Short-range Stiffness Research Articles

The Effects of Titin Degradation on Passive Stiffness Properties of Skinned Rabbit Psoas Fibers during Osmotic Compression

Muscle is disproportionately stiff over short stretch distances. This effect is largely calcium-sensitive, and can be eliminated in actively contracting skinned fibers by inhibiting crossbridge formation with pharmacological agents such as 2,3-butanedione monoxime (BDM). However, elevated short range stiffness can be induced in the absence of calcium in relaxed skinned muscle fibers by osmotic compression, an effect which is not attenuated by BDM. This study aimed to determine if titin, the primary source of tension and stiffness in relaxed muscle, could account for the elevated short range stiffness in osmotically compressed, relaxed skinned fibers. Accordingly, skinned rabbit psoas fibers (n=9) were held at 2.8 μm sarcomere length and osmotically compressed using 7.5% dextran-T-500 in calcium-free relaxing solution. Muscles were stretched by 2% (∼0.057 μm/sarcomere) over 0.66 s, and returned to initial length over another 0.66 s. Dextran treatment resulted in a 5.6 ± 0.6 fold increase in short range stiffness, with an apparent length limit of 4.5±0.2 nm per sarcomere (Phase 1). Beyond this limit (Phase 2), stiffness was not different (100.4±3.0%) than dextran-free stiffness. To selectively degrade titin, fibers were held in relaxing solution containing both dextran and trypsin (0.25 μg/mL) until mechanical failure, with periodic application of the stretch protocol. In the stretch immediately preceding failure, Phase 2 stiffness was 86±3% lower than initial values, while Phase 1 stiffness only decreased by 31±3%. Additionally, tension at peak stretch increased 12.5±2.1 μN (33±3%) upon osmotic compression, matching the 12.6±1.1 μN intercept of the Phase 2 regression line on the Δtension versus Δlength graph. Thus, short range stiffness appears additive to Phase 2, titin-based stiffness, and is best accounted for as a separate entity acting in parallel to titin.

Dynamic Contractility and Efficiency Impairments in Stretch-Shortening Cycle Are Stretch-Load-Dependent After Training-Induced Muscle Damage

To determine the acute task and stretch-load dependency of neuromuscular impairments after muscle-damaging exercises, we examined the magnitude of strength deficits in isometric and stretch-shortening cycle (SSC) contractions after a single bout of exercise. Ten trained men performed 90 unilateral isokinetic eccentric-concentric knee extensions on a dynamometer. Plasma creatine kinase activity, muscle soreness, maximal isometric torque, short-range stiffness, and peak torque in the eccentric phase of the SSC contraction at 3 stretch-loads (120, 150, and 180 J) were determined in the quadriceps before and 24 hours after exercise. During SSC, positive mechanical work and efficiency were also calculated. Creatine kinase and soreness increased at 24 hours (p < 0.05). In each of the 3 stretch-load conditions, muscle damage affected short-range stiffness less than isometric and peak SSC torque (p < 0.05), providing evidence for a selective impairment in contractile function after muscle damage. With greater SSC stretch-load peak, SSC torque deficit increased linearly, whereas short-range stiffness deficit was unaffected. Efficiency declined only at the 180-J condition (p < 0.05) as a result of decreased positive work (p < 0.05). It was concluded that intense exercise produced microtrauma in the muscle, and a selective loss of force generating capacity, which suggests greater damage to the contractile machinery. Practitioners may expect greater acute impairment of force generation in movements that use large loads in their daily training drills. However, altered knee flexion strategy during SSC may compensate for the force deficit, preserving mechanical efficiency at smaller stretch-loads.

