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1.
Reconstruction of equilibrium trajectories and joint stiffness patterns during single-joint voluntary movements under different instructions 总被引:2,自引:0,他引:2
Mark L. Latash 《Biological cybernetics》1994,71(5):441-450
A method for reconstructing joint compliant characteristics during voluntary movements was applied to the analysis of oscillatory
and unidirectional elbow flexion movements. In different series, the subjects were given one of the following instructions:
(1) do not intervene voluntarily; (2) keep the trajectory; (3) in cases of perturbations, return back to the starting position
as quickly as possible (only during unidirectional movements). Under the instruction ‘keep trajectory’, the apparent joint
stiffness increased by 50% to 250%. During oscillatory movements, this was accompanied by a decrease in the maximal difference
between the actual and equilibrium joint trajectories and, in several cases, led to a change in the phase relation between
the two trajectories. The coefficients of correlation between joint torque and angle were very high (commonly, over 0.9) under
the ‘do not intervene’ instruction. They dropped to about 0.6 under the ‘keep trajectory’ and to about 0.3 under the ‘return
back’ instructions. Under these two instructions, the low values of the coefficients of correlation did not allow reconstruction
of segments of equilibrium trajectories and joint stiffness values in all the subjects. The results provide further support
for the λ-version of the equilibrium-point hypothesis and for using the instruction ‘do not intervene voluntarily’ to obtain
reproducible time patterns of the central motor command.
Received: 14 December 1993/Accepted in revised form: 16 April 1994 相似文献
2.
Mark L. Latash 《Biological cybernetics》1992,67(4):377-384
In the framework of the equilibrium-point hypothesis, virtual trajectories and joint stiffness patterns have been reconstructed
during two motor tasks practiced against a constant bias torque. One task required a voluntary increase in joint stiffness
while preserving the original joint position. The other task involved fast elbow flexions over 36°. Joint stiffness gradually
subsided after the termination of fast movements. In both tasks, the external torque could slowly and unexpectedly change.
The subjects were required not to change their motor commands if the torque changed, i.e. “to do the same no matter what the
motor did”. In both tasks, changes in joint stiffness were accompanied by unchanged virtual trajectories that were also independent
of the absolute value of the bias torque. By contrast, the intercept of the joint compliant characteristic with the angle
axis,r(t)-function, has demonstrated a clear dependence upon both the level of coactivation and external load. We assume that a template
virtual trajectory is generated at a certain level of the motor hierarchy and is later scaled taking into account some commonly
changing dynamic factors of the movement execution, for example, external load. The scaling leads to the generation of commands
to the segmental structures that can be expressed, according to the equilibrium-point hypothesis, as changes in the thresholds
of the tonic stretch reflex for corresponding muscles. 相似文献
3.
The aim of this study was to describe and explain how individual muscles control mediolateral balance during normal walking. Biomechanical modeling and experimental gait data were used to quantify individual muscle contributions to the mediolateral acceleration of the center of mass during the stance phase. We tested the hypothesis that the hip, knee, and ankle extensors, which act primarily in the sagittal plane and contribute significantly to vertical support and forward progression, also accelerate the center of mass in the mediolateral direction. Kinematic, force plate, and muscle EMG data were recorded simultaneously for five healthy subjects who walked at their preferred speeds. The body was modeled as a 10-segment, 23 degree-of-freedom skeleton, actuated by 54 muscles. Joint moments obtained from inverse dynamics were decomposed into muscle forces by solving an optimization problem that minimized the sum of the squares of the muscle activations. Muscles contributed significantly to the mediolateral acceleration of the center of mass throughout stance. Muscles that generated both support and forward progression (vasti, soleus, and gastrocnemius) also accelerated the center of mass laterally, in concert with the hip adductors and the plantarflexor everters. Gravity accelerated the center of mass laterally for most of the stance phase. The hip abductors, anterior and posterior gluteus medius, and, to a much lesser extent, the plantarflexor inverters, actively controlled balance by accelerating the center of mass medially. 相似文献
4.
