the summary electromyogram (EMG) to the force momentum impulse. The asymmetries of the integrated estimates of visuomotor tracking with the use of isometric control organs have been analyzed in 23 patients (6 men and 17 women) aged 15–35 years in different periods, after surgical elimination of the difference between the lengths of their lower extremities. A slightly distinct zone of minimization of the integrated estimates of discrete visuomotor tracking in the range of 25–35% of the maximum force of the muscle group tested (foot dorsal flexors) has been found in healthy subjects (26 healthy men aged 19–39 years) and orthopedic patients (the intact extremity). The zone of optimal loads is more distinct in patents on the side of lengthening; this zone tends to shift towards the region of weaker forces.     

A strategy to determine operating parameters in tissue engineering hollow fiber bioreactors

The development of tissue engineering hollow fiber bioreactors (HFB) requires the optimal design of the geometry and operation parameters of the system. This article provides a strategy for specifying operating conditions for the system based on mathematical models of oxygen delivery to the cell population. Analytical and numerical solutions of these models are developed based on Michaelis–Menten kinetics. Depending on the minimum oxygen concentration required to culture a functional cell population, together with the oxygen uptake kinetics, the strategy dictates the model needed to describe mass transport so that the operating conditions can be defined. If c_min ≫ K_m we capture oxygen uptake using zero‐order kinetics and proceed analytically. This enables operating equations to be developed that allow the user to choose the medium flow rate, lumen length, and ECS depth to provide a prescribed value of c_min. When , we use numerical techniques to solve full Michaelis–Menten kinetics and present operating data for the bioreactor. The strategy presented utilizes both analytical and numerical approaches and can be applied to any cell type with known oxygen transport properties and uptake kinetics. Biotechnol. Bioeng. 2011; 108:1450–1461. © 2011 Wiley Periodicals, Inc.     

Skeletal muscle fiber atrophy: altered thin filament density changes slow fiber force and shortening velocity

Single skinned fibers from soleus and adductor longus (AL) muscles of weight-bearing control rats and rats after 14-day hindlimb suspension unloading (HSU) were studied physiologically and ultrastructurally to investigate how slow fibers increase shortening velocity (V₀) without fast myosin. We hypothesized that unloading and shortening of soleus during HSU reduces densities of thin filaments, generating wider myofilament separations that increase V₀ and decrease specific tension (kN/m²). During HSU, plantarflexion shortened soleus working length 23%. AL length was unchanged. Both muscles atrophied as shown by reductions in fiber cross-sectional area. For AL, the 60% atrophy accounted fully for the 58% decrease in absolute tension (mN). In the soleus, the 67% decline in absolute tension resulted from 58% atrophy plus a 17% reduction in specific tension. Soleus fibers exhibited a 25% reduction in thin filaments, whereas there was no change in AL thin filament density. Loss of thin filaments is consistent with reduced cross bridge formation, explaining the fall in specific tension. V₀ increased 27% in soleus but was unchanged in AL. The V₀ of control and HSU fibers was inversely correlated (R = –0.83) with thin filament density and directly correlated (R = 0.78) with thick-to-thin filament spacing distance in a nonlinear fashion. These data indicate that reduction in thin filament density contributes to an increased V₀ in slow fibers. Osmotically compacting myofilaments with 5% dextran returned density, spacing, and specific tension and slowed V₀ to near-control levels and provided evidence for myofilament spacing modulating tension and V₀. rat; soleus; adductor longus; fiber length; electron microscopy; hindlimb suspension unloading     

Design and operating criteria for hollow fiber bioreactors

We have analyzed the design and operation of hollow fiber bioreactors for cell culture on the basis of cell growth efficiency and non-uniform fiber spacing. Operating diagrams are presented that describe reactor performance as a function of important operating variables like oxygen concentration and the fiber packing density. The diagrams allow one to find the best set of operating conditions for a fixed reactor design, or to rationally adjust the design parameters for fixed operating conditions.     

Individual muscle contributions to the axial knee joint contact force during normal walking

Muscles are significant contributors to the high joint forces developed in the knee during human walking. Not only do muscles contribute to the knee joint forces by acting to compress the joint, but they also develop joint forces indirectly through their contributions to the ground reaction forces via dynamic coupling. Thus, muscles can have significant contributions to forces at joints they do not span. However, few studies have investigated how the major lower-limb muscles contribute to the knee joint contact forces during walking. The goal of this study was to use a muscle-actuated forward dynamics simulation of walking to identify how individual muscles contribute to the axial tibio-femoral joint force. The simulation results showed that the vastii muscles are the primary contributors to the axial joint force in early stance while the gastrocnemius is the primary contributor in late stance. The tibio-femoral joint force generated by these muscles was at times greater than the muscle forces themselves. Muscles that do not cross the knee joint (e.g., the gluteus maximus and soleus) also have significant contributions to the tibio-femoral joint force through their contributions to the ground reaction forces. Further, small changes in walking kinematics (e.g., knee flexion angle) can have a significant effect on the magnitude of the knee joint forces. Thus, altering walking mechanics and muscle coordination patterns to utilize muscle groups that perform the same biomechanical function, yet contribute less to the knee joint forces may be an effective way to reduce knee joint loading during walking.     

