共查询到20条相似文献,搜索用时 78 毫秒
1.
P. C. Teerhuis M. Rodenburg A. J. J. Maas 《Medical & biological engineering & computing》1979,17(2):249-256
Two main sources of disturbances can degrade the motion performance of the room (and so limit the possibilities of simulation),
namely, the bearing, and the driving unit with its transmission system. Mostly, the bearings are affected with more or less
Coulomb friction. This dry friction, which is far from constant, introduces acceleration disturbances that are considerably
higher than the perception level of man, especially at very low velocities where Coulomb friction is most pronounced (stick
slip). In order to control accelerations below the perception level, Coulomb friction must be minimised. For this reason hydrostatic
(externally pressurised) bearings are employed, with a consequent absence of Coulomb friction. The bearing consists of an
axial and a radial part. The axial forces consist of the weight of the room and its contents. The radial force is mainly determined
by the stretching force of the drive belt. The other source of disturbance may arise in the driving motor and its transmission.
In order to avoid discontinuities in torque transmission a flat belt is used in combination with a high power (17 kW) electrohydraulic
servomotor with main properties of easily controllable speed (input power 200 mW) from +700 r/min to −700 r/min (and thus
changing of the direction of rotation of the room) with almost constant torque, and superior dynamic behaviour because of
the small rotating mass (90° phase lag at 110 Hz). The dynamic behaviour of the room is that of a lightly damped 2nd-order
system with characteristic frequency of 1·6 Hz determined by the stiffness of the belt and the rotating mass of the room (J=2150
kgm2). The damping originates from the viscous friction of the oil film in the bearing and is very low (between 0·1–0·15). The
damping factor has been increased by applying acceleration feedback so that the transfer characteristic is that of a nearly
critical damped 2nd-order system. The frequency range of the room is very large 0–3 Hz. The maximum adjustable acceleration
and velocity amount 5°/s2 and +120°/s to −120°/s, respectively. The acceleration disturbances in the frequency range of 0–1 Hz are <0·05°/s2 at a velocity of 50°/s. 相似文献
2.
Potential roles of force cues in human stance control 总被引:1,自引:1,他引:0
Christian Cnyrim Thomas Mergner Christoph Maurer 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》2009,194(3):419-433
Human stance is inherently unstable. A small deviation from upright body orientation is enough to yield a gravitational component
in the ankle joint torque, which tends to accelerate the body further away from upright (‘gravitational torque’; magnitude
is related to body-space lean angle). Therefore, to maintain a given body lean position, a corresponding compensatory torque
must be generated. It is well known that subjects use kinematic sensory information on body-space lean from the vestibular
system for this purpose. Less is known about kinetic cues from force/torque receptors. Previous work indicated that they are
involved in compensating external contact forces such as a pull or push having impact on the body. In this study, we hypothesized
that they play, in addition, a role when the vestibular estimate of the gravitational torque becomes erroneous. Reasons may
be sudden changes in body mass, for instance by a load, or an impairment of the vestibular system. To test this hypothesis,
we mimicked load effects on the gravitational torque in normal subjects and in patients with chronic bilateral vestibular
loss (VL) with eyes closed. We added/subtracted extra torque to the gravitational torque by applying an external contact force
(via cable winches and a body harness). The extra torque was referenced to body-space lean, using different proportionality
factors. We investigated how it affected body-space lean responses that we evoked using sinusoidal tilts of the support surface
(motion platform) with different amplitudes and frequencies (normals ±1°, ±2°, and ±4° at 0.05, 0.1, 0.2, and 0.4 Hz; patients
±1° and ±2° at 0.05 and 0.1 Hz). We found that added/subtracted extra torque scales the lean response in a systematic way,
leading to increase/decrease in lean excursion. Expressing the responses in terms of gain and phase curves, we compared the
experimental findings to predictions obtained from a recently published sensory feedback model. For the trials in which the
extra torque tended to endanger stance control, predictions in normals were better when the model included force cues than
without these cues. This supports our notion that force cues provide an automatic ‘gravitational load compensation’ upon changes
in body mass in normals. The findings in the patients support our notion that the presumed force cue mechanism provides furthermore
vestibular loss compensation. Patients showed a body-space stabilization that cannot be explained by ankle angle proprioception,
but must involve graviception, most likely by force cues. Our findings suggest that force cues contribute considerably to
the redundancy and robustness of the human stance control system. 相似文献
3.