Systematic nonlinear relations between joint mechanics and the neural reflex response with changes in stretch amplitude at the wrist

The effects on both joint mechanics and the neural stretch reflex of changes in amplitude of stretch ranging from within the region of short range stiffness up to 3% of resting muscle length were quantified. The level of activation of the primary agonist was held constant but there was a small but systematic increase in activation of the other flexor and extensor muscles as stretch amplitude increased. The patterns of change with stretch amplitude in both the joint mechanics and stretch reflex properties were non-linear but systematic and were well described by power relationships that accounted for high proportions of variance. With an increase in stretch amplitude, joint stiffness, viscosity, damping ratio and natural frequency decreased. In parallel with these mechanical changes, the neural reflex coherence increased and the phase advance and gain decreased. Multiple regression analyses partitioned the variation in the joint mechanics that was attributable to the various factors and it was found that the variation of the mechanics was almost entirely attributable to the variation of the neural reflex gain, which accounted for 93% of the changes in the stiffness, viscosity and natural frequency and 82% of the changes in the damping ratio. It is concluded that joint mechanics are significantly affected by changes in the neural reflex gain.

Neuromuscular and stiffness adaptations in division I collegiate baseball players

To compare bi-lateral shoulder EMG, active and short range glenohumeral stiffness, and examine its correlation to posterior capsule thickness (PCT) in collegiate baseball players. Surface and fine wire EMG was recorded on shoulder and scapular musculature during stiffness testing. Posterior capsule thickness was assessed separately using a diagnostic ultrasound. Serratus anterior EMG area and peak on the dominant arm was significantly greater compared to the non-dominant arm. The dominant arm had significantly greater active and short range glenohumeral stiffness compared to the non-dominant arm. Active glenohumeral stiffness was significantly correlated with PCT, however short range glenohumeral stiffness was not significantly correlated with PCT. Healthy collegiate baseball players present with adaptations of their stiffness regulation strategies. There were also correlations between stiffness and morphologic changes. Our results support the theory that PCT has an impact on the energy absorption capabilities of the shoulder during the deceleration phase of throwing. It also seems that tightening of the series elastic component within the posterior rotator cuff may be causing the increase in short range stiffness on the dominant arm.

Transmural Heterogeneity and Depressed Function in the Mechanical Properties of Ventricular Tissue from Patients with End-Stage Heart Failure

Heart failure is a progressive condition in which the ventricles can no longer pump enough blood to meet the body's basal demands. Our laboratory is investigating whether the transmural variation in cellular contractile properties that occurs in normal hearts (and which is thought to be important for ventricular function) is altered in heart failure. We procured through wall samples of failing left ventricle from patients receiving transplants at the University of Kentucky and non-failing samples from brain dead organ donors. The tissue was divided into epicardial, midmyocardial and endocardial layers and frozen in liquid nitrogen within 30 minutes. Multicellular chemically permeabilized preparations were subsequently obtained from these samples by mechanical homogenization and triton treatment. The samples were attached between a force transducer and a motor and subjected to two mechanical protocols: 1) a stretch-restretch protocol in solutions with different activating Ca2+ concentrations and 2) a force-velocity protocol in which maximally-activated preparations were allowed to shorten against pre-set loads. Parameters including steady-state force, short-range stiffness, short-range force and maximum power output were measured using these two protocols. The results suggested a 30% decrease in maximum power output (p-value = 0.01) and steady-state force (p-value=0.005) in heart failure patients (n=8, total of 72 preparations) as compared to non-failing (n=4, total of 36 preparations). Short-range stiffness (p-value=0.003) and short-range force (p-value=0.002) also significantly decreased in heart failure vs. non-failing. Transmurally there was a significant difference in maximum power output between the regions (p-value=0.02). The data suggests that mechanically the mid myocardium maybe affected the most in heart failure. Further studies need to be done to understand the protein modifications that may be responsible for these variations.

Short range stiffness elastic limit depends on joint velocity

Muscles behave as elastic springs during the initial strain phase, indicated as short range stiffness (SRS). Beyond a certain amount of strain the muscle demonstrates a more viscous behavior. The strain at which the muscle transits from elastic- to viscous-like behavior is called the elastic limit and is believed to be the result of breakage of cross-bridges between the contractile filaments. The aim of this study was to test whether the elastic limit, measured in vivo at the wrist joint, depended on the speed of lengthening. Brief extension rotations were imposed to the wrist joint (n=8) at four different speeds and at three different levels of voluntary torque using a servo controlled electrical motor. Using a recently published identification scheme, we quantified the elastic limit from measured joint angle and torque. The results showed that the elastic limit significantly increased with speed in a linear way, indicating to a constant time of approximately 30ms before cross-bridges break. The implications for movement control of the joint are discussed.