Inferences on force transmission from muscle fiber architecture of the canine diaphragm

Functional properties of the diaphragm are mediated by muscle structure. Modeling of force transmission necessitates a precise knowledge of muscle fiber architecture. Because the diaphragm experiences loads both along and transverse to the long axes of its muscle fibers in vivo, the mechanism of force transmission may be more complex than in other skeletal muscles that are loaded uniaxially along the muscle fibers. Using a combination of fiber microdissections and histological and morphological methods, we determined regional muscle fiber architecture and measured the shape of the cell membrane of single fibers isolated from diaphragm muscles from 11 mongrel dogs. We found that muscle fibers were either spanning fibers (SPF), running uninterrupted between central tendon (CT) and chest wall (CW), or were non-spanning fibers (NSF) that ended within the muscle fascicle. NSF accounted for the majority of fibers in the midcostal, dorsal costal, and lateral crural regions but were only 25-41% of fibers in the sternal region. In the midcostal and dorsal costal regions, only approximately 1% of the NSF terminated within the fascicle at both ends; the lateral crural region contained no such fibers. We measured fiber length, tapered length, fiber diameters along fiber length, and the taper angle for 271 fibers. The lateral crural region had the longest mean length of SPF, which is equivalent to the mean muscle length, followed by the costal and sternal regions. For the midcostal and crural regions, the percentage of tapered length of NSF was 45.9 +/- 5.3 and 40.6 +/- 7.5, respectively. The taper angle was approximately 0.15 degrees for both, and, therefore, the shear component of force was approximately 380 times greater than the tensile component. When the diaphragm is submaximally activated, as during normal breathing and maximal inspiratory efforts, muscle forces could be transmitted to the cell membrane and to the extracellular intramuscular connective tissue by shear linkage, presumably via structural transmembrane proteins.     

A micromechanical muscle model for determining the impact of motor unit fiber clustering on force transmission in aging skeletal muscle

This study used a micromechanical finite element muscle model to investigate the effects of the redistribution of spatial activation patterns in young and old muscle. The geometry consisted of a bundle of 19 active muscle fibers encased in endomysium sheets, surrounded by passive tissue to model a fascicle. Force was induced by activating combinations of the 19 active muscle fibers. The spacial clustering of muscle fibers modeled in this study showed unbalanced strains suggesting tissue damage at higher strain levels may occur during higher levels of activation and/or during dynamic conditions. These patterns of motor unit remodeling are one of the consequences of motor unit loss and reinnervation associated with aging. The results did not reveal evident quantitative changes in force transmission between old and young adults, but the patterns of stress and strain distribution were affected, suggesting an uneven distribution of the forces may occur within the fascicle that could provide a mechanism for muscle injury in older muscle.

     

Determining the optimal flexion-extension axis of the elbow in vivo - a study of interobserver and intraobserver reliability

In the current study the interobserver and intraobserver reliability of a recently developed method to obtain the position and orientation vectors of the flexion-extension axis of the elbow in vivo is determined. The method uses the Flock of Birds six degrees-of-freedom electromagnetic tracking device. Ten subjects performed three trials comprising five flexion and extension cycles. The movements of the forearm with respect to the upper arm were recorded. Observer A measured two trials and observer B measured one trial. Optimal instantaneous helical axes were calculated in a humeral coordinate system for each trial. Intraclass correlation coefficients and 99% confidence intervals were computed to compare the three measurements. Zero was in the range of all the narrow confidence intervals, which is strong indication for resemblance. Interobserver intraclass correlation coefficients values for orientation vectors were good to excellent and intraobserver values were fair to good. The intraclass correlation coefficients values for position vectors were lower, probably due to the lack of variance between subjects. It is concluded that the method is reliable and can be used in certain clinical settings.     

Sensitivity of muscle force estimations to changes in muscle input parameters using nonlinear optimization approaches.

The purpose of this study was to analyze the sensitivity of muscle force calculations to changes in muscle input parameters. Force sharing between two synergistic muscles was derived analytically for a one-degree-of-freedom system using three nonlinear optimization approaches. Changes in input parameters that are within normal anatomical variations often caused changes in muscular forces exceeding 100 percent. These results indicate that errors in muscle force calculations may depend as much on inadequate muscle input parameters as they may on the choice of the objective and constraint functions of the optimization approach.     