In experiments on isolated animal muscle, the force produced during active lengthening contractions can be up to twice the isometric force, whereas in human experiments lengthening force shows only modest, if any, increase in force. The presence of synergist and antagonist muscle activation associated with human experiments in situ may partly account for the difference between animal and human studies. Therefore, this study aimed to quantify the force–velocity relationship of the human soleus muscle and assess the likelihood that co-activation of antagonist muscles was responsible for the inhibition of torque during submaximal voluntary plantar flexor efforts. Seven subjects performed submaximal voluntary lengthening, shortening(at angular, velocities of +5, –5, +15, –15 and +30, and –30° s–1) and isometric plantar flexor efforts against an ankle torque motor. Angle-specific (90°) measures of plantar flexor torque plus surface and intramuscular electromyography from soleus, medial gastrocnemius and tibialis anterior were made. The level of activation (30% of maximal voluntary isometric effort) was maintained by providing direct visual feedback of the soleus electromyogram to the subject. In an attempt to isolate the contribution of soleus to the resultant plantar flexion torque, activation of the synergist and antagonist muscles were minimised by: (1) flexing the knee of the test limb, thereby minimising the activation of gastrocnemius, and (2) applying an anaesthetic block to the common peroneal nerve to eliminate activation of the primary antagonist muscle, tibialis anterior and the synergist muscles, peroneus longus and peroneus brevis. Plantar flexion torque decreased significantly (P<0.05) after blocking the common peroneal nerve which was likely due to abolishing activation of the peroneal muscles which are synergists for plantar flexion. When normalised to the corresponding isometric value, the force–velocity relationship between pre- and post-block conditions was not different. In both conditions, plantar flexion torques during shortening actions were significantly less than the isometric torque and decreased at faster velocities. During lengthening actions, however, plantar flexion torques were not significantly different from isometric regardless of angular velocity. It was concluded that the apparent inhibition of lengthening torques during voluntary activation is not due to co-activation of antagonist muscles. Results are presented as mean (SEM). 相似文献
4.
在康复机器人辅助脑卒中患者进行康复训练时,为激发患者的主动参与意识,康复机器人应按照患者康复需求提供其所需的辅助力矩。本文针对腕功能康复机器人提出一种按需辅助控制策略:首先制定能力评估规则,并依据该规则评估患者能力;然后设计控制器,控制器可基于评估结果求解出患者完成康复训练任务所需的辅助力矩,并下发指令至电机;最后控制电机输出指令值,辅助患者完成康复训练任务。将该控制策略应用于腕功能康复机器人,不仅实现了按需辅助的训练模式,而且能够避免辅助力矩激增,同时康复治疗师可在线调节能力评估规则中的多个参数,为不同康复状态的患者定制任务难度。本文所提方法不依赖于力学传感器信息,降低了开发成本且易于实现,具有一定的工程应用价值。 相似文献
5.
Peter N. Ayittey John S. Walker Jeremy J. Rice Pieter P. de Tombe 《Pflügers Archiv : European journal of physiology》2009,457(6):1415-1422
Changes in developed force (0.1–3.0 μN) observed during contraction of single myofibrils in response to rapidly changing calcium
concentrations can be measured using glass microneedles. These microneedles are calibrated for stiffness and deflect on response
to developed myofibril force. The precision and accuracy of kinetic measurements are highly dependent on the structural and
mechanical characteristics of the microneedles, which are generally assumed to have a linear force–deflection relationship.
