Biomedical subjects

M F Bobbert

Publications and source records attributed to M F Bobbert.

At least 37 records · Page 2Linked to original sources

PubMed1999

Jumping for distance: control of the external force in squat jumps.

PURPOSE: It was investigated whether control in jumps for distance is related to control in jumps for height. METHODS: Five male subjects performed maximum squat jumps in the following conditions: VJ (vertical jump), LJ (long jump), and two conditions with inclination angles of the body relative to the horizontal of 75 and 65 degrees, respectively. An inverse dynamics analysis was performed using measured kinematics and ground reaction forces. In addition, jumps were simulated with a forward dynamic model of the musculoskeletal system, comprising four segments and six muscles. First, VJ was simulated by finding the optimal stimulation pattern, i.e., the pattern resulting in a maximum height of the mass center of the body (MCB). Subsequently, LJ was simulated using a "rotation-extension" strategy, i.e., by applying the optimal stimulation pattern for VJ to the system after imposing an initial angular velocity. RESULTS: In the experiments, no significant differences were found among jumps with different inclination angles in the magnitude of the peak ground reaction force. The same was true for the magnitude of the velocity of MCB and the distance of MCB from the center of pressure at the instance of take-off. As the inclination angle became smaller, i.e., jumps were directed more forward, the net knee joint moment increased whereas net hip and ankle moments decreased. Also, the peak angular velocity in the hip joint was higher and the joint was more extended at take-off. The opposite was true for the knee joint. In the simulation study, using the "rotation-extension" strategy for simulating VJ, these adaptations in kinematics and net joint moments were reproduced satisfactorily. CONCLUSION: By virtue of the stabilizing effect of intrinsic muscle properties, a jump for distance may be achieved using control of a vertical jump according to a "rotation-extension" strategy.

Adult↗

PubMed1998

From twitch to tetanus for human muscle: experimental data and model predictions for m. triceps surae.

In models describing the excitation of muscle by the central nervous system, it is often assumed that excitation during a tetanic contraction can be obtained by the linear summation of responses to individual stimuli, from which the active state of the muscle is calculated. We investigate here the extent to which such a model describes the excitation of human muscle in vivo. For this purpose, experiments were performed on the calf muscles of four healthy subjects. Values of parameters in the model describing the behaviour of the contractile element (CE) and the series elastic element (SEE) of this muscle group were derived on the basis of a set of isokinetic release contractions performed on a special-purpose dynamometer as well as on the basis of morphological data. Parameter values describing the excitation of the calf muscles were optimized such that the model correctly predicted plantar flexion moment histories in an isometric twitch, elicited by stimulation of the tibial nerve. For all subjects, the model using these muscle parameters was able to make reasonable predictions of isometric moment histories at higher stimulation frequencies. These results suggest that the linear summation of responses to individual stimuli can indeed give an adequate description of the process of human muscle excitation in vivo.

Adult↗

PubMed1997

Evaluation of a self-consistent method for calculating muscle parameters from a set of isokinetic releases.

A new method for calculating parameters describing the force-velocity relationship of the contractile element and the force-extension relationship of the series elastic element of skeletal muscle from a set of isokinetic release contractions is evaluated using experimental and numerical techniques. The method calculates from the set of isokinetic releases those force-velocity and force-extension relationships that give a self-consistent description of the data set. The self-consistent calculation method is applied to data obtained from the gastrocnemius medialis muscle of the rat, since for such an animal model both relationships can be independently derived from a set of isotonic release contractions. For the two animals studied, the force-velocity and force-extension relationships calculated by the self-consistent method were in good agreement with the ones derived from isotonic releases performed on the same muscle. The statistical properties of the estimates obtained by the calculation method were investigated using a Monte Carlo technique. The method was found to yield results which were biased by less than 2% and which possessed a coefficient of variation smaller than 5%. These findings indicate that the proposed calculation method can be a useful tool for determining the contractile properties of skeletal muscle as reflected in the force-velocity and force-extension relationships.

