Virtual trajectories of single-joint movements performed under two basic strategies

BackgroundThe development of osteoarthritis (OA) in the hand results in increased joint stiffness, which in turn affects the grip strength. The goal of the present study is to theoretically analyze the muscle forces in a thumb in response to the increased joint stiffness.MethodsThe thumb was modeled as a linkage system consisting of a trapezium, a metacarpal bone, a proximal and a distal phalanx. Nine muscles were included in the model: flexor pollicis longus (FPL), extensor pollicis longus (EPL), extensor pollicis brevis (EPB), abductor pollicis longus (APL), flexor pollicis brevis (FPB), abductor pollicis brevis (APB), the transverse head of the adductor pollicis (ADPt), the oblique head of the adductor pollicis (ADPo), and opponens pollicis (OPP). Numerical tests were performed using an inverse dynamic approach. The joints were prescribed to an angular motion at one degree-of-freedom (DOF) each time with all other DOFs of the joints being mechanically constrained, while the muscle forces in response to the joint motions were predicted. The normal joint stiffness was assumed to be 0.05, 0.10, and 0.15 N m/rad for interphalangeal (IP), metacarpophalangeal (MCP), and carpometacarpal (CMC) joint, respectively. The joint stiffness was assumed to increase by 50% and 100%, simulating the biomechanical consequences of OA.ResultsOur simulations indicated that the increase in joint stiffness induced substantial increases in muscle forces, especially in the EPL and FPL muscles in response to IP, MCP, or CMC extension/flexion motions.ConclusionsBecause the strength of the muscles in the fingers is limited, the muscles will not be able to overcome joint resistance if joint stiffness is increased to its limit due to OA. This may contribute to the reduced range of motion typically seen in OA.

Background Torsional joint stiffness is thought to play a role in the observed sex bias in noncontact anterior cruciate ligament injury rates. Hypothesis Women will exhibit lower torsional stiffness values of the knee in response to varus/valgus and internal/external rotations than will men. Study Design Controlled laboratory study. Methods Knee kinematics of 20 university students (10 men, 27.3 ± 3.4 years, 177.3 ± 6.8 cm, 81.1 ± 7.0 kg; 10 women, 22.9 ± 1.5 years, 169.0 ± 7.1 cm, 66.1 ±11.4 kg) were measured while 0 to 10 Nm of varus and valgus torques were applied with the subject nonweightbearing and while 0 to 5 N.m of internal and external torques were applied with the subject nonweightbearing and weightbearing with the use of a custom joint testing device. Joint stiffness values were calculated at 1-N.m increments. Results When low magnitudes of torque were applied to the knee, women had significantly lower stiffness values than did men-With the exception of applied external torque with the joint weightbearing and varus torque with the joint nonweightbearing, women demonstrated an increase in joint stiffness as the magnitude of torque increased from lower to higher magnitudes. In contrast, for the men, joint stiffness values remained unchanged as the magnitude of applied torque increased. Conclusion Women exhibited lower knee stiffness in response to low magnitudes of applied torque compared to men and demonstrated an increase of joint stiffness as the magnitude of applied torque increased. Clinical Relevance The decreased stiffness behavior of the knee in response to low torques that was observed for women may have a role in detrimentally affecting knee biomechanics and resulting neuromuscular function, particularly when an individual transitions from nonweightbearing to weightbearing.

Use of an objective measure of articular stiffness to record changes in finger joints after intra-articular injection of corticosteroid

Management of intraarticular fractures is often complicated by joint stiffness long after bony healing has occurred. The following experiments provide biomechanical evidence for the effectiveness of CPM in maintaining joint function in stabilized articular injuries. The ankles of ten adolescent New Zealand white rabbits were penetrated by Steinmann pins to create an intraarticular injury. One limb of each animal was immobilized, and the contralateral limb was placed in a continuous passive motion (CPM) machine for three weeks. Joint stiffness of each ankle was quantified with an arthrograph before injury and after three weeks of treatment. Hindlimb volumes were recorded before injury and monitored at weekly intervals using a water-displacement method. Joint stiffness increased 2.6 times the preinjury levels in limbs that were immobilized for three weeks. There was no statistically significant increase in joint stiffness in ankles treated with CPM compared to preinjury values. The posttraumatic difference between the CPM-treated and immobilized limbs was highly significant; limb swelling was not affected by CPM.

