Jw. van den Berg

EMG to force processing I: An electrical analogue of the Hill muscle model.

Abstract A processing method is presented by which the surface electromyogram can be processed to the muscle force. The smoothed rectified EMG and the joint angle (proportional to the muscle length) are the inputs of an electrical analogue of the Hill muscle model. The output of the analogue is the torque around the joint due to the muscle force and, by multiplication with the joint rotation, the muscle work. Both can be obtained instantaneously, irrespective of the type of contraction (isometric, isotonic, auxotonic, etc.). The processor is described and some potentialities are shown. Methods by which to find the parameter values of the muscle model and methods for the evaluation of the processor performance are discussed. The relevant experiments, on the human calf muscles (M. triceps surae), will be described in the subsequent parts of this series of papers.

Linearity between the weighted sum of the EMGs of the human triceps surae and the total torque

Abstract The relation between mean rectified EMG and muscle torque was investigated on the muscles of the human triceps surae for slowly varying isometric contractions. It was found that the mean rectified EMG of any of the muscles involved (soleus, gastrocnemius medial head and lateral head) is linearly proportional to the torque it develops. As a consequence the sum of the individual musle torques as derived from their EMGs was equal to the mechanically measured total torque, though the ratio of the contributing individual torques was far from constant. A method is given for the determination of the proportionality factor for each muscle. This method is based on the fact that it is possible to prevent the gastrocnemius from developing torque by shortening it strongly. This was brought about by bending the knee, which changes gastrocnemius length without altering soleus length. The influence of fatigue was negligible during the experiments.

EMG to force processing II: Estimation of parameters of the Hill muscle model for the human triceps surae by means of a calfergometer

On a calfergometer isotonic contractions were performed by the calf muscles of human subjects. The measured torque Mm was compared to the torque M processed from the EMG and the joint angle by means of an electronic processor based on the Hill muscle model. By means of these experiments the model parameters were determined for the torque-angle relation, the torque-angular velocity relation and the parallel elastic component (PEC). Experiments were done (a) for the soleus only (4 subjects) and (b) for the combination of soleus and gastrocnemius (8 subjects). It turned out that: • — the torque-angle parameters varied among subjects; • — the torque-angular velocity parameters for soleus only varied also among subjects; • — for the combined calf muscle group the interindividual differences in the torque-angular velocity parameters could be neglected; • — one of the parameters of the PEC was the same for all subjects, while the other was variable among the subjects. The obtained parameter values are discussed and compared with available literature data. Values for the r.m.s. and the peak error in the isotonic phases of the contraction are given and compared with values predicted from the stochastic properties of the EMG signal.

EMG to force processing III: Estimation of model parameters for the human triceps surae muscle and assessment of the accuracy by means of a torque plate.

The parameters of the active state and the series-elastic component of the Hill muscle model were estimated for the human calf muscles by means of a ‘torque plate’ on seven normal subjects. Together with the results of the calfergometer experiments (Part II) this yields the complete set of parameters for the EMG to force processor based on the Hill model (Part I). The accuracy of the processor was assessed by means of a set of severe test contractions in which the subjects tilted on one foot at various velocities (slow to as fast as possible). The mechanical work ∫ M ϕdt and the integrated torque ∫ M dt were calculated for these contractions, M being the torque calculated by the EMG processor and ϕ the measured angular velocity. These values of the work and the integrated torque were compared to those obtained with the measured torque Mm instead of M. The relative error, for positive work, negative work, and integrated torque was + 6.2% ± 14% (means ± s.d.). Data on the peak values of M – Mm are also given.

The Origin of the Variations of Body Impedance Occurring during the Cardiac Cycle

If the trunk is placed between two electrodes of a high frequency circuit, changes in impedance occur during the cardiac cycle. Experiments are presented which show that these variations do not result from the actual volume changes of the heart, as has been suggested in the literature. The changes in impedance are caused by the rhythmic variations in blood content of the vessels.