Rate of Muscle Force Development and Instability in Knee Osteoarthritis

People with knee OA experience pain and the sensation of buckling in their knees that leads to diminished function. Maximum quadriceps force production relates to high function in this group, however, maximum force is rarely used during daily activities. Rate of muscle force development (RFD) is important in maintaining balance and declines with age and disuse, but it is unclear what relationship RFD has with function in people with knee OA. PURPOSE: To examine the relationship between quadriceps RFD and knee function in 3 groups: people with knee OA whose knees were stable (OAS) unstable (OAU) and controls (C). METHODS: Knee joint stiffness and maximum volitional isometric force was measured in 28 people with knee OA (15 OAS and 13 OAU) and 21 C. Functional measures included the KOOS and stair climbing test. The peak instantaneous RFD (RFDi) and average RFD (RFDa) over the interval from 0-200 msec were compared between groups using a one-way ANOVA and post-hoc tests. Pearson's correlation coefficients were used to examine relationships. RESULTS: Significant differences between groups were found in the RFDa (F=5.563, p=.007) and in both relaxed short range and total stiffness (F=4.205, p=.022 and 4.916, p=.012). Post hoc tests revealed that the OAU subjects had higher RFDa and had a higher short range stiffness than controls (p=.006 and p=.021). OAU also had a greater relaxed total stiffness than the OA stable group (p=.010). RFDa correlated negatively with the stair climbing time (r= -0.544, p=.036) in the OAS group. The peak RFDi correlated positively with the KOOS -ADL scores in the OAU group only (r=0.570, p=.042). In the OAU group the peak RFDi and RFDa significantly correlated with the KOOS symptom scores (r=-0.622, p=.023, r=0.606, p=.028) respectively. CONCLUSION: The relationship between RFD and knee function and symptoms in people with knee OA who experience buckling suggests an attempt to stabilize the knee joint. Higher knee joint stiffness in the OAU subjects may allow for these increases in RFD. Accurately assessing and understanding the role of RFD in knee OA may indicate that rehabilitation should involve activities relating to speed in addition to the force of quadriceps contraction. Funded By: NIH P20 RR16458, NIH S10RR022396, ACR REF Health Professional Graduate Student Research Preceptorship

Muscle short-range stiffness can be used to estimate the endpoint stiffness of the human arm

The mechanical properties of the human arm are regulated to maintain stability across many tasks. The static mechanics of the arm can be characterized by estimates of endpoint stiffness, considered especially relevant for the maintenance of posture. At a fixed posture, endpoint stiffness can be regulated by changes in muscle activation, but which activation-dependent muscle properties contribute to this global measure of limb mechanics remains unclear. We evaluated the role of muscle properties in the regulation of endpoint stiffness by incorporating scalable models of muscle stiffness into a three-dimensional musculoskeletal model of the human arm. Two classes of muscle models were tested: one characterizing short-range stiffness and two estimating stiffness from the slope of the force-length curve. All models were compared with previously collected experimental data describing how endpoint stiffness varies with changes in voluntary force. Importantly, muscle properties were not fit to the experimental data but scaled only by the geometry of individual muscles in the model. We found that force-dependent variations in endpoint stiffness were accurately described by the short-range stiffness of active arm muscles. Over the wide range of evaluated arm postures and voluntary forces, the musculoskeletal model incorporating short-range stiffness accounted for 98 ± 2, 91 ± 4, and 82 ± 12% of the variance in stiffness orientation, shape, and area, respectively, across all simulated subjects. In contrast, estimates based on muscle force-length curves were less accurate in all measures, especially stiffness area. These results suggest that muscle short-range stiffness is a major contributor to endpoint stiffness of the human arm. Furthermore, the developed model provides an important tool for assessing how the nervous system may regulate endpoint stiffness via changes in muscle activation.