We present a finite-element analysis (FEA) model used to simulate the effects of measurable geometry on stiffness as a function
of applied force and validate our model with actual measured needle properties. In addition, we developed a simple heuristic
constitutive equation that best describes the stiffness of our range of microneedles used and define limits of geometry parameters
within which our predictions hold true. Our model also maps a relation between the geometry parameters and natural frequencies
in air, enabling optimum parametric combinations for microneedle fabrication that would reflect more reliable force measurement
in fluids and physiological environments. We propose a use for this model to aid in the design of microneedles to improve
calibration time, reproducibility, and precision for measuring myofibrillar, cellular, and supramolecular kinetic forces. 相似文献
6.
Kawakami Y Kubo K Kanehisa H Fukunaga T 《European journal of applied physiology》2002,87(4-5):381-387
The influence of muscle series elasticity on the relationship between torque and joint angle during dynamic contractions
was studied. The torque–angle relationship during the maximal isokinetic knee extension was determined for six male subjects
(25–45 years) at 0.52, 1.05, 1.57, 2.09, 2.62, 3.49 rad/s. The knee joint angle at which peak torque was observed showed a
systematic shift to more extended positions, i.e., the quadriceps muscle–tendon unit length became shorter as the velocity
increased [from 1.01 (0.12) rad (0.52 rad/s) to 0.75 (0.14) rad (3.49 rad/s), mean (SD)]. The corresponding difference in
muscle–tendon unit length between 0.52 and 3.49 rad/s, estimated from the angle shift at peak torque and the moment arm length
change of the quadriceps muscles, was 9 (4) mm. The relationship between estimated changes in muscle–tendon unit length and
muscle force of the vastus lateralis and intermedius (VLI) over the seven velocities (including isometric contraction, 0 rad/s)
coincided with the load-elongation properties of the series elastic component of VLI, determined separately in vivo by ultrasonography
when the same subjects performed a ramp isometric knee extension. The results suggest that the torque–angle relationship is
affected by the interaction between contractile and elastic components, and that peak torque angle shift is attributable to
the elongation of tendinous tissues as a function of force applied to them.
Electronic Publication 相似文献
7.
J. Konczak Maike Borutta Johannes Dichgans 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》1997,113(3):465-474
Nine young infants were followed longitudinally from 4 to 15 months of age. They performed multijoint reaching movements
to a stationary target presented at shoulder height. Time-position data of the hand, shoulder, and elbow were collected using
an optoelectronic measurement system. In addition, we recorded electromyographic activity (EMG) from arm extensors and flexors.
This paper documents how control problems of proximal torque generation may account for the segmented hand paths seen during
early reaching. Our analysis revealed the following results: first, muscular impulse (integral of torque) increased significantly
between the ages of 20 (reaching onset) and 64 weeks. That is, as infants got older they produced higher levels of mean muscular
flexor torque during reaching. Data were normalized by body weight and movement time, so differences are not explained by
anthropometric changes or systematic variations in movement time. Second, while adults produced solely flexor muscle torque
to accomplish the task, infants generated flexor and extensor muscle torque at shoulder and elbow throughout a reach. At reaching
onset more than half of the trials revealed this latter kinetic profile. Its frequency declined systematically as infants
got older. Third, we examined the pattern of muscle coordination in those trials that exhibited elbow extensor muscle torque.
We found that during elbow extension coactivation of flexor and extensor muscles was the predominant pattern in 67% of the trials. This pattern was notably absent in comparable
adult reaching movements. Fourth, fluctuations in force generation, as measured by the rate of change of total torque (NET)
and muscular torque (MUS), were more frequent in early reaching (20–28 weeks) than in the older cohort (52–64 weeks), indicating
that muscular torque production became increasingly smoother and task-efficient. Our data demonstrate that young infants have
problems in generating smooth profiles of proximal joint torques. One possible reason for this imprecision in infant force
control is their inexperience in predicting the magnitude and direction of external forces. That infants learned to consider
external forces is documented by their increasing reliance on these forces when performing voluntary elbow extensions. The
patterns of muscle coordination underlying active elbow extensions were basically the same as during the prereaching phase,
indicating that the formation of functional synergies is based on a basal repertoire of innervation patterns already observable
in very early, spontaneous movements.