Stretch-induced,steady-state force enhancement in single skeletal muscle fibers exceeds the isometric force at optimum fiber length

Stretch-induced force enhancement has been observed in a variety of muscle preparations and on structural levels ranging from single fibers to in vivo human muscles. It is a well-accepted property of skeletal muscle. However, the mechanism causing force enhancement has not been elucidated, although the sarcomere-length non-uniformity theory has received wide support. The purpose of this paper was to re-investigate stretch-induced force enhancement in frog single fibers by testing specific hypotheses arising from the sarcomere-length non-uniformity theory. Single fibers dissected from frog tibialis anterior (TA) and lumbricals (n=12 and 22, respectively) were mounted in an experimental chamber with physiological Ringer's solution (pH=7.5) between a force transducer and a servomotor length controller. The tetantic force-length relationship was determined. Isometric reference forces were determined at optimum length (corresponding to the maximal, active, isometric force), and at the initial and final lengths of the stretch experiments. Stretch experiments were performed on the descending limb of the force-length relationship after maximal tetanic force was reached. Stretches of 2.5-10% (TA) and 5-15% lumbricals of fiber length were performed at 0.1-1.5 fiber lengths/s. The stretch-induced, steady-state, active isometric force was always equal or greater than the purely isometric force at the muscle length from which the stretch was initiated. Moreover, for stretches of 5% fiber length or greater, and initiated near the optimum length of the fiber, the stretch-enhanced active force always exceeded the maximal active isometric force at optimum length. Finally, we observed a stretch-induced enhancement of passive force. We conclude from these results that the sarcomere length non-uniformity theory alone cannot explain the observed force enhancement, and that part of the force enhancement is associated with a passive force that is substantially greater after active compared to passive muscle stretch.     

The essential light chain N-terminal extension alters force and fiber kinetics in mouse cardiac muscle

The functional significance of the actin-binding region at the N terminus of the cardiac myosin essential light chain (ELC) remains elusive. In a previous experiment, the endogenous ventricular ELC was replaced with a protein containing a 10-amino acid deletion at positions 5-14 (ELC1vDelta5-14, referred to as 1vDelta5-14), a region that interacts with actin. 1vDelta5-14 mice showed no discernable mutant phenotype in skinned ventricular strips. However, because the myofilament lattice swells upon skinning, the mutant phenotype may have been concealed by the inability of the ELC to reach the actin-binding site. Using the same mouse model, we repeated earlier measurements and performed additional experiments on skinned strips osmotically compressed to the intact lattice spacing as determined by x-ray diffraction. 1vDelta5-14 mice exhibited decreased maximum isometric tension without a change in calcium sensitivity. The decreased force was most evident in 5-6-month-old mice compared with 13-15-month-old mice and may account for the greater ventricular wall thickness in young 1vDelta5-14 mice compared with age-matched controls. No differences were observed in unloaded shortening velocity at maximum calcium activation. However, 1vDelta5-14 mice exhibited a significant difference in the frequency at which minimum complex modulus amplitude occurred, indicating a change in cross-bridge kinetics. We hypothesize that the ELC N-terminal extension interaction with actin inhibits the reversal of the power stroke, thereby increasing isometric force. Our results strongly suggest that an interaction between residues 5-14 of the ELC N terminus and the C-terminal residues of actin enhances cardiac performance.     

Finite element analysis of mechanics of lateral transmission of force in single muscle fiber

Most of the myofibers in long muscles of vertebrates terminate within fascicles without reaching either end of the tendon, thus force generated in myofibers has to be transmitted laterally through the extracellular matrix (ECM) to adjacent fibers; which is defined as the lateral transmission of force in skeletal muscles. The goal of this study was to determine the mechanisms of lateral transmission of force between the myofiber and ECM. In this study, a 2D finite element model of single muscle fiber was developed to study the effects of mechanical properties of the endomysium and the tapered ends of myofiber on lateral transmission of force. Results showed that most of the force generated is transmitted near the end of the myofiber through shear to the endomysium, and the force transmitted to the end of the model increases with increased stiffness of ECM. This study also demonstrated that the tapered angle of the myofiber ends can reduce the stress concentration near the myofiber end while laterally transmitting force efficiently.     

Influence of contraction force and speed on muscle fiber conduction velocity during dynamic voluntary exercise.

Before using electromyographic (EMG) variables such as muscle fiber conduction velocity (MFCV) and the mean or median frequency (MDF) of an EMG power spectrum as indicators of muscular fatigue during dynamic exercises, it is necessary to determine the influence of a joint angle, contraction force and contraction speed on the EMG variables. If these factors affect the EMG variables, their influence must be removed or compensated for before discussing fatigue. The vastus lateralis of eight normal healthy male adults was studied. EMG signals during non-fatiguing dynamic knee extension exercises were detected with a three-bar active surface electrode array. EMG variables were calculated from the detected signals and compared with the angle of the knee joint, the extension torque and the extension speed. The extension torque was set at four levels with 10% intervals between 40 and 70% of the maximum voluntary contraction. The extension speed was set at five levels with 60 degrees /s intervals between 0 and 240 degrees /s. Because the joint angle unsystematically affected the MFCV, EMG variables at a given joint angle were extracted for comparison. The influence of the extension torque and speed on the extracted EMG variables was clarified with an ANOVA and a regression analysis. The statistical analyses showed that MFCV increased with the extension torque but did not depend on the extension speed. In contrast, MDF was independent of the extension torque but was dependent on the extension speed. MDF thus showed a behavior different from that of MFCV. It became clear that if MFCV is used as an indicator of muscular fatigue during dynamic exercises, it is at least necessary to extract MFCV at a predetermined joint angle and then remove the influence of extension torque on MFCV.