In vivo estimation of the short-range stiffness of cross-bridges from joint rotation

Short-range stiffness (SRS) is a mechanical property of muscles that is characterized by a disproportionally high stiffness within a short length range during both lengthening and shortening movements. SRS is attributed to the cross-bridges and is beneficial for stabilizing a joint during, e.g., postural conditions. Thus far, SRS has been estimated mainly on isolated mammalian muscles. In this study we presented a method to estimate SRS in vivo in the human wrist joint. SRS was estimated at joint level in the angular domain (N m/rad) for both flexion and extension rotations of the human wrist in nine healthy subjects. Wrist rotations of 0.15 rad at 3 rad/s were imposed at eight levels of voluntary contraction ranging from 0 to 2.1 N m by means of a single axis manipulator. Flexion and extension SRS of the wrist joint was estimated consistently and accurately using a dynamic nonlinear model that was fitted onto the recorded wrist torque. SRS increased monotonically with torque in a way consistent with previous studies on isolated muscles. It is concluded that in vivo measurement of joint SRS represents the population of coupled cross-bridges in wrist flexor and extensor muscles. In its current form, the presented technique can be used for clinical applications in many neurological and muscular diseases where altered joint torque and (dissociated) joint stiffness are important clinical parameters.

Short-Range Mechanical Properties of Myocardium from Diabetic Rats

Diabetes mellitus is often associated with abnormalities in active relaxation and passive stiffness of the left ventricle but the molecular mechanisms responsible for the dysfunction are not yet clear. This study was designed to identify the molecular components that are responsible for the increased myocardial stiffness associated with diabetes. Multicelluar myocardial preparations were isolated from control Sprague-Dawley rats and an experimental group of rats injected 4 weeks previously with streptozotocin (model of Type I Diabetes). Preparations were subjected to paired ramp stretches/releases imposed under fiber length control in a series of calcium activations (pCa 4.5 - 9.0). The relative short-range force and elastic limits were substantially higher in the diabetic groups. The rate of tension recovery (ktr) was considerably lower in the diabetic groups. Short range stiffness values did not differ in the control and diabetic animals. Gel electrophoresis showed that the relative content of slower beta Myosin heavy chain increased from 34±15% in control hearts to 100% in the diabetic rat hearts. These results support the hypothesis that pathological changes in the mechanical properties of diabetic rat myocardium are mostly due to alterations in the active component (cycling cross-bridges) of ventricular stiffness.View Large Image | View Hi-Res Image | Download PowerPoint Slide

In Vivo Assessment of Both Active and Passive Parts of the Plantarflexors Series Elastic Component Stiffness Using the Alpha Method: A Reliability Study

The aim of this study was to investigate the reliability of an in vivo adaptation of the short range stiffness experiment associated with the application of a mathematical model to determine the stiffness of both torque dependent and independent components of the plantarflexors series elastic component. Fourteen subjects participated in this study. The experimental protocol consisted of quickly moving the ankle joint in dorsiflexion during constant voluntary isometric plantarflexion at 7 submaximal torque levels. Relationships between joint stiffness and torque were established and the stiffness of both torque dependent and independent components were determined using the alpha method. The day-to-day reliability was assessed for joint stiffness and stiffness of both torque dependent and independent components (ICC higher than 0.88 and CVs lower than 6.0%). This method could then be used to better understand adaptive subjacent mechanisms to assess the effects of training protocols, and the rehabilitation of neuromuscular pathologies or traumatisms.

The Consequences of Short-Range Stiffness and Fluctuating Muscle Activity for Proprioception of Postural Joint Rotations: The Relevance to Human Standing