Animals↗

PubMed1996

From twitch to tetanus: performance of excitation dynamics optimized for a twitch in predicting tetanic muscle forces.

In models of the excitation of muscles it is often assumed that excitation during a tetanic contraction can be obtained by the linear summation of responses to individual stimuli from which the active state of the muscle is calculated. The purpose of this study was to investigate whether such a model adequately describes the process of excitation of muscle. Parameters describing the contraction dynamics of the muscle model used were derived from physiological and morphological measurements made on the gastrocnemius medialis muscle of three adult Wistar rats. Parameters pertaining to the excitation dynamics were optimized such that the muscle model correctly predicted force histories recorded during an isometric twitch. When a relationship between intracellular calcium and active state from literature on rat muscle was used, the muscle model was capable of generating force histories at stimulation frequencies of 20, 40, 60 and 80 Hz and other muscle-tendon complex lengths which closely matched those measured experimentally - albeit forces were underestimated slightly in all cases. Differences in responses to higher stimulation frequencies between animals could be traced back to differences in twitch dynamics between the animals and adequate predictions of muscle forces were obtained for all animals. These results suggest that the linear summation of responses to individual stimuli indeed gives an adequate description of the excitation of muscle.

Animals↗

PubMed1996

Mechanical output from individual muscles during explosive leg extensions: the role of biarticular muscles.

The main result of this study is that biarticular leg muscles contribute significantly to the work done at joints, due to transfer of power during explosive leg extensions. In particular, a net power transfer was shown from hip to knee joint during jumping and sprinting. Seven elite athletes performed explosive one legged jump and spring push offs. Kinematics, ground reaction forces and electromyography (EMG) of leg muscles were recorded. The mechanical output of six individual muscle groups was estimated by using Hill-based muscle models. The EMG and kinematics served as input to these models. For jumping as well as for sprinting, the model estimated similar results for the relative work contribution done about a joint due to transfer of power by the biarticular muscles. Rectus femoris showed a power transfer from hip to knee joint, while in contrast hamstrings showed a power transfer from knee to hip joint. Regardless of these opposite directions of power transfer, a net transfer occurred from the hip to the knee joint. The relative work contribution of hamstrings done in hip extension was 7% in jumping and 11% in sprinting. For rectus femoris, the relative work contribution done in knee extension was 21% in jumping and 31% in sprinting. Power transferring actions by gastrocnemius from knee to ankle contributed 25% in jumping and 28% in sprinting to the work done in plantar flexion. These results support the hypothesis that the action of biarticular muscles contributes to a net transfer of power from proximal to distal joints during explosive leg extensions. This action of the biarticular muscles causes an efficient conversion of body segment rotations into the desired translation of the body centre of gravity.

Adult↗

PubMed1996

Why is countermovement jump height greater than squat jump height?

In the literature, it is well established that subjects are able to jump higher in a countermovement jump (CMJ) than in a squat jump (SJ). The purpose of this study was to estimate the relative contribution of the time available for force development and the storage and reutilization of elastic energy to the enhancement of performance in CMJ compared with SJ. Six male volleyball players performed CMJ and SJ. Kinematics, kinetics, and muscle electrical activity (EMG) from six muscles of the lower extremity were monitored. It was found that even when the body position at the start of push-off was the same in SJ as in CMJ, jump height was on average 3.4 cm greater in CMJ. The possibility that nonoptimal coordination in SJ explained the difference in jump height was ruled out: there were no signs of movement disintegration in SJ, and toe-off position was the same in SJ as in CMJ. The greater jump height in CMJ was attributed to the fact that the countermovement allowed the subjects to attain greater joint moments at the start of push-off. As a consequence, joint moments were greater over the first part of the range of joint extension in CMJ, so that more work could be produced than in SJ. To explain this finding, measured and manipulated kinematics and electromyographic activity were used as input for a model of the musculoskeletal system. According to simulation results, storage and reutilization of elastic energy could be ruled out as explanation for the enhancement of performance in CMJ over that in SJ. The crucial contribution of the countermovement seemed to be that it allowed the muscles to build up a high level of active state (fraction of attached cross-bridges) and force before the start of shortening, so that they were able to produce more work over the first part of their shortening distance.