Until now, the equilibrium-point hypothesis (λ model) of motor control has assumed nonintersecting force-length characteristics
of the tonic stretch reflex for individual muscles. Limited data from animal experiments suggest, however, that such intersections
may occur. We have assumed the possibility of intersection of the characteristics of the tonic stretch reflex and performed
a computer simulation of movement trajectories and electromyographic patterns. The simulation has demonstrated, in particular,
that a transient change in the slope of the characteristic of an agonist muscle may lead to temporary movement reversals,
hesitations, oscillations, and multiple electromyographic bursts that are typical of movements of patients with dystonia.
The movement patterns of three patients with idiopathic dystonia during attempts at fast single-joint movements (in the elbow,
wrist, and ankle) were recorded and compared with the results of the computer simulation. This approach considers that motor
disorders in dystonia result from faulty control patterns that may not correlate with any morphological or neurophysiological
changes. It provides a basis for the high variability of dystonic movements. The uniqueness of abnormal motor patterns in
dystonia, that precludes statistical analysis across patients, may result from subtle differences in the patterns of intersecting
characteristics of the tonic stretch reflex. The applicability of our analysis to disordered multijoint movement patterns
is discussed.
Received: 26 July 1993/Accepted in revised form: 22 December 1993 相似文献
5.
Six subjects performed a planar reaching arm movement to a target while unpredictable perturbations were applied to the endpoint;
the perturbations consisted of pulling springs having different stiffness. Two conditions were applied; in the first, subjects
had to reach for the target despite the perturbation, in the second condition, the subjects were asked to not correct the
motion as a perturbation was applied. We analyzed the kinematics profiles of the three arm segments and, by means of inverse
dynamics, calculated the joint torques. The framework of the equilibrium-point (EP) hypothesis, the λ model, allowed the reconstruction
of the control variables, the “equilibrium trajectories”, in the “do not correct” condition for the wrist and the elbow joints
as well as for the end point final position, while for the other condition, the reconstruction was less reliable. The findings
support and extend to a multiple-joint planar movement, the paradigm of the EP hypothesis along with the “do not correct”
instruction.
Received: 3 May 1999 / Accepted in revised form: 19 May 2000 相似文献
6.
For the control of actuated orthoses, or gait rehabilitation robotics, kinematic reference trajectories are often required. These trajectories, consisting of joint angles, angular velocities and accelerations, are highly dependent on walking-speed. We present and evaluate a novel method to reconstruct body-height and speed-dependent joint trajectories. First, we collected gait kinematics in fifteen healthy (middle) aged subjects (47–68), at a wide range of walking-speeds (0.5–5 kph). For each joint trajectory multiple key-events were selected (among which its extremes). Second, we derived regression-models that predict the timing, angle, angular velocity and acceleration for each key-event, based on walking-speed and the subject?s body-height. Finally, quintic splines were fitted between the predicted key-events to reconstruct a full gait cycle. Regression-models were obtained for hip ab-/adduction, hip flexion/extension, knee flexion/extension and ankle plantar-/dorsiflexion. Results showed that the majority of the key-events were dependent on walking-speed, both in terms of timing and amplitude, whereas the body-height had less effect. The reconstructed trajectories matched the measured trajectories very well, in terms of angle, angular velocity and acceleration. For the angles the RMSE between the reconstructed and measured trajectories was 2.6°. The mean correlation coefficient between the reconstructed and measured angular trajectories was 0.91. The method and the data presented in this paper can be used to generate speed-dependent gait patterns. These patterns can be used for the control of several robotic gait applications. Alternatively they can assist the assessment of pathological gait, where they can serve as a reference for “normal” gait. 相似文献
7.