Proprioception comes from muscles and tendons. Tendon compliance, muscle stiffness, and fluctuating activity complicate transduction of joint rotation to a proprioceptive signal. These problems are acute in postural regulation because of tiny joint rotations and substantial short-range muscle stiffness. When studying locomotion or perturbed balance these problems are less applicable. We recently measured short-range stiffness in standing and considered the implications for load stability. Here, using an appropriately simplified model we analyze the conversion of joint rotation to spindle input and tendon tension while considering the effect of short-range stiffness, tendon compliance, fluctuating muscle activity, and fusimotor activity. Basic principles determine that when muscle stiffness and tendon compliance are high, fluctuating muscle activity is the greatest factor confounding registration of postural movements, such as ankle rotations during standing. Passive and isoactive muscle, uncomplicated by active length fluctuations, enable much better registration of joint rotation and require fewer spindles. Short-range muscle stiffness is a degrading factor for spindle input and enhancing factor for Golgi input. Constant fusimotor activity does not enhance spindle registration of postural joint rotations in actively modulated muscle: spindle input remains more strongly associated with muscle activity than joint rotation. A hypothesized rigid alpha-gamma linkage could remove this association with activity but would require large numbers of spindles in active postural muscles. Using microneurography, the existence of a rigid alpha-gamma linkage could be identified from the correlation between spindle output and muscle activity. Basic principles predict a proprioceptive "dead zone" in the active agonist muscle that is related to the short-range muscle stiffness.

In situ estimation of tendon material properties: Differences between muscles of the feline hindlimb

Recent experiments to characterize the short-range stiffness (SRS)–force relationship in several cat hindlimb muscles suggested that the there are differences in the tendon elastic moduli across muscles [Cui, L., Perreault, E.J., Maas, H., Sandercock, T.G., 2008. Modeling short-range stiffness of feline lower hindlimb muscles. J. Biomech. 41 (9), 1945–1952.]. Those conclusions were inferred from whole muscle experiments and a computational model of SRS. The present study sought to directly measure tendon elasticity, the material property most relevant to SRS, during physiological loading to confirm the previous modeling results. Measurements were made from the medial gastrocnemius (MG), tibialis anterior (TA) and extensor digitorum longus (EDL) muscles during loading. For the latter, the model indicated a substantially different elastic modulus than for MG and TA. For each muscle, the stress–strain relationship of the external tendon was measured in situ during the loading phase of isometric contractions conducted at optimum length. Young's moduli were assessed at equal strain levels (1%, 2% and 3%), as well as at peak strain. The stress–strain relationship was significantly different between EDL and MG/TA, but not between MG and TA. EDL had a more apparent toe region (i.e., lower Young's modulus at 1% strain), followed by a more rapid increase in the slope of the stress–strain curve (i.e., higher Young's modulus at 2% and 3% strain). Young's modulus at peak strain also was significantly higher in EDL compared to MG/TA, whereas no significant difference was found between MG and TA. These results indicate that during natural loading, tendon Young's moduli can vary considerably across muscles. This creates challenges to estimating muscle behavior in biomechanical models for which direct measures of tendon properties are not available.

Effects Of Blebbistatin And BDM (2,3-Butanedione Monoxime) On The Short-range Mechanical Properties Of Murine Diaphragm Muscle Fibers

Blebbistatin (BLEB) and 2,3-butanedione monoxime (BDM) are well-know inhibitors of myofilament force production and useful tools in structural and functional studies of cell motility and muscle contraction. In this study, we investigated the effects of BLEB and BDM on the short-range mechanical properties of single chemically permeabilized murine diaphragm fibers. BLEB and BDM were used in separate sets of experiments to reduce isometric force in saturating Ca2+ solution to approximately 50% of the control value. Muscle fibers were subjected to repeated triangular length changes (paired ramp stretches/releases, 0.04 l0, 0.33 l0 s−1) imposed under fiber length control in solutions with free Ca2+ concentrations ranging from pCa 9.0 to pCa 4.5. Short-range stiffness values were calculated from the slopes of regression lines fitted to the first 15 ms of XY plots of force against muscle length for each stretch response and expressed as Young's Moduli. Analysis of results obtained in control Ca2+ solutions (without BLEB or BDM) showed that short-range stiffness increased proportionately with the level of isometric force. Experiments performed with BLEB showed that short-range stiffness declined in proportion with the reduction in isometric force (no change in the stiffness/force ratio). In contrast, BDM produced a disproportionately large decrease in isometric force (that is, the stiffness/force ratio increased in the presence of BDM, ANACOVA test, p<0.001). These results support the hypothesis that BLEB and BDM reduce isometric force in skeletal muscles by different mechanisms. BLEB seems to prevent myosin heads from attaching to the thin filament while BDM probably reduces force by decreasing the rate at which myosin heads undergo tension-generating biochemical transitions.