Adult↗

PubMed1996

Influence of the parameters of a human triceps surae muscle model on the isometric torque-angle relationship.

This study investigates the influence of parameter values of the human triceps surae muscle on the torque-angle relationship. The model used consisted of three units, each containing a contractile, a series elastic and a parallel elastic element. Parameter values were based on morphological characteristics, which made it possible to model individual units. However, for a number of parameters the values reported in the literature vary considerably. It was investigated how sensitive model results were for variation of these parameters. Slack length of the series elastic element, mean moment arm, maximum force, and length of the contractile element appeared to be the most important determinants of the behavior. For mean moment arm and contractile element length, morphology-based methods of estimation could be recommended. Slack length and maximum force were obtained through optimization. It was concluded that the model does not contain parameters on which its output depends strongly and which are difficult to estimate as well, with two exceptions: slack length of the series elastic element and maximum force.

Ankle↗

PubMed1994

A control strategy for the execution of explosive movements from varying starting positions.

1. Humans can execute explosive movements such as jumping and hitting an object irrespective of the starting position from which these movements have to be initiated; in fact, variability of kinematic parameters has been shown to decrease in the course of the movement. 2. We address the question of whether it is necessary to adapt the stimulation pattern of the muscles to such variations in starting position or whether the stabilizing effect of intrinsic muscle properties is such that one single muscle stimulation pattern might be used for a wide range of starting positions. 3. Specifically, we address this question for maximum-height human vertical squat jumping, using an approach based on mathematical modeling and computer simulation. The stimulation pattern of the muscles is the input of the model and the resulting movement is the output. 4. The optimal stimulation pattern for a starting position in the middle of the range of starting positions considered does not lead to adequate performance for other starting positions in that range. 5. However, a muscle stimulation pattern can be found that does result in close to optimal achievement for a wide range of starting positions. This muscle stimulation pattern, which is not optimal for any specific starting position, may be considered as "control that works" as opposed to "optimal control." 6. The latter muscle stimulation pattern also leads to adequate behavior for "new" starting positions both within and outside the range considered.

Adult↗

PubMed1994

Effects of muscle strengthening on vertical jump height: a simulation study.

In this study the effects of systematic manipulations of control and muscle strength on vertical jump height were investigated. Forward dynamic simulations of vertical squat jumps were performed with a model of the human musculoskeletal system. Model input was STIM(t), stimulation of six lower extremity muscles as function of time; model output was body motion. The model incorporated all features of the musculoskeletal system of human test subjects considered salient for vertical jumping, and the initial body configuration was set equal to that of the test subjects. First, optimal STIM(t) was found for a standard version of the model (experiment A). A satisfactory correspondence was found between simulation results and kinematics, kinetics and electromyograms of the test subjects. Subsequently, optimal STIM(t) for the standard model was used to drive a model with strengthened muscles (experiment B). Jump height was now lower than that found in experiment A. Finally, optimal STIM(t) was found for the model with strengthened muscles (experiment C). Jump height was now higher than that found in experiment A. These results suggest that in order to take full benefit of an increase in muscle strength, control needs to be adapted. It is speculated that in training programs aimed at improving jumping achievement, muscle training exercises should be accompanied by exercises that allow athletes to practice with their changed muscles.

Adult↗

PubMed1993

The contribution of muscle properties in the control of explosive movements.