Ole Rintek Madsen Ulrik Birk Lauridsen Andreas Hartkopp Ole Helmer S?rensen 《European journal of applied physiology and occupational physiology》1997,75(3):239-245
Dual energy x-ray absorptiometry (DEXA) offers the possibility of assessing regional soft tissue composition, i.e. lean mass (LM) and fat
mass : LM may be considered a measure of muscle mass. We examined age-related differences in LM, percentage fat (%fat) and
muscle strength in 100 healthy non-athletic women aged 18–87 years. Relationships between muscle strength and leg LM in 20
elite female weight lifters and in 18 inactive women with previous hip fractures were also studied. The LM and %fat of the
whole body, trunk, arms and legs were derived from a whole body DEXA scan. Isokinetic knee extensor strength (KES) and flexor
strength (KFS) at 30° · s–1 were assessed using an isokinetic dynamometer. The women aged 71–87 years had 35% lower KES and KFS than the women aged 18–40
years (P < 0.0001). Differences in LM were less pronounced. The LM of the legs, for instance, was 15% lower in the old than in the
young women (P < 0.0001). In a multiple regression analysis with age, body mass, height and leg LM or KES as independent variables and KES
or leg LM as the dependent variable, age was the most important predictor of KES (r
partial = −0.74, P < 0.0001). The same applied to KFS. Body mass, not age, was the most important predictor of leg LM (r
partial = 0.65, P < 0.0001) and of LM at all other measurement sites. The LM measured at different regions decreased equally with increasing
age. The KES:leg LM ratio was negatively correlated with age (r = −0.70, P < 0.0001). The weight lifters had significantly higher KES:leg LM ratios than age-matched controls (+12%, P < 0.0001) and vice versa for the women with previous hip fractures (–36%, P < 0.0001). In conclusion, from our study it would seem that in healthy nonathletic women, age is a more important determinant
of muscle strength than is LM as measured by DEXA. Muscle strengthening exercises and inactivity seem to have a considerably
stronger influence on muscle strength than on LM.
Accepted: 27 August 1996 相似文献
8.
During human movement, muscle activation and limb movement result in subtle changes in muscle mass distribution. Muscle mass redistribution can affect limb inertial properties and limb dynamics, but it is not currently known to what extent. The objectives of this study were to investigate: (1) how physiological alterations of muscle and tendon length affect limb inertial characteristics, and (2) how such changes affect dynamic simulations of human movement. To achieve these objectives, a digital model of a human leg, custom software, and Software for interactive musculoskeletal modeling were used to simulate mass redistribution of muscle–tendon structures within a limb segment during muscle activation and joint movement. Thigh and shank center of mass and moments of inertia for different muscle activation and joint configurations were determined and compared. Limb inertial parameters representing relaxed muscles and fully active muscles were input into a simulated straight-leg movement to evaluate the effect inertial parameter variations could have on movement simulation results. Muscle activation and limb movement altered limb segment center of mass and moments of inertia by less than 0.04 cm and 1.2%, respectively. These variations in limb inertial properties resulted in less than 0.01% change in maximum angular velocity for a simulated straight-leg hip flexion task. These data demonstrate that, for the digital human leg model considered, assuming static quantities for segment center of masses and moments of inertia in movement simulations appear reasonable and induce minimal errors in simulated movement dynamics. 相似文献
9.
Acetabular dysplasia is a known cause of hip osteoarthritis. In addition to abnormal anatomy, changes in kinematics, joint reaction forces (JRFs), and muscle forces could cause tissue damage to the cartilage and labrum, and may contribute to pain and fatigue. The objective of this study was to compare lower extremity joint angles, moments, hip JRFs and muscle forces during gait between patients with symptomatic acetabular dysplasia and healthy controls. Marker trajectories and ground reaction forces were measured in 10 dysplasia patients and 10 typically developing control subjects. A musculoskeletal model was scaled in OpenSim to each subject and subject-specific hip joint centers were determined using reconstructions from CT images. Joint kinematics and moments were calculated using inverse kinematics and inverse dynamics, respectively. Muscle forces and hip JRFs were estimated with static optimization. Inter-group differences were tested for statistical significance (p ≤ 0.05) and large effect sizes (d ≥ 0.8). Results demonstrated that dysplasia patients had higher medially directed JRFs. Joint angles and moments were mostly similar between the groups, but large inter-group effect sizes suggested some restriction in range of motion by patients at the hip and ankle. Higher medially-directed JRFs and inter-group differences in hip muscle forces likely stem from lateralization of the hip joint center in dysplastic patients. Joint force differences, combined with reductions in range of motion at the hip and ankle may also indicate compensatory strategies by patients with dysplasia to maintain joint stability. 相似文献
10.
