Active Muscle Stiffness Research Articles

Active Stiffness and Strength in People With Unilateral Anterior Shoulder Instability: A Bilateral Comparison

Active muscle stiffness might protect the unstable shoulder from recurrent dislocation. To compare strength and active stiffness in participants with unilateral anterior shoulder instability and to examine the relationship between active stiffness and functional ability. Cross-sectional study. University research laboratory. Participants included 16 males (age range, 16-40 years; height = 179.4 ± 6.1 cm; mass = 79.1 ± 6.8 kg) with 2 or more episodes of unilateral traumatic anterior shoulder instability. Active stiffness and maximal voluntary strength were measured bilaterally in participants. In addition, quality of life, function, and perceived instability were measured using the Western Ontario Stability Index, American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form, and Single Alpha Numeric Evaluation, respectively. We found less horizontal adduction strength (t(15) = -4.092, P = .001) and less stiffness at 30% (t(14) = -3.796, P = .002) and 50% (t(12) = -2.341, P = .04) maximal voluntary strength in the unstable than stable shoulder. Active stiffness was not correlated with quality of life, function, or perceived instability (r range, 0.0-0.25; P > .05). The observed reduction in stiffness in the unstable shoulder warrants inclusion of exercises in the rehabilitation program to protect the joint from perturbations that might lead to dislocation. The lack of association between active stiffness and quality of life, function, or perceived instability might indicate that stiffness plays a less direct role in shoulder stability.

Electromyography and mechanomyography of elbow agonists and antagonists in Parkinson disease

The purpose of this study was to assess the electromyographic (EMG) and mechanomyographic (MMG) activities of agonist and antagonist muscles in Parkinson disease patients during maximal isometric elbow contraction in flexion and extension. Ten elderly females with Parkinson disease (average age 75 years) and 10 age-matched healthy females were tested. The torque and the EMG and MMG signals from biceps brachii and triceps brachii were recorded during sustained maximal voluntary isometric contraction of the elbow flexors and extensors. There were no intergroup differences in the EMG and MMG activities of agonist and antagonist muscles or in torque. This might be because the Parkinson subjects were tested during their medication "ON" phase, or perhaps maximal isometric contraction (MVC) induced greater active muscle stiffness that affected the MMG signal. Muscle Nerve 40: 240-248, 2009.

Reliability And Validity Of A Hand-held Myotonometer For Assessing Skeletal Muscle Stiffness

Active stiffness of the muscles surrounding a joint has been shown to be an essential component for maintaining joint stability. To date, methods for measuring active muscle stiffness have utilized complicated computer algorithms and sophisticated laboratory hardware, making accurate assessment of muscle stiffness for the clinician nearly impossible. PURPOSE: To examine the reliability and validity of a hand-held device for assessing skeletal muscle stiffness. METHODS: Active muscle stiffness of the rectus femoris muscle was collected on 20 subjects (21.88 ± 2.86 yrs, 169.96 ± 9.29 cm, 71.17 ± 11.82 kg) over loads of 10, 20, 30, 40, and 50% of their maximum voluntary isometric contraction. Subjects were seated in an isokinetic dynamometer with their knee flexed to 60 degrees. Measurements were collected from the mid-bellies of the rectus femoris with a myotonometer that exerted a local impact on the tissue by means of a brief mechanical impulse. Five trials were measured in each condition. Intraclass correlations (ICC(2,1)) were performed across the five trials in each load condition to assess trial reliability. RESULTS: Mean stiffness values for each of the load conditions (10% = 314.52 ± 69.11, 20% = 344.74 ± 85.96, 30% = 362.26 ± 99.28, 40% = 371.76 ± 99.11, 50% = 386.79 ± 120.40 N/rad) were consistent with previous literature. Post-hoc analysis revealed that stiffness values at 10% load were significantly different than those at 30, 40, and 50% loads, and that 20% was significantly different than stiffness levels at 50% MVIC. Trial reliability was excellent for each load condition with ICCs ranging from 0.86-0.96 with SEMs ranging from 19.38-46.78 N/rad. CONCLUSION: The results suggest that a hand-held myotonometer is a reliable tool for measuring muscle stiffness over a range of contraction levels. Stiffness levels increased with the proportional increase in force output, which agrees with previous literature showing an increase in active muscle stiffness values with an increase in contraction level. While we did not find a direct linear relationship between level of contraction and measured stiffness values, the data trend suggests the myotonometer measurements are also valid.