Received: 5 January 1996 / Accepted: 19 August 1996 相似文献
8.
Abe M Yamada N 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》2001,139(1):120-125
Voluntary arm movements frequently perturb body equilibrium in an upright posture. The motions of leg joints need to be coordinated
according to the properties of voluntary arm movements in order to maintain body equilibrium, and this may cause a change
in postural pattern. The purpose of this study was to determine whether the kinematic pattern generation of upright posture
is influenced by a change in the swinging frequency of arm movements and whether the pattern generation is correlated with
a change in joint torque about the shoulder joint. Four male subjects in an upright posture were instructed to swing their
arms at seven different frequencies, determined by the maximum swinging frequency of each subject (35%max, 40~60%max, 65%max).
Segment rotations around the shoulder, hip, and ankle joints were analyzed at kinematic and kinetic levels. The results of
kinematic analysis indicated that tight coupling between motions of the shoulder and hip joints was generated in lower-frequency
trials (under 40–45%max), whereas tight coupling between motions of the shoulder and ankle joints was generated in higher
frequency trials (more than 40–45%max). Furthermore, the results of kinetic analysis revealed that changes in the joint torque
patterns about the shoulder and hip joints occurred in trials at 40–45%max. The mean value of 40–45%max was close to the eigenfrequency
of each subject's arm. We concluded that (1) postural patterns associated with a gradual change in the swinging frequency
of the arms can be divided into two coordination modes (a hip-shoulder in-phase mode and an ankle-shoulder in-phase mode),
and (2) these two patterns may be divided by the eigenfrequency of the arm.
Electronic Publication 相似文献
9.
J. N. van der Meer A. C. Schouten L. J. Bour E. de Vlugt A. F. van Rootselaar F. C. T. van der Helm M. A. J. Tijssen 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》2010,202(3):681-691
In dystonia, both sensory malfunctioning and an abnormal intermuscular low-frequency drive of 3–7 Hz have been found, although
cause and effect are unknown. It is hypothesized that sensory processing is primarily disturbed and induces this drive. Accordingly,
experimenter-controlled sensory input should be able to influence the frequency of the drive. In six genetically confirmed
myoclonus-dystonia (MD) patients and six matched controls, the low-frequency drive was studied with intermuscular coherence
analysis. External perturbations were applied mechanically to the wrist joint in small frequency bands (0–4, 4–8 and 8–12 Hz;
‘angle protocol) and at single frequencies (1, 5, 7 and 9 Hz; ‘torque’ protocol). The low-frequency drive was found in the
neck muscles of 4 MD patients. In these patients, its frequency did not shift due to the perturbation. In the torque protocol,
the externally applied frequencies could be detected in all controls and in the two patients without the common drive. The
common low-frequency drive was not be affected by external perturbations in MD patients. Furthermore, the torque protocol
did not induce intermuscular coherences at the applied frequencies in these patients, as was the case in healthy controls
and in patients without the drive. This suggests that the dystonic 3–7 Hz drive is caused by a sensory-independent motor drive
and sensory malfunctioning in MD might rather be a consequence than a cause of dystonia. 相似文献
10.
Summary Muscle force recovery from short term intense exercise was examined in 16 physically active men. They performed 50 consecutive
maximal voluntary knee extensions. Following a 40-s rest period five additional maximal contractions were executed. The decrease
in torque during the 50 contractions and the peak torque during the five contractions relative to initial torque were used
as indices for fatigue and recovery, respectively. Venous blood samples were collected repeatedly up to 8 min post exercise
for subsequent lactate analyses. Muscle biopsies were obtained from m. vastus lateralis and analysed for fiber type composition,
fiber area, and capillary density.
Peak torque decreased 67 (range 47–82%) as a result of the repeated contractions. Following recovery, peak torque averaged
70 (47–86%) of the initial value.