Two behavioral goals are achieved simultaneously during forward trunk bending in humans: the bending movement per se and
equilibrium maintenance. The objective of the present study was to understand how the two goals are achieved by using a biomechanical
model of this task. Since keeping the center of pressure inside the support area is a crucial condition for equilibrium maintenance
during the movement, we decided to model an extreme case, called “optimal bending”, in which the movement is performed without
any center of pressure displacement at all, as if standing on an extremely narrow support. The “optimal bending” is used as
a reference in the analysis of experimental data in a companion paper. The study is based on a three-joint (ankle, knee, and
hip) model of the human body and is performed in terms of “eigenmovements”, i.e., the movements along eigenvectors of the
motion equation. They are termed “ankle”, “hip”, and “knee” eigenmovements according to the dominant joint that provides the
largest contribution to the corresponding eigenmovement. The advantage of the eigenmovement approach is the presentation of
the coupled system of dynamic equations in the form of three independent motion equations. Each of these equations is equivalent
to the motion equation for an inverted pendulum. Optimal bending is constructed as a superposition of two (hip and ankle)
eigenmovements. The hip eigenmovement contributes the most to the movement kinematics, whereas the contributions of both eigenmovements
into the movement dynamics are comparable. The ankle eigenmovement moves the center of gravity forward and compensates for
the backward center of gravity shift that is provoked by trunk bending as a result of dynamic interactions between body segments.
An important characteristic of the optimal bending is the timing of the onset of each eigenmovement: the ankle eigenmovement
onset precedes that of the hip eigenmovement. Without an earlier onset of the ankle eigenmovement, forward bending on the
extremely narrow support results in falling backward. This modeling approach suggests that during trunk bending, two motion
units – the hip and ankle eigenmovements – are responsible for the movement and for equilibrium maintenance, respectively.
Received: 1 July 1999 / Accepted in revised form: 23 October 2000 相似文献
11.
Biomechanical analysis of movement strategies in human forward trunk bending. II. Experimental study
The large mass of the human upper trunk, its elevated position during erect stance, and the small area limited by the size
of the feet, stress the importance of equilibrium control during trunk movements. The objective of the present study was to
perform a biomechanical analysis of fast forward trunk movements in order to understand the coordination between movement
and posture. The analysis is based on a comparison between experimentally observed bending and hypothetical “optimal bending”
performed on an infinitely narrow support, as presented in a companion paper. The experimental data were obtained from 16
subjects who performed fast forward bending while standing on a wide platform or on a narrow beam. The analysis is performed
by decomposition of the movement into three dynamically independent components, each representing a movement along one of
the three eigenvectors of the motion equation. The eigenmovements are termed “hip”, “ankle”, and “knee” eigenmovements, according
to the dominant joint. The experimentally observed movement is characterized mainly by the hip and ankle eigenmovements, whereas
the knee eigenmovement is negligible. Similarly to the “optimal bending” the ankle eigenmovement starts earlier and lasts
longer than the hip eigenmovement. An early forward acceleration of the center of gravity in the ankle eigenmovement is caused
by anticipatory changes in the ankle joint torque. This clarifies the role of the early tibialis anterior burst and/or soleus
inhibition usually observed in electromyographic recordings during forward bending. The results suggest that the hip and the
ankle eigenmovements can be treated as independently controlled motion units aimed at functionally different behavioral goals:
the bending per se and postural adjustment. It is proposed that the central nervous system has to control these motion units
sequentially in order to perform the movement and maintain equilibrium. It is also suggested that the hip and ankle eigenmovements
can be regarded as a biomechanical background for the hip and ankle strategies introduced by Horak and Nashner (1986) on the
basis of electromyographic recordings and kinematic patterns in response to postural perturbations.
Received: 1 July 1999 / Accepted in revised form: 23 October 2000 相似文献
12.