A Novel Theoretical Framework for the Dynamic Stability Analysis, Movement Control, and Trajectory Generation in a Multisegment Biomechanical Model

We consider a simplified characterization of the postural control system that embraces two broad components: one representing the musculoskeletal dynamics in the sagittal plane and the other representing proprioceptive feedback and the central nervous system (CNS). Specifically, a planar four-segment neuromusculoskeletal model consisting of the ankle, knee, and hip degrees-of-freedom (DOFs) is described in this paper. The model includes important physiological constructs such as Hill-type muscle model, active and passive muscle stiffnesses, force feedback from the Golgi tendon organ, muscle length and rate feedback from the muscle spindle, and transmission latencies in the neural pathways. A proportional-integral-derivative (PID) controller for each individual DOF is assumed to represent the CNS analog in the modeling paradigm. Our main hypothesis states that all stabilizing PID controllers for such multisegment biomechanical models can be parametrized and analytically synthesized. Our analytical and simulation results show that the proposed representation adequately shapes a postural control that (a) possesses good disturbance rejection and trajectory tracking, (b) is robust against feedback latencies and torque perturbations, and (c) is flexible to embrace changes in the musculoskeletal parameters. We additionally present detailed sensitivity analysis to show that control under conditions of limited or no proprioceptive feedback results in (a) significant reduction in the stability margins, (b) substantial decrease in the available stabilizing parameter set, and (c) oscillatory movement trajectories. Overall, these results suggest that anatomical arrangement, active muscle stiffness, force feedback, and physiological latencies play a major role in shaping motor control processes in humans.

KINEMATIC TRAJECTORY GENERATION IN A NEUROMUSCULOSKELETAL MODEL WITH SOMATOSENSORY AND VESTIBULAR FEEDBACK

Mechanisms of posture and movement control in the human body are investigated via a four-segment neuro-musculoskeletal model characterizing the action of the central nervous system (CNS). The model includes position, velocity, force, and vestibular feedback, the intrinsic stiffness properties of the muscle, and the physiological latencies of the motor servo-system. Prediction of limb kinematics is achieved through a combination of analytical methods and computer simulations. Our simulation results confirm that the anatomical arrangement, active muscle stiffness, force and vestibular feedback, and physiological latencies of the body segments play a major role in shaping motor control processes in the human body.

Comparison of triceps surae structural stiffness and material modulus across sex

Dynamic joint stability is derived, in part, from active muscle stiffness. Previous research has identified greater structural musculotendinous stiffness in males than in females, suggesting potential sex differences in joint stability. However, structural stiffness is influenced by anthropometrics, and it is currently unclear if sex differences in musculotendinous stiffness are purely functions of anthropometrics or related to additional factors. The purpose of this investigation was to compare structural stiffness and material modulus of the triceps surae between sexes to determine the relative influence of anthropometrics. It was hypothesized that males would demonstrate greater structural stiffness and material modulus. Twenty male and 20 female individuals volunteered for participation. Active triceps surae structural stiffness was estimated from the damped frequency of oscillation of the shank about the ankle. Material modulus was calculated as the ratio of stress to strain, derived from estimates of triceps surae length and physiological cross-sectional area. Structural stiffness was significantly greater in males [mean (SD)] [137.41 (26.99) N/cm] than females [91.06 (20.10) N/cm]. Similarly, material modulus was significantly greater in males [2778.51 (549.95) Pa] than females [1968.58 (439.61) Pa]. Greater structural stiffness in males identified in previous literature appears to be a true phenomenon. Identification of greater material modulus in males suggests that these differences are likely attributable to sex differences in tendon stiffness and muscle architecture. These results indicate that male musculature is more effective at resisting changes in its length, a finding which may have implications for joint stability.