Lactate concentration after the 50 contractions was 2.9±1.3 mmol·l−1 and the peak post exercise value averaged 8.7±2.1 mmol·l−1. Fatigue and recovery respectively were correlated with capillary density (r=−0.71 and 0.69) but not with fiber type distribution. A relationship was demonstrated between capillary density and post
exercise/peak post exercise blood lactate concentration (r=0.64). Based on the present findings it is suggested that lactate elimination from the exercising muscle is partly dependent
upon the capillary supply and subsequently influences the rate of muscle force recovery.
Dr. Tesch was on leave from Department of Clinical Physiology, Karolinska Hospital, Stockholm, Sweden 相似文献
11.
Reflex responses to unexpected stretches are well documented for selected muscles in both animal and human. Moreover, investigations
of their possible functional significance have revealed that stretch reflexes can contribute substantially to the overall
stiffness of a joint. In the lower extremity only the muscles spanning the human ankle joint have been investigated in the
past. This study implemented a unique hydraulic actuator to study the contributions of the knee extensor stretch reflex to
the overall knee joint torque. The quadriceps muscles were stretched at various background torques, produced either voluntarily
or by electrical stimulation, and thus the purely reflex mediated torque could be calculated. The stretch had a velocity of
67°/s and an amplitude of 20°. A reflex response as measured by electromyography (EMG) was observed in all knee extensors
at latencies of 26 – 36 ms. Both phasic and tonic EMG stretch responses increased with increasing background torques. Lines
of best fit produced correlation coefficients of 0.59 – 0.78. This study is the first to examine the reflex contribution of
the knee extensors to the total torque at background torques of 0 – 90% MVC. The contribution of the reflex mediated torque
is initially low and peaked at background torques of 20 – 40% MVC. In terms of the total torque the reflex contributed 16
– 52% across all levels of background torque. It is concluded that during medium background torque levels such as those obtained
during walking, the stretch reflex of the quadriceps muscle group contributes substantially to the total torque around the
knee joint. 相似文献
12.
For comparative purposes, normalisation of strength measures to body size using allometric scaling is recommended. A wide
range of scaling exponents have been suggested, typically utilising body mass, although a comprehensive evaluation of different
body size variables has not been documented. Differences between force (F) and torque (T) measurements of strength, and the velocity of measurement might also explain some of the variability in the scaling exponents
proposed. Knee extensor strength of 86 young men was assessed with measurement of torque at four velocities (0–4.19 rad s−1) and force measured isometrically. Body size variables included body mass, height and fat-free mass. Scaling exponents for
torque were consistently higher than for force, but the velocity of torque measurement had no influence. As the confounding
effects of fat mass were restricted, scaling exponents and the strength of the power-function relationships progressively
increased. Fat-free mass determined a surprisingly high proportion of the variance in measured strength (F, 31%; T, 52–58%). Absolute force and torque measurements, and even torque normalised for body mass, were significantly influenced
by height, although strength measures normalised to fat-free mass were not. To normalise strength measurements to body mass,
for relatively homogenous lean populations (body fat <20%), exponents of 0.66 (F) and 1.0 (T) are appropriate. For more adipose populations (body fat >20%) lower body mass exponents appear more suitable (F, 0.45; T, 0.68). Nevertheless, fat-free mass is the recommended index for scaling strength to body size, and higher exponents (F, 0.76; T, 1.12) are advocated in this case. 相似文献
13.
Carpenter MG Thorstensson A Cresswell AG 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》2005,167(3):433-445
Understanding the physiological and psychological factors that contribute to healthy and pathological balance control in man
has been made difficult by the confounding effects of the perturbations used to test balance reactions. The present study
examined how postural responses were influenced by the acceleration–deceleration interval of an unexpected horizontal translation.