Alterations of stiffness and resting position of the elbow joint following flexors resistance training

Equifinality, during arm reaching movements, relates to the capacity of the neuromuscular system to attain the same final position in the presence or absence of transient perturbations. There have been several controversies regarding equifinality in the literature. A brief elastic perturbation, applied during a fast arm movement or just before its initiation, typically does not affect final arm position. On the other hand, several experiments have shown that velocity-dependent perturbations, such as Coriolis force or negative damping, while transient in nature, have a significant effect on final arm position when compared to unperturbed movements. In this study, an unloading paradigm was used to study the role of reflexes with respect to equifinality. The effects on final arm position of suddenly decreasing a static load maintained by fourteen subjects were analyzed. Subjects maintained an initial load produced by a double-joint manipulandum moving in the horizontal plane. The load was suddenly decreased, either in one or in two successive steps with different time intervals, resulting in a rapid reflex-mediated change in arm position. Unloading led to short-latency changes in the activity of shoulder and elbow muscles and significant variations in tonic activity. It was found that the final hand position was shorter for double- versus single-step unloading if the time between two successive changes in load was greater than 100 ms. With a shorter time interval, the final hand positions were the same. This difference in final hand positions was inversely proportional to the hand velocity at the time of the second change in load. Further, agonist/antagonist co-activation increased in double-step unloading. Thus, the change in both the load and the movement velocity may influence the magnitude of the unloading reflex. This may be indicative of a dependence of stretch reflexes on velocity. Perturbation may cause a reflex-mediated increase in joint stiffness, which could explain why equifinality is not preserved after some perturbations, such as velocity-dependant external forces.

Because of its simplicity, static optimization (SO) is frequently used to resolve the muscle redundancy problem (i.e., more muscles than degrees-of-freedom (DOF) in the human musculoskeletal system). However, SO minimizes antagonistic co-activation and likely joint stiffness as well, which may not be physiologically realistic since the body modulates joint stiffness during movements such as walking. Knowledge of joint stiffness is limited due to the difficulty of measuring it experimentally, leading researchers to estimate it using computational models. This study explores how imposing a synergy structure on the muscle activations estimated by optimization (termed "synergy optimization," or SynO) affects calculated lower body joint stiffnesses during walking. By limiting the achievable muscle activations and coupling all time frames together, a synergy structure provides a potential mechanism for reducing indeterminacy and improving physiological co-activation but at the cost of a larger optimization problem. To compare joint stiffnesses produced by SynO (2-6 synergies) and SO, we used both approaches to estimate lower body muscle activations and forces for sample experimental overground walking data obtained from the first knee grand challenge competition. Both optimizations used a custom Hill-type muscle model that permitted analytic calculation of individual muscle contributions to the stiffness of spanned joints. Both approaches reproduced inverse dynamic joint moments well over the entire gait cycle, though SynO with only two synergies exhibited the largest errors. Maximum and mean joint stiffnesses for hip and knee flexion in particular decreased as the number of synergies increased from 2 to 6, with SO producing the lowest joint stiffness values. Our results suggest that SynO increases joint stiffness by increasing muscle co-activation, and furthermore, that walking with a reduced number of synergies may result in increased joint stiffness and perhaps stability.

Load to Failure of the Ankle Joint Complex After Fusion of the Subtalar and Talonavicular Joints: A Cadaveric Study

Spasticity is often seen in patients with central nervous system lesion, such as stroke. It hinders functional movement and may induce pain. Current measures for assessing spasticity are either quantitative but not convenient to use or convenient to use in clinics but lack of objective quantification. We developed a manual spasticity evaluator (MSE) to evaluate the spasticity quantitatively and potentially suitable for a clinical setting. Joint position and torque from 10 subjects with right hemiplegia and 9 healthy subjects were measured conveniently and used to evaluate spasticity and determine the catch angle. EMG signal was obtained from the biceps brachii and triceps brachii to corroborate the mechanical measurement of the MSE. Results showed that the MSE provided a convenient and quantitative measurement of spasticity, including presence of catch angle, increase in joint stiffness, and decrease in joint range of motion in the stroke patients, as compared with healthy subjects. EMG signals corroborated MSE assessment of the catch angle.

ABSTRACTA striking cultural feature of most of Western society is the expectation that the elderly will become progressively more sedentary, increasingly indolent and gradually more febrile until they are eventually unable to cope for themselves. There is the perception that old people do not need to exercise to any great extent, that exercise may be harmful to the elderly and that older people in exercise are most unattractive. Advertising emphasises that exercise is for the young and encourages a belief that the elderly should take life easily as reward for a lifetime's work.Research into musculo‐skeletal structure and performance has clearly shown the necessity for high levels of exercise activity through the whole life span. Inactivity is associated with a decline in muscle bulk, strength and endurance; an increase in joint stiffness and a decline in the individual's ability to respond to rapid environment change.All of the above changes are markedly improved by exercise, even in the very elderly. This paper discusses current research into the effect of ageing and exercise on the musculo‐skeletal system of the body, and demonstrates the necessity for continued high levels of physical activity into old age.

The use of reinforcements in adhesive joints makes the stress distribution more uniform, improving their mechanical performance and adhesion. The present paper aims to verify the effectiveness and efficiency of the insertion of nylon 6 fabric in the adhesive layer, to study their applicability and functionality in building components. The increase in stiffness achieved by applying nylon 6 fabric in the adhesive layer between glass and GFRP pultruded profiles and steel laminates applied to GFRP beams is investigated. Three different epoxy adhesives and one epoxy resin are used and compared. Three different types of tests are carried out in order to study the different properties of the reinforcement system: tensile tests on GFRP/GFRP single-lap adhesive joints, with and without nylon fabric reinforcement; tensile tests on double-lap adhesive joints between float glass and pultruded GFRP profiles reinforced with nylon fabric according to four configurations (in the middle plane of the adhesive layer, on the glass surfaces, on the GFRP surfaces, on both GFRP and glass configurations) to verify the influence of its position; three-point bending tests on long GFRP tubular profiles reinforced with steel plates and nylon fabric in different configurations, to study resistance to bending loads. The results from the experimental campaign show the effectiveness of the reinforcement system using nylon fabric 6. In general, both a reduction in ultimate strength and an increase in joint stiffness compared to unreinforced configurations are observed.