Neuromuscular Control and Active Trunk Stiffness during Isometric Flexion and Extension

Pushing and pulling tasks account for 20% of low-back injury claims. Torso flexion necessary for pushing exertions requires different muscle recruitment than for extension exertions typical of lifting tasks. These differences in recruitment and control may influence spinal stability and associated risk of injury. Active muscle stiffness is considered the primary stabilizing mechanism for spinal stability. Therefore, active trunk stiffness was recorded while subjects generated upright isometric trunk flexion and extension exertions against an isotonic preload. Small pseudo-random force disturbances were superimposed on the preloads causing small amplitude trunk movements. Trunk stiffness was computed from systems identification of the measured force and trunk motion data. Results demonstrated significantly greater stiffness during flexion exertions as compared to extension exertions. EMG data suggest this difference was due to increased co-contraction during the flexion exertions. These behaviours were attributed to the need to augment neuromuscular control of spinal stability during pushing tasks. Keywords: Spine; Co-contraction; Push; Manual Materials Handling; Biomechanics

Effects of a Myosin-II Inhibitor (N-benzyl-p-toluene Sulphonamide, BTS) on Contractile Characteristics of Intact Fast-twitch Mammalian Muscle Fibres

We have examined the effects of N-benzyl-p-toluene sulphonamide (BTS), a potent and specific inhibitor of fast muscle myosin-II, using small bundles of intact fibres or single fibres from rat foot muscle. BTS decreased tetanic tension reversibly in a concentration-dependent manner with half-maximal inhibition at approximately approximately 2 microM at 20 degrees C. The inhibition of tension with 10 microM BTS was marked at the three temperatures examined (10, 20 and 30 degrees C), but greatest at 10 degrees C. BTS decreased active muscle stiffness to a lesser extent than tetanic tension indicating that not all of the tension inhibition was due to a reduced number of attached cross-bridges. BTS-induced inhibition of active tension was not accompanied by any change in the free myoplasmic Ca2+ transients. The potency and specificity of BTS make it a very suitable myosin inhibitor for intact mammalian fast muscle and should be a useful tool for the examination of outstanding questions in muscle contraction.

Isokinetic measurements of calf muscle stiffness following fatiguing exertion

Objective: To determine the influence of fatigue on active and passive muscle stiffness when the muscle/tendon complex has been exposed to high-intensity dynamic exercises of a short duration. Subjects and methods: Fifteen (see: Methods: 2.1 subjects) healthy women aged 23 ± 2 years participated. Active and passive stiffness of the calf muscle was measured on an isokinetic dynamometer using electrical stimulation and passive stretch (200°/s). The stiffness test was conducted before and immediately after high-intensity dynamic exercises of the plantar flexors performed until exhaustion. Results: The forces generated by the same electrical stimulation level were significantly higher before than after the fatigue test. No significant changes could be demonstrated regarding active or passive stiffness before and after the fatigue protocol. However individual changes in active stiffness of about 40% were noted. Conclusion: Even though no significant changes in stiffness were apparent after the fatiguing exertion large individual changes could be noted. This is an indication that stiffness in the calf muscle after high-intensity dynamic exercises may not correspond to the peripheral fatigue.

Trunk stiffness and dynamics during active extension exertions

Spinal stability is related to the recruitment and control of active muscle stiffness. Stochastic system identification techniques were used to calculate the effective stiffness and dynamics of the trunk during active trunk extension exertions. Twenty-one healthy adult subjects (10 males, 11 females) wore a harness with a cable attached to a servomotor such that isotonic flexion preloads of 100, 135, and 170 N were applied at the T10 level of the trunk. A pseudorandom stochastic force sequence (bandwidth 0–10 Hz, amplitude ±30 N) was superimposed on the preload causing small amplitude trunk movements. Nonparametric impulse response functions of trunk dynamics were computed and revealed that the system exhibited underdamped second-order behavior. Second-order trunk dynamics were determined by calculating the best least-squares fit to the IRF. The quality of the model was quantified by comparing estimated and observed displacement variance accounted for (VAF), and quality of the second-order fits was calculated as a percentage and referred to as fit accuracy. Mean VAF and fit accuracy were 87.8 ± 4.0 % and 96.0 ± 4.3 % , respectively, indicating that the model accurately represented active trunk kinematic response. The accuracy of the kinematic representation was not influenced by preload or gender. Mean effective stiffness was 2.78 ± 0.96 N / mm and increased significantly with preload ( p < 0.001 ), but did not vary with gender ( p = 0.425 ). Mean effective damping was 314 ± 72 N s/m and effective trunk mass was 37.0 ± 9.3 kg . We conclude that stochastic system identification techniques should be used to calculate effective trunk stiffness and dynamics.