Twelve adult males maintained balance during unexpected forward and backward surface translations with two different acceleration–deceleration
intervals and presentation orders (serial or random). “SHORT” perturbations consisted of an initial acceleration (peak acceleration
1.3 m s−2; duration 300 ms) followed 100 ms later by a deceleration. “LONG” perturbations had the same acceleration as SHORT perturbations,
followed by a 2-s interval of constant velocity before deceleration. Surface and intra-muscular electromyography (EMG) from
the leg, trunk, and shoulder muscles were recorded along with motion and force plate data. LONG perturbations induced larger
trunk displacements compared to SHORT perturbations when presented randomly and larger EMG responses in proximal and distal
muscles during later (500–800 ms) response intervals. During SHORT perturbations, activity in some antagonist muscles was
found to be associated with deceleration and not the initial acceleration of the support surface. When predictable, SHORT
perturbations facilitated the use of anticipatory mechanisms to attenuate early (100–400 ms) EMG response amplitudes, ankle
torque change and trunk displacement. In contrast, LONG perturbations, without an early deceleration effect, did not facilitate
anticipatory changes when presented in a predictable order. Therefore, perturbations with a short acceleration–deceleration
interval can influence triggered postural responses through reactive effects and, when predictable with repeated exposure,
through anticipatory mechanisms. 相似文献
14.
Gür H Gransberg L vanDyke D Knutsson E Larsson L 《European journal of applied physiology》2003,88(6):487-496
In an attempt to explore the relationship between force production during voluntary contractions at different speeds of isokinetic
movement and the myofibrillar protein isoform expression in humans, an improved isokinetic dynamometer that corrects for gravitation,
controls for acceleration and deceleration, and identifies a maximum voluntary activation was used. Muscle torque recordings
were compared at the same muscle length (knee angle) and the torque was calculated as the average torque at each angle over
a large knee angle range (75°–25°) to reduce the influence of small torque oscillation on the calculated torque. Muscle torque
at fast (240° s−1) versus slow (30° s−1) speeds of movement, torque normalized to muscle cross-sectional area (specific tension), and absolute torque at fast speeds
of movement were measured in 34 young healthy male and female short-, middle-, and long-distance runners. The relationship
between the different measures of muscle function and the expression of myosin heavy chain (MyHC) isoforms using enzyme–histochemical
and electrophoretic protein separation techniques were investigated. A significant correlation between the 240° s−1 vs 30° s−1 torque ratio and the relative area of the type II fibers and type II MyHC isoforms were observed in both the men (r=0.74;P<0.001) and the women (r=0.81; P<0.05). Thus, the present results confirm a significant relationship between in vivo human muscle function and the MyHC isoform
expression in the contracting muscle.
Electronic Publication 相似文献
15.
The purpose of this study was to examine in vivo whether passive stiffness in human muscles was related to the elasticity
of tendon structures and to performance during stretch-shortening cycle exercise. Passive torque of plantar flexor muscles
was measured during passive stretch from 90° (anatomical position) to 65° of dorsiflexion at a constant velocity of 5°·s–1. The slope of the linear portion of the passive torque-angle curve during stretching was defined as the passive stiffness
of the muscle. The elongation of the tendon and aponeurosis of the medial gastrocnemius muscle (MG) was directly measured
using ultrasonography during ramp isometric plantar flexion up to the voluntary maximum. The relationship between the estimated
muscle force of MG and tendon elongation was fitted to a linear regression, the slope of which was defined as the stiffness
of the tendon. In addition, the dynamic torques during maximal voluntary concentric plantar flexion with and without prior
eccentric contraction were determined at a constant velocity of 120°·s–1. There were no significant correlations between passive stiffness and either the tendon stiffness (r=0.19, P>0.05) or the relative increase in torque with prior eccentric contraction (r=–0.19, P>0.05). However, tendon stiffness was negatively correlated to the relative increase in torque output (r=–0.42, P<0.05). The present results suggested that passive stiffness was independent of the elasticity of tendon structures, and had
no favourable effect on the muscle performance during stretch-shortening cycle exercise.
Electronic Publication 相似文献
16.