This study examined the effect of hip flexion angle on the stiffness of the adductor longus (AL) muscle during isometric hip flexion. Seventeen men were recruited. Ten participants performed submaximal voluntary contraction at 0%, 25%, 50%, and 75% of maximal voluntary contraction (MVC) during isometric hip flexion after performing MVC at 0°, 40°, and 80° of hip flexion. Seven participants performed submaximal voluntary tasks during isometric hip extension in addition to hip flexion task. The shear modulus of the AL muscle was used as the index of muscle stiffness, and was measured using ultrasound shear-wave elastography during the tasks at each contraction intensity for each hip flexion angle. During hip flexion, the shear modulus of the AL muscle was higher at 0° than at 40° and 80° of hip flexion at each contraction intensity (p < 0.016). Conversely, a significant effect was not found among hip flexion angle during hip extension at 75% of MVC (p = 0.867). These results suggest that mechanical stress of the AL muscle may be higher at 0° of hip flexion during isometric hip flexion. 相似文献
13.
Background: Footwear-generated medio-lateral foot center of pressure manipulation has been shown to have potential positive effects on gait parameters of hip osteoarthritis patients, ultimately reducing maximum joint reaction forces. The objective of this study was to investigate effects of medio-lateral foot center of pressure manipulation on muscle activity of hip-spanning and back muscles during gait in bilateral hip osteoarthritis patients. Methods: Foot center of pressure was shifted along the medio-lateral foot axis using a foot-worn biomechanical device allowing controlled center of pressure manipulation. Sixteen female bilateral hip osteoarthritis patients underwent electromyography analysis while walking in the device set to three parasagittal configurations: neutral (control), medial, and lateral. Seven hip-spanning muscles (Gluteus Medius, Gluteus Maximus, Tensor Fascia Latae, Rectus Femoris, Semitendinosis, Biceps Femoris, Adductor Magnus) and one back muscle (Erector Spinae) were analyzed. Magnitude and temporal parameters were calculated. Results: The amplitude and temporal parameter varied significantly between foot center of pressure positions for 5 out of 8 muscles each for either the more or less symptomatic leg in at least one subphase of the gait cycle. Conclusion: Medio-lateral foot center of pressure manipulation significantly affects neuromuscular pattern of hip and back musculature during gait in female hip bilateral osteoarthritis patients. 相似文献
14.
The hypothesis was put forward that, along with the regulation of mass center projection, the system of upright posture control stabilizes the deviation of pressure center from the position of the mass center projection. The regularities in the behavior of the trajectories of pressure center and mass center projection were analysed. Experimental evidence was obtained supporting the validity of the hypothesis. The structure of the control system that corresponds to the new understanding of the variables being regulated during the maintenance of vertical posture was considered. 相似文献
15.
This study examined the effect of body segment parameter (BSP) perturbations on joint moments calculated using an inverse dynamics procedure and muscle forces calculated using computed muscle control (CMC) during gait. BSP (i.e. segment mass, center of mass location (com) and inertia tensor) of the left thigh, shank and foot of a scaled musculoskeletal model were perturbed. These perturbations started from their nominal value and were adjusted to ±40% in steps of 10%, for both individual as well as combined perturbations in BSP. For all perturbations, an inverse dynamics procedure calculated the ankle, knee and hip moments based on an identical inverse kinematics solution. Furthermore, the effect of applying a residual reduction algorithm (RRA) was investigated. Muscle excitations and resulting muscle forces were calculated using CMC. The results show only a limited effect of an individual parameter perturbation on the calculated moments, where the largest effect is found when perturbing the shank com (MScom,shank, the ratio of absolute difference in torque and relative parameter perturbation, is maximally −7.81 N m for hip flexion moment). The additional influence of perturbing two parameters simultaneously is small (MSmass+com,thigh is maximally 15.2 N m for hip flexion moment). RRA made small changes to the model to increase the dynamic consistency of the simulation (after RRA MScom,shank is maximally 5.01 N m). CMC results show large differences in muscle forces when BSP are perturbed. These result from the underlying forward integration of the dynamic equations. 相似文献
16.