An ultrasonic detector for microgasemboli in a bloodflow line

Abstract A detector for small gas bubbles in the blood-flow line from a heart-lung machine was constructed and tested. The way in which a passing bubble forms a signal is analyzed. The relation between signal size and bubble size is established experimentally and confirmed by theoretical calculations. It is shown that solid and soft (tissuelike) particles give much smaller signals than gaseous particles. The influence of the measuring frequency on the detectability of particles is derived from theory and shown experimentally.

EMG to force processing IV: Eccentric-concentric contractions on a spring-flywheel set up

Abstract In walking, and in many other common activities, the calf muscles contract in such a way that muscle stretching (eccentric contraction) precedes muscle shortening (concentric contraction). The performance of the EMG to force processor described in Part I of this series of papers is evaluated for this kind of contraction by means of a pedal driven spring-flywheel set-up. Values for the work ∫ M ϕ d t and the integrated torque ∫ M dt were both measured and obtained from the EMG processor. The results indicate that the error of the processor for eccentric-concentric contractions is of the same order of magnitude as found for concentric-eccentric contractions (Part III). This finding, together with the satisfactory degree of accuracy, suggests that the EMG to force processing method can be a useful tool for the measurement of the force and work of a single muscle, or synergistic muscle group, in biomechanics.

SOUND PRODUCTION IN ISOLATED HUMAN LARYNGES

Isolated human larynges were first used to study the function of the larynx by Johannes Muller in his classical experiments described in 1840. A century before Miiller, Ferrein (1741) was the first to use isolated larynges, those of dogs, to investigate whether they should be classified as instruments “a cordes” or “a vent,” i.e., stringed or wind instruments. His report is still of interest. TO his surprise he found an instrument which should be classified as “a cordes et vent,” which meant that the larynx was actuated by air, but that the vibrations of the vocal folds were also essential, in contrast to the complex hypotheses of Dodart (1700) who tried to explain everything by means of eddies generated in the glottis. The accuracy of Ferrein’s conclusions is very limited, because he had virtually no equipment. Apparently he studied only the falsetto voice. He also introduced the misleading term “cordes vocales,” vocal cords, but his notion that the vibration of the vocal folds is essential was a fundamental one. However, he over-emphasized its importance in his belief that the sound was directly due to the vibrating folds. Muller’s data are much more detailed and accurate than those of Ferrein, but it took Helmholtz (1863) to define clearly that the puffs of air escaping through the vibrating glottis were the primary source of sound. Miiller investigated the influence of a number of parameters on the response of the larynx, primarily in midand falsetto voice, but his equipment was very limited. H e had to rely cpon the acoustic quality of the sounds produced by the larynges without their attached resonating vocal cavities. Nobody had seen a living larynx at work, as the laryngeal mirror and the stroboscopic principle had not yet been invented. Since Miiller’s time, a number of investigators have used isolated human larynges for various purposes, especially W. Trendelenburg and associates (1937), but none of them have undertaken a systematic analysis like Miiller’s. Therefore, we decided in 1956 to reinvestigate the influence of the parameters with modern equipment (van den Berg & Tan, 1959; van den Berg, 1960, 1961, 1962; van den Berger al., 1960).

Detection of csf flow in ventriculo-atrial shunts by cold transfer.

A method of detecting CSF flow in ventriculo‐atrial shunts by means of a thermosensitive device recording cold transfer was described in a previous paper. The method was not as reliable as desired because of the effect of the intervening tissue between the thermistor and the cathétér. A new method using two thermistors instead of one has now been developed and makes it possible to determine the rate of CSF flow more precisely.

How much energy can be stored in human muscle elasticity?: Comment on: ‘An alternative view of the concept of utilisation of elastic energy in human movements’

Abstract In a paper by G.J. van Ingen Schenau (Human Movement Science 3, 301–336) it is stated that the amount of elastic energy that can be stored in muscles and tendons under tension is far too small to explain the reported advantages of pre-stretch before a concentric contraction in a number of human movements. In our opinion the estimates of the maximal amount of stored elastic energy can be substantially higher, if it is considered that the muscle may first ‘take up the slack’ at the onset of a contraction. The amounts of elastic energy are then sufficient for a complete transfer of negative work into positive work for the human ankle plantarflexors and knee extensors in walking and running.