The relationships between active extensibility, and passive and active stiffness of the knee flexors

Insufficient active knee flexor stiffness may predispose the anterior cruciate ligament to injury. Insufficient passive stiffness may result in insufficient active stiffness. Similarly, higher levels of musculotendinous extensibility may inhibit active and passive muscle stiffness, potentially contributing to an increased risk of injury. The literature is both limited and inconsistent concerning relationships between extensibility, passive stiffness, and active stiffness. Extensibility was measured as the maximal active knee extension angle from a supine position with the hip flexed to 90°. Passive stiffness was calculated as the slope of the moment–angle curve resulting from passive knee extension. Active stiffness was assessed via acceleration associated with damped oscillatory motion about the knee. Stepwise multiple regression indicated that passive stiffness accounted for 25% of active muscle stiffness variance. The linear combination of extensibility and passive stiffness explained only 2% more variance compared to passive stiffness alone. Musculotendinous extensibility was moderately related to passive muscle stiffness, and weakly related to active muscle stiffness. The moderate relationship observed between active and passive stiffness emphasizes the dependence of active muscle stiffness on cross-bridge formation, and the relatively smaller contribution from parallel elastic tissues. Additionally, heightened extensibility does not appear to be a predisposing factor for reduced muscle stiffness.

Influence of fatigue in neuromuscular control of spinal stability.

Lifting-induced fatigue may influence neuromuscular control of spinal stability. Stability is primarily controlled by muscle recruitment, active muscle stiffness, and reflex response. Fatigue has been observed to affect each of these neuromuscular parameters and may therefore affect spinal stability. A biomechanical model of spinal stability was implemented to evaluate the effects of fatigue on spinal stability. The model included a 6-degree-of-freedom representation of the spine controlled by 12 deformable muscles from which muscle recruitment was determined to simultaneously achieve equilibrium and stability. Fatigue-induced reduction in active muscle stiffness necessitated increased antagonistic cocontraction to maintain stability resulting in increased spinal compression with fatigue. Fatigue-induced reduction in force-generating capacity limited the feasible set of muscle recruitment patterns, thereby restricting the estimated stability of the spine. Electromyographic and trunk kinematics from 21 healthy participants were recorded during sudden-load trials in fatigued and unfatigued states. Empirical data supported the model predictions, demonstrating increased antagonistic cocontraction during fatigued exertions. Results suggest that biomechanical factors including spinal load and stability should be considered when performing ergonomic assessments of fatiguing lifting tasks. Potential applications of this research include a biomechanical tool for the design of administrative ergonomic controls in manual materials handling industries.

Gender differences in active musculoskeletal stiffness. Part I.: Quantification in controlled measurements of knee joint dynamics

Active females demonstrate increased risk for musculoskeletal injuries relative to equivalently-trained males. Although gender differences in factors such as passive laxity, skeletal geometry and kinematics have been examined, the effect of gender on active muscle stiffness has not been reported. Stiffness of the active quadriceps and hamstrings musculature were recorded during isometric knee flexion and extension exertions from twelve male and eleven female subjects. A second-order biomechanical model of joint dynamics was used to quantify stiffness from the transient motion response to an angular perturbation of the lower-leg. Female subjects demonstrated reduced active stiffness relative to male subjects at all torque levels, with levels 56–73% of the males. Effective stiffness increased linearly with the torque load, with stiffness increasing at a rate of 3.3 Nm/rad per unit of knee moment in knee flexion exertions (hamstrings) and 6.6 Nm/rad per unit of knee moment extension exertions (quadriceps). To account for gender differences in applied moment associated with leg mass, regressions analyses were completed that demonstrated a gender difference in the slope of stiffness-versus-knee moment relation. Further research is necessary to identify the cause of the observed biomechanical difference and implications for controlling injury.