Marius Henriksen Jens Aaboe Erik B. Simonsen Tine Alkjær Henning Bliddal 《Journal of biomechanics》2009,42(9):1236-1240
Hip and knee functions are intimately connected and reduced hip abductor function might play a role in development of knee osteoarthritis (OA) by increasing the external knee adduction moment during walking. The purpose of this study was to test the hypothesis that reduced function of the gluteus medius (GM) muscle would lead to increased external knee adduction moment during level walking in healthy subjects. Reduced GM muscle function was induced experimentally, by means of intramuscular injections of hypertonic saline that produced an intense short-term muscle pain and reduced muscle function. Isotonic saline injections were used as non-painful control. Fifteen healthy subjects performed walking trials at their self-selected walking speed before and immediately after injections, and again after 20 min of rest, to ensure pain recovery. Standard gait analyses were used to calculate three-dimensional trunk and lower extremity joint kinematics and kinetics. Surface electromyography (EMG) of the glutei, quadriceps, and hamstring muscles were also measured. The peak GM EMG activity had temporal concurrence with peaks in frontal plane moments at both hip and knee joints. The EMG activity in the GM muscle was significantly reduced by pain (?39.6%). All other muscles were unaffected. Peaks in the frontal plane hip and knee joint moments were significantly reduced during pain (?6.4% and ?4.2%, respectively). Lateral trunk lean angles and midstance hip joint adduction and knee joint extension angles were reduced by ?1°. Thus, the gait changes were primarily caused by reduced GM function. Walking with impaired GM muscle function due to pain significantly reduced the external knee adduction moment. This study challenge the notion that reduced GM function due to pain would lead to increased loads at the knee joint during level walking. 相似文献
17.
Richard Quint van der Linde 《Biological cybernetics》1999,81(3):227-237
The existence of self-organizing walking patterns is often considered the result of a mechanical system interacting with
the environment and a (neural) oscillating unit. The pattern generators might be thought of as an indispensable component
for the existence of limit cycle behavior. This paper shows that this is not a necessity for the existence of a self-organizing
bipedal walking pattern. Stable walking cycles emerge from a simple passive bipedal structure, with an energy source inevitably
present to sustain the oscillation. In this work the energy source is chosen to be phasic muscle contraction. A two-dimensional
model is composed of two legs and a hip mass, symbolizing the trunk. The stance leg stiffness is generated by two muscles.
The hip stiffness is generated by four muscles. Muscle activation is caused by two reflex-like trigger signals, without feedback
control. Human equivalent model parameters such as geometry and mass distribution were assumed. With return map analysis,
the model is analyzed on periodic behavior. Stable walking cycles were found and could be manipulated during walking by varying
the muscle or reflex parameters, forcing the oscillation to converge to a new attractor.
Received: 5 November 1998 / Accepted in revised form: 26 March 1999 相似文献
18.
Shahid Hussain Prashant K. Jamwal Mergen H. Ghayesh 《Computer methods in biomechanics and biomedical engineering》2017,20(6):626-635
Background and Objectives: While body weight support (BWS) intonation is vital during conventional gait training of neurologically challenged subjects, it is important to evaluate its effect during robot assisted gait training. In the present research we have studied the effect of BWS intonation on muscle activities during robotic gait training using dynamic simulations. Methods: Two dimensional (2-D) musculoskeletal model of human gait was developed conjointly with another 2-D model of a robotic orthosis capable of actuating hip, knee and ankle joints simultaneously. The musculoskeletal model consists of eight major muscle groups namely; soleus (SOL), gastrocnemius (GAS), tibialis anterior (TA), hamstrings (HAM), vasti (VAS), gluteus maximus (GLU), uniarticular hip flexors (iliopsoas, IP), and Rectus Femoris (RF). BWS was provided at levels of 0, 20, 40 and 60% during the simulations. In order to obtain a feasible set of muscle activities during subsequent gait cycles, an inverse dynamics algorithm along with a quadratic minimization algorithm was implemented. Results: The dynamic parameters of the robot assisted human gait such as joint angle trajectories, ground contact force (GCF), human limb joint torques and robot induced torques at different levels of BWS were derived. The patterns of muscle activities at variable BWS were derived and analysed. For most part of the gait cycle (GC) the muscle activation patterns are quite similar for all levels of BWS as is apparent from the mean of muscle activities for the complete GC. Conclusions: Effect of BWS variation during robot assisted gait on muscle activities was studied by developing dynamic simulation. It is expected that the proposed dynamic simulation approach will provide important inferences and information about the muscle function variations consequent upon a change in BWS during robot assisted gait. This information shall be quite important while investigating the influence of BWS intonation on neuromuscular parameters of interest during robotic gait training. 相似文献
19.