Modeling short-range stiffness of feline lower hindlimb muscles

The short-range stiffness (SRS) of skeletal muscles is a critical property for understanding muscle contributions to limb stability, since it represents a muscle's capacity to resist external perturbations before reflexes or voluntary actions can intervene. A number of studies have demonstrated that a simple model, consisting of a force-dependent active stiffness connected in series with a constant passive stiffness, is sufficient to characterize the SRS of individual muscles over the entire range of obtainable forces. The purpose of this study was to determine if such a model could be used to characterize the SRS–force relationship in a number of architecturally distinct muscles. Specifically, we hypothesized that the active and passive stiffness components for a specific muscle can be estimated from anatomical measurements, assuming uniform active and passive stiffness properties across all muscles. This hypothesis was evaluated in six feline lower hindlimb muscle types with different motor unit compositions and architectures. The SRS–force relationships for each muscle type were predicted based on anatomical measurements and compared to experimental data. The model predictions were accurate to within 30%, when uniform scaling properties were assumed across all muscles. Errors were the greatest for the extensor digitorum longus (EDL). When this muscle was removed from the analysis, prediction errors dropped to less than 8%. Subsequent analyses suggested that these errors might have resulted from differences in the tendon elastic modulus, as compared to the other muscles tested.

The passive, human calf muscles in relation to standing: the short range stiffness lies in the contractile component.

Using short duration perturbations, previous attempts to measure the intrinsic ankle stiffness during human standing have revealed a substantial stabilizing contribution (65-90% normalized to load stiffness 'mgh'). Others regard this method as unsuitable for the low-frequency conditions of quiet standing and believe the passive contribution to be small (10-15%). This latter view, consistent with a linear Hill-type model, argues that during standing, the contractile portion of the muscle is much less stiff than the tendon. Here, for upright subjects, we settle this issue by measuring the stiffness of the contractile portion of the passive calf muscles using low-frequency ankle rotations. Using ultrasound we tracked the changes in muscle contractile length and partitioned the ankle rotation into contractile and extra-contractile (series elastic) portions. Small ankle rotations of 0.15 and 0.4 deg show a contractile to series elastic stiffness ratio (K(ce)/K(se)) of 12 +/- 9 and 6.3 +/- 10, respectively, with both elements displaying predominantly elastic behaviour. Larger, 7 deg rotations reveal the range of this ratio. It declines in a non-linear way from a high value (K(ce)/K(se) = 18 +/- 11) to a low value (K(ce)/K(se) = 1 +/- 0.4) as rotation increases from 0.1 to 7 deg. There is a marked transition at around 0.5 deg. The series elastic stiffness (K(se)/mgh) remains largely constant (77 +/- 13%) demonstrating the contractile component origin of passive, short range stiffness. The linear Hill-type model does not describe the range-related stiffness relevant to the progression from quiet standing to perturbed balance and movement and can lead to inaccurate predictions regarding human balance.

The passive, human calf muscles in relation to standing: the non‐linear decrease from short range to long range stiffness

During human standing, tonic ankle extensor torque is required to support the centre of mass (CoM) forward of the ankles, and dynamic torque modulation is required to maintain unstable balance. Passive mechanisms contribute to both but the extent is controversial. Some groups have revealed a substantial intrinsic stiffness (65-90%) normalized to load stiffness, 'mgh'. Others regard their methodology as unsuitable for the low-frequency conditions of quiet standing and believe the passive contribution to be small (10-15%). Here we applied low-frequency ankle rotations to upright subjects who were supported at the waist allowing the leg muscles to be passive and we report normalized stiffness. The passive calf muscles provided: (i) an extensor torque capable of sustaining unstable balance without tonic activity at a mean CoM-ankle angle of 1.6 deg, (ii) a long range stiffness of 13 +/- 2% and (iii) a short range (< 0.2 deg) stiffness of 67 +/- 8%. Chordal ankle stiffness, derived from the torque versus angle relationship for 7 deg rotations, shows a non-linear decrease (stiffness alpha rotation(-0.33+/-0.04)) from 101 +/- 9% to 19 +/- 5% for rotations of 0.03-7 deg, respectively. Thus, passive stiffness is well adapted for the continuum of postural and movement activity and has a substantial postural role eliminating the need for continuous muscle activity and increasing the unstable time constant of the human inverted pendulum. Ignoring the non-linear dependence of passive stiffness on sway size could lead to serious misinterpretation of experiments using perturbations and sensory manipulations such as eye closure, sway referencing and altered support surfaces.