Gender differences in active musculoskeletal stiffness. Part II. Quantification of leg stiffness during functional hopping tasks

Leg stiffness was compared between age-matched males and females during hopping at preferred and controlled frequencies. Stiffness was defined as the linear regression slope between the vertical center of mass (COM) displacement and ground-reaction forces recorded from a force plate during the stance phase of the hopping task. Results demonstrate that subjects modulated the vertical displacement of the COM during ground contact in relation to the square of hopping frequency. This supports the accuracy of the spring–mass oscillator as a representative model of hopping. It also maintained peak vertical ground-reaction load at approximately three times body weight. Leg stiffness values in males (33.9± 8.7 kN/m) were significantly ( p<0.01) greater than in females (26.3± 6.5 kN/m) at each of three hopping frequencies, 3.0, 2.5 Hz, and a preferred hopping rate. In the spring–mass oscillator model leg stiffness and body mass are related to the frequency of motion. Thus male subjects necessarily recruited greater leg stiffness to drive their heavier body mass at the same frequency as the lighter female subjects during the controlled frequency trials. However, in the preferred hopping condition the stiffness was not constrained by the task because frequency was self-selected. Nonetheless, both male and female subjects hopped at statistically similar preferred frequencies (2.34± 0.22 Hz) , therefore, the females continued to demonstrate less leg stiffness. Recognizing the active muscle stiffness contributes to biomechanical stability as well as leg stiffness, these results may provide insight into the gender bias in risk of musculoskeletal knee injury.

Dynamically determined contractile states of airway smooth muscle.

The contractile state of maximally activated bovine airway smooth muscle is altered during imposed tidal stretches that simulate the action of breathing. When the amplitude of imposed tidal stretch is very small (0.25% of muscle optimal length), the steady-state value of the muscle force, F, approximates the isometric force, the muscle stiffness, E, is large, and the muscle hysteresivity, eta, is small. When the amplitude is increased beyond 1%, however, F and E promptly decrease and eta promptly increases. The provocative stretch amplitude required to cause active force or muscle stiffness to fall by half, or hysteresivity to double, is slightly greater than 2%. By contrast, the stretch amplitude expected during quiet breathing at rest is 4%. Therefore, the isometric force generating capacity of airway smooth muscle may not be applicable to the force generated in normal physiologic circumstances, even during maximal bronchial provocation.

Airway smooth muscle, tidal stretches, and dynamically determined contractile states.

In the classic theory of airway lumen narrowing in asthma, active force in airway smooth muscle is presumed to be in static mechanical equilibrium with the external load against which the muscle has shortened. This theory is useful because it identifies the static equilibrium length toward which activated airway smooth muscle would tend if given enough time. The corresponding state toward which myosin-actin interactions would tend is called the latch state. But are the concepts of a static mechanical equilibrium and the latch state applicable in the setting of tidal loading, as occurs during breathing? To address this question, we have studied isolated, maximally contracted bovine tracheal smooth muscle subjected to tidal stretches imposed at 0.33 Hz. We measured the active force (F) and stiffness (E), which reflect numbers of actin-myosin interactions, and hysteresivity (eta) which reflects the rate of turnover of those interactions. When the amplitude of imposed tidal stretch (epsilon) was very small, 0.25% of muscle optimal length, the steady-state value of F approximated the isometric force, E was large, and eta was small. When epsilon was increased beyond 1%, however, F and E promptly decreased and eta promptly increased. The muscle could be maintained in these steady, dynamically determined contractile states for as long as the tidal stretches were sustained; when epsilon subsequently decreased back to 0.25%, F, E, and eta returned slowly toward their previous values. The provocative stretch amplitude required to cause active force or muscle stiffness to fall by half, or hysteresivity to double, was slightly greater than 2%. These observations are consistent with a direct effect of stretch upon bridge dynamics in which, with increasing tidal stretch amplitude, the number of actin-myosin interactions decreases and their rate of turnover increases. We conclude that the interactions of myosin with actin are at every instant tending toward those that would prevail in the isometric steady state, but tidal changes of muscle length cause an excess in the rate of detachment. These stretch-induced detachment events can come so fast compared with the rate of attachment that static equilibrium conditions are never attained. If so, then airway lumenal narrowing and the underlying contractile state would be governed by a dynamic mechanical process rather than by a mechanical equilibrium of static forces.