M.-T. Linossier D. Dormois P. Brégère A. Geyssant C. Denis 《European journal of applied physiology and occupational physiology》1997,76(1):48-54
The aim of this study was to examine whether the alkalosis-induced improvement in supramaximal performance could be explained
by a less-altered muscle metabolic status. Eight subjects first performed exhausting exercise at 120% peak oxygen uptake after
ingesting either a placebo (PLC) or sodium citrate (CIT) at a dose of 0.5 g · kg−1 body mass to determine exhaustion time (t
exh). They then, performed exercise (Lim-EX) at the same relative intensity lasting PLCt
exh minus 20 s in both treatments. Samples were taken from vastus lateralis muscle at rest (90-min after the ingestion) and at
the end of Lim-EX. Arterial blood samples were obtained at rest (immediately prior to and 90 min after ingesting the drug)
and during the 20-min post-exercise recovery. The t
exh was significantly increased by CIT [PLC 258 (SD 29) s, CIT 297 (SD 45) s]. The CIT raised the rest [citrate] in blood [PLC
0.11 (SD 0.01) mmol · l−1, CIT 0.34 (SD 0.07) mmol · l−1] and in muscle [PLC 0.78 (SD 0.23) mmol · kg−1 dry mass, CIT 1.00 (SD 0.21) mmol · kg−1 dry mass]. Resting muscle pH and buffering capacity were unchanged by CIT. The same fall in muscle pH was observed during
Lim-EX in the two conditions. This was associated with similar variations in both the cardio-respiratory response and muscle
energy and metabolism status in spite of a better blood acid-base status after CIT. Thus, CIT would not seem to allow the
alkalinization of the muscle cytosolic compartment. Though sodium citrate works in a similar way to NaHCO3 on plasma alkalinization and exercise performance, the exact nature of the mechanisms involved in the delay of exhaustion
could be different and remains to be elucidated.
Accepted: 26 November 1996 相似文献
20.
Improved understanding is required of how the mechanics of the fall affect hip fracture risk. We used a hip impact simulator to determine how peak stresses at the femoral neck were affected by pelvis impact angle, hip abductor muscle force, and use of a wearable hip protector.We simulated falls from standing (2 m/s impact velocity) involving initial hip abductor muscle forces of 700 or 300 N. Trials were acquired for impact to the lateral aspect of the greater trochanter, and impact to the pelvis rotated 5°, 10° and 15° anteriorly (positive) or posteriorly (negative). Measures were acquired with and without a commercially available hip protector. During trials, we measured three-dimensional forces with a load cell at the femoral neck, and derived peak compressive and tensile stresses.Peak compressive stress increased 37% (5.91 versus 4.31 MPa; p < 0.0005) and peak tensile stress increased 209% (2.31 versus 0.75 MPa; p < 0.0005) when the pelvis impact angle changed from 15° anterior to −15° posterior. For lateral impacts, the peak tensile and compressive stresses averaged 73% and 8% lower, respectively, in the 700 N than 300 N muscle force condition, but the effect was reversed for anteriolateral or posteriolateral impacts. The attenuation in peak compressive stress from the hip protector was greatest for posteriolateral impacts (−15 to −5°; 36–41%), and least for anteriolateral (+15°; 10%).These results clarify the effects on hip fracture risk during a fall of pelvis impact angle and muscle forces, and should inform the design of improved hip protectors. 相似文献