Gajdosik RL 《European journal of applied physiology》2002,87(3):220-227
The purpose of this study was to use a model of aging to examine the relationships between passive properties of the calf
muscles and plantarflexion concentric isokinetic torque characteristics. Eighty-one active women 20–84 years of age were tested
using a Kin-Com isokinetic dynamometer interfaced with electromyography (EMG). The passive properties were tested by stretching
the muscles from relaxed plantarflexion to a maximal dorsiflexion (DF) angle at a rate of 5°·s–1 (0.087 rad·s–1) with minimal raw EMG activity (<0.05 mV). The maximal concentric torque was tested from maximal passive DF into plantarflexion
at four randomly ordered velocities of 30, 60, 120, and 180°·s–1. Pearson correlation coefficients (Bonferroni adjusted) indicated a hierarchical order of high to moderate positive correlations
between four passive properties and the peak and mean concentric torque for all test velocities. Correlation coefficients
for the four passive properties ranged from 0.50 to 0.78 (P<0.001), and the coefficients of determination (r
2) from higher to lower were: (1) maximal DF passive resistive torque (r
2: 0.50–0.62), (2) length extensibility (r
2: 0.40–0.49), (3) maximal muscle length (r
2: 0.28–0.41), and (4) passive elastic stiffness in the last half of the full-stretch range of motion (r
2: 0.25–0.31). The maximal DF passive resistive torque and the length extensibility accounted for 50–62% and 40–49% of the
variability in the concentric torque, respectively. The results indicate that the concentrically stronger calf muscles of
active women were positively correlated with passively stronger, longer, and stiffer calf muscles, which are characteristics
of the calf muscles of younger women. Further studies are needed to examine whether therapeutic interventions, such as stretching
and strengthening, can promote adaptations in the calf muscles of older women to attain these more youthful characteristics.
Electronic Publication 相似文献
17.
Aoki T Niu X Latash ML Zatsiorsky VM 《Experimental brain research. Experimentelle Hirnforschung. Expérimentation cérébrale》2006,172(4):425-438
The effects of surface friction at the digit-object interface on digit forces were studied when subjects (n=8) statically held an object in a five-digit grasp. The friction conditions were SS (all surfaces are sandpaper), RR (all are rayon), SR (S for the thumb and R for the four fingers), and RS (the reverse of SR). The interaction effects of surface friction and external torque were also examined using five torques (–0.5, –0.25, 0, +0.25, +0.5 Nm). Forces and moments exerted by the digits on a handle were recorded. At zero torque conditions, in the SS and RR (symmetric) tasks the normal forces of the thumb and virtual finger (VF, an imagined finger with the mechanical effect equal to that of the four fingers) were larger for the RR than the SS conditions. In the SR and RS (asymmetric) tasks, the normal forces were between the RR and SS conditions. Tangential forces were smaller at the more slippery side than at the less slippery side. According to the mathematical optimization analysis decreasing the tangential forces at the more slippery sides decreases the cost function values. The difference between the thumb and VF tangential forces, ΔF
t, generated a moment of the tangential forces (friction-induced moment). At non-zero torque conditions the friction-induced moment and the moment counterbalancing the external torque (equilibrium-necessitated moment) could be in same or in opposite directions. When the two moments were in the same direction, the contribution of the moment of tangential forces to the total moment was large, and the normal forces were relatively low. In contrast, when the two moments were in opposite directions, the contribution of the moment of tangential forces to the total moment markedly decreased, which was compensated by an increase in the moment of normal forces. The apparently complicated results were explained as the result of summation of the friction-related (elemental) and torque-related (synergy) components of the central commands to the individual digits. 相似文献
18.