Explosive movements such as throwing, kicking, and jumping are characterized by high velocity and short movement time. Due to the fact that latencies of neural feedback loops are long in comparison to movement times, correction of deviations cannot be achieved on the basis of neural feedback. In other words, the control signals must be largely preprogrammed. Furthermore, in many explosive movements the skeletal system is mechanically analogous to an inverted pendulum; in such a system, disturbances tend to be amplified as time proceeds. It is difficult to understand how an inverted-pendulum-like system can be controlled on the basis of some form of open loop control (albeit during a finite period of time only). To investigate if actuator properties, specifically the force-length-velocity relationship of muscle, reduce the control problem associated with explosive movement tasks such as human vertical jumping, a direct dynamics modeling and simulation approach was adopted. In order to identify the role of muscle properties, two types of open loop control signals were applied: STIM(t), representing the stimulation of muscles, and MOM(t), representing net joint movements. In case of STIM control, muscle properties influence the joint moments exerted on the skeleton; in case of MOM control, these moments are directly prescribed. By applying perturbations and comparing the deviations from a reference movement for both types of control, the reduction of the effect of disturbances due to muscle properties was calculated. It was found that the system is very sensitive to perturbations in case of MOM control; the sensitivity to perturbations is markedly less in case of STIM control. It was concluded that muscle properties constitute a peripheral feedback system that has the advantage of zero time delay. This feedback system reduces the effect of perturbations during human vertical jumping to such a degree that when perturbations are not too large, the task may be performed successfully without any adaptation of the muscle stimulation pattern.

Computer Simulation↗

PubMed1993

The influence of the biarticularity of the gastrocnemius muscle on vertical-jumping achievement.

Hypotheses concerning the influence of changes in the design of the human musculoskeletal system on performance cannot be tested experimentally. Computer modelling and simulation provide a research methodology that does allow manipulation of the system's design. In the present study this methodology was used to test a recently formulated hypothesis concerning the role of the biarticularity of the gastrocnemius muscle (GAS) in vertical jumping [Bobbert and van Ingen Schenau, J. Biomechanics 21, 249-262 (1988)]. This was done by comparing maximal jump heights for a model equipped with biarticular GAS with a model equipped with a monoarticular GAS. It was found that jump height decreased by 10 mm when GAS was changed into a monoarticular muscle. Thus, the hypothesis formulated by Bobbert was substantiated, although quantitatively the effect is small. Our result differs from that of Pandy and Zajac [J. Biomechanics 24, 1-10 (1991)], who performed similar model calculations. It is shown that the results described by these authors can be explained from the moment-arm-joint-angle relation of GAS at the knee in their model.

Ankle Joint↗

PubMed1993

The global design of the hindlimb in quadrupeds.

In the hindlimb of quadrupeds three major segments can be distinguished which can rotate with respect to each other in the hip, knee and ankle joints. Movements in a sagittal plane appear to be controlled by three sets of antagonistic mono-articular muscle groups (one set per joint) and two sets of antagonistic bi-articular muscles. This design allows co-activations of mono-articular agonists and their bi-articular antagonists. These co-contractions occur for example in ballistic tasks such as jumping where the transfer of angular acceleration in the joints into the translational acceleration of the hip decreases as a function of joint extension. Well-trained humans appear to solve this problem by a distinct proximodistal sequence in the timing of the joint extensions and a transport of energy via bi-articular muscles. This can also be observed in a number of animals. A second example has to do with the necessity for a certain distribution of net moments in the joints which is required to control the direction and magnitude of the external force on the ground. The control of these moments can to a large extent be judged as a requirement which is independent of the joint displacements required in the same task. Many walking, running or pulling tasks, for example, require knee extension combined with a net knee flexing moment. Co-activation of mono-articular agonists and their bi-articular antagonists appear to solve these problems in an effective and efficient way.

Animals↗

PubMed1993

Function of mono- and biarticular muscles in running.