Motor unit composition has little effect on the short-range stiffness of feline medial gastrocnemius muscle

Studies on skinned fibers and single motor units have indicated that slow-twitch fibers are stiffer than fast-twitch fibers. This suggests that skeletal muscles with different motor unit compositions may have different short-range stiffness (SRS) properties. Furthermore, the natural recruitment of slow before fast motor units may result in an SRS-force profile that is different from electrical stimulation. However, muscle architecture and the mechanical properties of surrounding tissues also contribute to the net SRS of a muscle, and it remains unclear how these structural features each contribute to the SRS of a muscle. In this study, the SRS-force characteristics of cat medial gastrocnemius muscle were measured during natural activation using the crossed-extension reflex, which activates slow before fast motor units, and during electrical activation, in which all motor units are activated synchronously. Short, rapid, isovelocity stretches were applied using a linear puller to measure SRS across the range of muscle forces. Data were collected from eight animals. Although there was a trend toward greater stiffness during natural activation, this trend was small and not statistically significant across the population of animals tested. A simple model, in which the slow-twitch fibers were assumed to be 30% stiffer than the fast-twitch fibers, was used to simulate the experimental results. Experimental and simulated results show that motor unit composition or firing rate has little effect on the SRS property of the cat MG muscle, suggesting that architectural features may be the primary determinant of SRS.

Postural Muscle Tone in the Body Axis of Healthy Humans

Across the entire human body, postural tone might play its most critical role in the body's axis because the axis joins the four limbs and head into a single functioning unit during complex motor tasks as well as in static postures. Although postural tone is commonly viewed as low-level, tonic motor activity, we hypothesized that postural tone is both tonically and dynamically regulated in the human axis even during quiet stance. Our results describe the vertical distribution of postural muscle tone in the neck, trunk, and hips of standing human adults. Each subject stood blindfolded on a platform that axially rotated the neck, trunk, or pelvis at 1 degrees /s and +/-10 degrees relative to the neutral position (i.e., facing forward). The measured resistance to axial rotation was highest in the trunk and lowest in the neck and was characterized by several nonlinear features including short-range stiffness and hysteresis. In half of the subjects, axial muscle activity was relatively constant during axial rotation, and in the other half, muscle activity was modulated by lengthening and shortening reactions, i.e., decreasing activity in lengthening muscles and increasing activity in shortening muscles, respectively. Axial resistance to rotation was reduced in subjects whose muscle activity was modulated. The results indicate that axial tone is modulated sensitively and dynamically, this control originates, at least in part, from tonic lengthening and shortening reactions, and a similar type of control appears to exist for postural tone in the proximal muscles of the arm.

Systematic nonlinear relations between displacement amplitude and joint mechanics at the human wrist

This study quantified the systematic effects on wrist joint mechanics of changes in amplitude of displacement ranging from within the region of short-range stiffness (0.2% of resting muscle length) up to 3% of resting muscle length. The joint mechanics were modelled using a second-order system from which estimates of joint stiffness, viscosity, inertia, natural resonant frequency and damping ratio were obtained. With increasing amplitude of displacement, the stiffness decreased by 31%, the viscosity decreased by 73%, the damping ratio decreased by 71% and the resonant frequency decreased from 10.5 to 7.3 Hz. The patterns of change in joint mechanics with displacement amplitude were nonlinear but systematic and were well described by power relationships with high R 2 values. These relationships provide normative data for the adult population and may be used in the modelling of human movement, in the study of neurological disorders and in robotics where human movement is simulated. The observed patterns of high initial stiffness and viscosity, decreasing progressively as displacement amplitude increases, may provide a good compromise between postural stability and liveliness of voluntary movement.