Summary Transversal torque between the stance leg and the ground was measured in 169 test persons with normal gait (91 women, 78 men; ages 15–79 years) using three-dimensional force plates. Taking the average of six to ten single steps produces reproducible person- and group-specific data. This permits the isolation and study of individual solutions to specific problems of locomotion, so-called gait mechanisms. The relatively great torque during the double-stance phase is caused mainly by the medially directed, short-term impact peak at heel strike, with the stride length as lever arm. Double-stance torque is therefore determined by motion dynamics and does not help in understanding individual gait mechanisms. Torque measured during the single-stance phase is, in comparison, rather small and is interindividually relatively variable. Experimentally measured torque is considered in terms of various hypotheses concerning the creation of torque in the human gait. Men obviously tend, for reasons of equilibrium and stability, to compensate the various torques. Better than the more known vertical, sagittal, and frontal force components, the remaining torque represents the individual problem solution and therefore permits the study of specific gait mechanisms, both physiological and pathological. Using the characteristic gait of patients with hemiplegia as an example, it is shown that pathological torque observed systematically on the nonaffected side in such patients is a quantitative measure of the trunk rotation necessary to move the plegic side forward. Measurement of this torque therefore permits precise determination of the degree of damage, as well as quantitative control and objective documentation of the rehabilitation progress.Dedicated to Prof. Dr. N. Zöllner on the occasion of his 70th birthday 相似文献
19.
The measurement of endurance time (t
lim) is the procedure commonly used to quantify the ability of a muscle to maintain force. The relationship between surface electromyographic
(sEMG) manifestations of localised muscle fatigue and t
lim during an effort at 50% of maximal voluntary isometric torque of the knee extensors (vastus lateralis and vastus medialis)
until exhaustion was studied in 14 healthy volunteers. It was carried out to test whether changes in sEMG computed over shorter
periods than expected t
lim could be used to predict t
lim. Changes in mean muscle fibre conduction velocity, mean power frequency , median frequency , root mean square ), in the relative
power in the 6–30 Hz and 30–60 Hz frequency bands were monitored using linear slope and area ratio index as statistical indicators.
These indicators were computed over fixed periods shorter than t
lim. The subjects were able to maintain the required force level for [mean (SD)] 78.8 (9.5) s. During the fatigue trial, it was
the greatest of the increases in the 6–30 Hz frequency band, recorded for either of the two muscles investigated, that was
the only variable which correlated with t
lim. Significant relationships between t
lim and changes in this low frequency band were observed as early as the first 15–30 s of the contraction. These results suggest
that sEMG frequency banding may predict mechanical endurance without the need to maintain the contraction until exhaustion.
From a clinical perspective, this could be an advantage for patients who might not be able to tolerate contractions to exhaustion.
Electronic Publication 相似文献
20.
Conventionally assessed voluntary activation does not represent relative voluntary torque production
The ability to voluntarily activate a muscle is commonly assessed by some variant of the twitch interpolation technique (ITT),
which assumes that the stimulated force increment decreases linearly as voluntary force increases. In the present study, subjects
(n = 7) with exceptional ability for maximal voluntary activation (VA) of the knee extensors were used to study the relationship
between superimposed and voluntary torque. This includes very high contraction intensities (90–100%VA), which are difficult
to consistently obtain in regular healthy subjects (VA of ∼90%). Subjects were tested at 30, 60, and 90° knee angles on two
experimental days. At each angle, isometric knee extensions were performed with supramaximal superimposed nerve stimulation
(triplet: three pulses at 300 Hz). Surface EMG signals were obtained from rectus femoris, vastus lateralis, and medialis muscles.
Maximal VA was similar and very high across knee angles: 97 ± 2.3% (mean ± SD). At high contraction intensities, the increase
in voluntary torque was far greater than would be expected based on the decrement of superimposed torque. When voluntary torque
increased from 79.6 ± 6.1 to 100%MVC, superimposed torque decreased from 8.5 ± 2.6 to 2.8 ± 2.3% of resting triplet. Therefore,
an increase in VA of 5.7% (from 91.5 ± 2.6 to 97 ± 2.3%) coincided with a much larger increase in voluntary torque (20.4 ± 6.1%MVC)
and EMG (33.9 ± 6.6%max). Moreover, a conventionally assessed VA of 91.5 ± 2.6% represented a voluntary torque of only 79.6 ± 6.1%MVC.
In conclusion, when maximal VA is calculated to be ∼90% (as in regular healthy subjects), this probably represents a considerable
overestimation of the subjects’ ability to maximally drive their quadriceps muscles. 相似文献