In this study the function of leg muscles during stretch-shortening cycles in fast running (6 m.s-1) was investigated. For a single stance phase, kinematics, ground reaction forces, and EMG were recorded. First, rough estimates of muscle force, obtained by shifting the EMG curves +90 ms, were correlated with origin-to-insertion velocity (VOI). Second, active state and internal muscle behavior were estimated by using a muscle model that was applied for soleus and gastrocnemius. High correlations were found between estimates of muscle force and VOI time curves for mono-articular hip, knee, and ankle extensor muscles. The correlation coefficients for biarticular muscles were low. The model results showed that active state of gastrocnemius was high during increase of origin-to-insertion length (LOI), whereas active state of soleus was low during the start of increase of LOI and rose to a plateau at the time lengthening ended and shortening started. It seems that the difference in stimulation between gastrocnemius and soleus is a compromise between minimizing energy dissipation and using the stretch-shortening cycle optimally. Furthermore, it was found that the net plantar flexion moment during running reached a value of 302 Nm, which was 158% and 127% higher than the peak values reached in maximal jump and sprint push-offs, respectively. It was argued that the higher mechanical output in running than in jumping could be ascribed to the utilization of the stretch-shortening cycle in running. The higher values in running compared with sprinting, however, may lie in a difference in muscle stimulation.

Adult↗

PubMed1993

Evaluation of moment-angle curves in isokinetic knee extension.

The purpose of this study was to challenge the notion that the relationship of moment of force to angular velocity, determined in isokinetic knee extension tests, reflects the force-velocity relationship of human knee extensors. For this purpose, maximum-effort isometric knee extension moments were collected at seven different knee joint angles, and maximum-effort isokinetic knee extension moments were measured as a function of knee joint angle at angular velocities of 30, 60, 120, and 210 degrees.s-1. For the isokinetic contractions two protocols were used with a different threshold moment, i.e., the minimum knee extension moment that had to be reached isometrically before knee extension was initiated. In one protocol (LTM, for low threshold moment) the threshold was set at 5% of the maximum isometric knee extension moment produced at the starting angle of 85 degrees (180 degrees being full knee extension), in the second protocol (HTM, for high threshold moment) it was set at 95% of this isometric moment. During all contractions, electromyograms were recorded from m. vastus medialis, m. vastus lateralis, m. rectus femoris, and m. biceps femoris. The EMG signals were rectified and smoothed to yield SREMG. Two hypotheses were specifically tested. The first was that the threshold moment does not affect knee extension moments in the middle of the range of motion. Between knee angles of 100 degrees and 145 degrees, no differences were found between moments produced in the HTM protocol and those produced in the LTM protocol. Thus, the first hypothesis was supported. The second hypothesis was that subjects achieve the same level of muscle excitation at different speeds.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1992

Mechanical analysis of the landing phase in heel-toe running.

Results of mechanical analyses of running may be helpful in the search for the etiology of running injuries. In this study a mechanical analysis was made of the landing phase of three trained heel-toe runners, running at their preferred speed and style. The body was modeled as a system of seven linked rigid segments, and the positions of markers defining these segments were monitored using 200 Hz video analysis. Information about the ground reaction force vector was collected using a force plate. Segment kinematics were combined with ground reaction force data for calculation of the net intersegmental forces and moments. The vertical component of the ground reaction force vector Fz was found to reach a first peak approximately 25 ms after touch-down. This peak occurs because, in the support leg, the vertical acceleration of the knee joint is not reduced relative to that of the ankle joint by rotation of the lower leg, so that the support leg segments collide with the floor. Rotation of the support upper leg, however, reduces the vertical acceleration of the hip joint relative to that of the knee joint, and thereby plays an important role in limiting the vertical forces during the first 40 ms. Between 40 and 100 ms after touch-down, the vertical forces are mainly limited by rotation of the support lower leg. At the instant that Fz reaches its first peak, net moments about ankle, knee and hip joints of the support leg are virtually zero. The net moment about the knee joint changed from -100 Nm (flexion) at touch-down to +200 Nm (extension) 50 ms after touch-down. These changes are too rapid to be explained by variations in the muscle activation levels and were ascribed to spring-like behavior of pre-activated knee flexor and knee extensor muscles. These results imply that the runners investigated had no opportunity to control the rotations of body segments during the first part of the contact phase, other than by selecting a certain geometry of the body and muscular (co-)activation levels prior to touch-down.