The movement control of articulated limbs in vertebrates has been explained in terms of equilibrium points and moving equilibrium points or virtual trajectories. These hypotheses state that the nervous system makes the control of multi-segment limbs easier by simply planning in terms of these equilibrium points and trajectories. The present paper elaborates on a planar computer simulation of an articulated three-segment limb, controlled by pairs of muscles. The nature of the virtual trajectory is analysed when the limb is required to make fast movements with endpoint movements along a straight line with bell-shaped velocity profiles. It appears that the faster the movement, the more the virtual trajectory deviates from the real trajectory, becoming up to eight times longer. The complexity of the shape of the virtual trajectories in these fast movements makes it unlikely that the nervous system plans to use these trajectories. It seems simpler to set up the required bursts of muscle activation, coupled in the nervous system to the direction of movement, the speed and the place in workspace. Finally, it is argued that the two types of explanation do not contradict each other: when a relation is established in the nervous system between muscle activation and movements, equilibrium points and virtual trajectories are implicitly part of that relation.

Introduction: During military activities, soldiers are often required to run at a fixed cadence with body borne load, but these loads purportedly increase leg stiffness, leading to increased risk of musculoskeletal injury. Yet, to date, it is unknown how altering stride length when running with body borne load affects lower limb stiffness for males and females. Purpose: To quantify leg stiffness, and lower limb joint (hip, knee and ankle) stiffness for males and females using different stride lengths to run with body borne loads of 20 kg, 25 kg, 30 kg, and 35 kg. Methods: Twenty-seven (17 males and 10 females) participants (age: 21.2 ± 2.3 years, height: 1.7 ± 0.1 m, and weight: 75.5 ± 11.3 kg) had leg and joint stiffness quantified while running at 4 m/s with four load conditions (20, 25, 30, and 35 kg). With each load condition, participants performed three run trials using either: their preferred stride length (PSL) and strides that are 15% longer (LSL) and shorter (SSL) than their PSL. Statistical Analysis: Leg and hip, knee, and ankle stiffness were submitted to a RM ANOVA to test the main effect and interaction of load (20, 25, 30, and 35 kg), stride length (PSL, PSL+15%, and PSL-15%), and sex (male vs female). Results: Body borne load increased leg stiffness (P=0.006). Male participants decreased leg stiffness as stride lengthened from SSL to PSL and PSL to LSL (P=0.026; P0.05). Body borne load increased peak vGRF (PP=0.010; P=0.011), while females only increased peak vGRF between PSL and LSL (PPP=0.013) stiffness increased with the addition of body borne load, but load had no significant effect on hip stiffness (P=0.723). Increasing stride length significant decreased ankle stiffness (P=0.003), but had no effect on hip (P=0.661) or knee (P=0.170) stiffness. Sex had no significant effect on hip (P=0.880), knee (P=0.234), or ankle (P=0.081) stiffness. Conclusion: Running with body borne load increased leg stiffness and potential risk of musculoskeletal injury. But, only male participants decreased leg stiffness and injury risk with longer strides. Both the knee and ankle increased joint stiffness, and risk of musculoskeletal injury with the addition of body borne load. The ankle, however, decreased joint stiffness with longer strides.

Due to the extensive area, lightweight construction, and hinged multi-panel connection, spacecraft solar panels exhibit low stiffness and weak damping properties, resulting in low-frequency vibration and nonlinear dynamic behavior. This paper proposes an adjustable-stiffness magnetic (ASM) joint as both structural and functional component for the drive and control of solar panels. First, joint structure design and adjustable-stiffness principle are introduced. Subsequently, a rigid, flexible coupling dynamic model of solar panel structure with ASM joints is established, considering the weight and rotational inertia of the joints. The eigen equation of the system is derived to obtain the natural frequencies and corresponding global modes of the system through Rayleigh-Ritz method. The inherent characteristics of the rigid flexible coupling system are explored. The extracted global modes effectively capture the characteristics of both rigid-body motion modes and elastic vibration modes, providing an accurate representation of its dynamic behaviour. Meanwhile, all the first four modes exhibit an increase in frequency with the increase of joint stiffness. Finally, a ground-scale experimental platform of panels with ASM joints is constructed. A significant frequency-shift phenomenon with variable stiffness is observed under impact excitation. Additionally, the accuracy of GMM is validated by experiments and finite element simulation. These findings will pave the way for further exploring vibration suppression methods for multiple spacecraft solar panels under ultra-low frequency excitation.

The basis of a simple synergy: reconstruction of joint equilibrium trajectories during unrestrained arm movements