Role of muscles in lumbar spine stability in maximum extension efforts.

Many problems of the lumbar spine that cause pain are attributed to instability. The ligamentous spine (without muscles) is unstable at very low compressive loads. This study examined the hypothesis that instability of the lumbar spine is prevented under normal circumstances by the stiffness of spinal musculature, without active responses from the neuromuscular control system. The effect of muscle activity (force and stiffness) on the stability of the lumbar spine was analyzed for maximum voluntary extension efforts with different spinal postures in the sagittal plane. The analysis included realistic three-dimensional representation of the muscular anatomy with muscles crossing several motion segments. The stiffness of motion segments was represented using in vitro measured properties. Under a range of conditions with maximum extension effort, active muscle stiffness was required to prevent the lumbar spine from buckling. The dimensionless value of the muscle stiffness parameter q as a function of activation and length had to be greater than a critical value in the range of 3.7-4.7 in order to stabilize the spine. Experimentally determined values of q ranged from 0.5 to 42. These analyses demonstrate how changes in motion segment stiffness, muscle activation strategy, or muscle stiffness (due to degenerative changes, injuries, fatigue, and so on) might lead to spinal instability and "self-injury."

Characterization of cross-bridge elasticity and kinetics of cross-bridge cycling during force development in single smooth muscle cells.

Force development in smooth muscle, as in skeletal muscle, is believed to reflect recruitment of force-generating myosin cross-bridges. However, little is known about the events underlying cross-bridge recruitment as the muscle cell approaches peak isometric force and then enters a period of tension maintenance. In the present studies on single smooth muscle cells isolated from the toad (Bufo marinus) stomach muscularis, active muscle stiffness, calculated from the force response to small sinusoidal length changes (0.5% cell length, 250 Hz), was utilized to estimate the relative number of attached cross-bridges. By comparing stiffness during initial force development to stiffness during force redevelopment immediately after a quick release imposed at peak force, we propose that the instantaneous active stiffness of the cell reflects both a linearly elastic cross-bridge element having 1.5 times the compliance of the cross-bridge in frog skeletal muscle and a series elastic component having an exponential length-force relationship. At the onset of force development, the ratio of stiffness to force was 2.5 times greater than at peak isometric force. These data suggest that, upon activation, cross-bridges attach in at least two states (i.e., low-force-producing and high-force-producing) and redistribute to a steady state distribution at peak isometric force. The possibility that the cross-bridge cycling rate was modulated with time was also investigated by analyzing the time course of tension recovery to small, rapid step length changes (0.5% cell length in 2.5 ms) imposed during initial force development, at peak force, and after 15 s of tension maintenance. The rate of tension recovery slowed continuously throughout force development following activation and slowed further as force was maintained. Our results suggest that the kinetics of force production in smooth muscle may involve a redistribution of cross-bridge populations between two attached states and that the average cycling rate of these cross-bridges becomes slower with time during contraction.

Nonlinear mechanical behavior in striated muscle and its relationship to underlying crossbridge activity.

Small muscles dissected from the frog (Rana pipien) toe were submersed in a cooled (0 degrees C) Ringers solution and attached at opposite ends to a tension transducer and a length regulating servo device. Muscle stiffness was measured by imposing a length disturbance (bandlimited (0-500 Hz) noise or single-frequency sinusoid) on an initially isometric muscle preparation and recording this disturbance and the concomitant tension response. If active muscle stiffness was to be measured, the muscle was electrically stimulated to produce a fused tetanus prior to imposing the length disturbance. The passive muscle was characterized as highly compliant, almost linear elasticity (elastic modulus 638 kN/m/sup 3/). Estimates of the muscle stiffness as a function of frequency were obtained using spectral density methods. The active muscle appears to 'soften' in response to release and 'harden' in response to stretch. The linear behavior of active muscle was modeled as a linear spring (elastic modulus 102040 kNm/sup 3/) in parallel with a linear dampener (viscous modulus 810 kN*s/m/sup 3/). >