Acceleration↗

PubMed1992

SPACAR: a software subroutine package for simulation of the behavior of biomechanical systems.

Direct dynamics computer simulation is gaining importance as a research tool in the biomechanical study of complex human movements. Therefore, the need for general-purpose software packages with which the equations of motion can be derived automatically and solved numerically is growing. In this paper such a method is described: SPACAR. The method is compared to well-known commercially available software packages. On the basis of the results obtained on a test problem simulated with both SPACAR and DADS, it is concluded that both methods are accurate; DADS is much faster. The user-friendliness of SPACAR is less than that of DADS. However, SPACAR has two major advantages. First is the basic deformability of all elements, which allows handling of all kinds of problems within a unified framework; second is the full availability of the source code, which allows the experienced user to broaden the scope of possibilities to any extent.

Biomechanical Phenomena↗

PubMed1991

Calculation of vertical ground reaction force estimates during running from positional data.

The purpose of this study was to calculate, as a function of time, segmental contributions to the vertical ground reaction force Fz from positional data for the landing phase in running. In order to evaluate the accuracy of the method, time histories of the sum of the segmental contributions were compared to Fz(t) measured directly by a force plate. The human body was modeled as a system of seven rigid segments. During running the positions of markers defining these segments were monitored using a video analysis system operating at 200 Hz. Special care was taken to minimize marker movement relative to the mass centers of segments, and low-pass cutoff frequencies of 50 Hz (markers defining leg segments) and 15-20 Hz (markers defining upper body) were used in filtering the position time histories so as to ensure that high signal frequencies were preserved. The magnitude of the high-frequency peak in Fz, also known as 'impact force peak', was estimated with errors less than 10%, while the time of occurrence of the peak was estimated with errors less than 5 ms. It would appear that the positional data were sufficiently accurate to be used for calculation of intersegmental forces and moments during the landing phase in running. Analysis of the segmental contributions to Fz(t) revealed that the first peak in Fz has its origin in the contribution of support leg segments, while its magnitude is determined primarily by the contribution of the rest of the body. These contributions could be varied independently by changing running style. It follows that if the possible relationship between 'impact force peaks' and injuries is to be investigated, or if the effects of running shoe and surface construction on these force peaks are to be evaluated, the calculation of segmental contributions to Fz(t) is a more suitable approach than measuring only Fz(t).

Acceleration↗

PubMed1990

Length and moment arm of human leg muscles as a function of knee and hip-joint angles.

Lengths of muscle tendon complexes of the quadriceps femoris muscle and some of its heads, biceps femoris and gastrocnemius muscles, were measured for six limbs of human cadavers as a function of knee and hip-joint angles. Length-angle curves were fitted using second degree polynomials. Using these polynomials the relationships between knee and hip-joint angles and moment arms were calculated. The effect of changing the hip angle on the biceps femoris muscle length is much larger than that of changing the knee angle. For the rectus femoris muscle the reverse was found. The moment arm of the biceps femoris muscle was found to remain constant throughout the whole range of knee flexion as was the case for the medial part of the vastus medialis muscle. Changes in the length of the lateral part of the vastus medialis muscle as well as the medial part of the vastus lateralis muscle are very similar to those of vastus intermedius muscle to which they are adjacent, while those changes in the length of the medial part of the vastus medialis muscle and the lateral part of the vastus lateralis muscle, which are similar to each other, differ substantially from those of the vastus intermedius muscle. Application of the results to jumping showed that bi-articular rectus femoris and biceps femoris muscles, which are antagonists, both contract eccentrically early in the push off phase and concentrically in last part of this phase.

Biomechanical Phenomena↗