Biomedical subjects

A Gollhofer

Publications and source records attributed to A Gollhofer.

28 records · Page 2Linked to original sources

PubMed1990

Reproducibility of electromyographic patterns in stretch-shortening type contractions.

Within a set of repeated measurements, 12 male subjects performed the following types of muscle contraction: (1) running at constant velocity (12 km.h-1); (2) one-leg hopping at the frequency preferred; (3) both-legs hopping at the frequency preferred; (4) drop jumps from a height of 0.40 m. The surface electromyographs (EMG) from the left and right legs were recorded, together with the averaged signals from vertical ground reaction forces and angular displacement of the ankle joint. Reliability coefficients for the day to day and the week to week comparison were calculated. In addition, on any one test day the positions of the electrodes were systematically changed. The analysis of various EMG parameters (integrated-EMG) revealed high coefficients for both comparisons as well as for the total set of measurements. Qualitative comparisons of the EMG showed a high reproducibility of the shape of the patterns. If the relative position of the electrodes with respect to the belly of the muscle is changed, the amplitudes of the EMG recordings are markedly reduced. It is concluded that surface EMG is a reliable method for studies of the neuromuscular system. On the basis of the data presented, it is suggested that the term "reliability" should be replaced by the term "reproducibility" in order to cover both the aspect of the relative stability plus the aspects of linear changes and the scattering of the data in repeated measurements.

Adult↗

PubMed1989

Human postural reflexes and gravity--an under water simulation.

This study represents the first attempt to investigate the influence of gravity on postural adjustments. Subjects were displaced while standing under water on a movable platform, while the buoyancy of the body was adjusted by using a variety of lead vests. Under water, an approximately linear relationship was found between body weight and impulse directed electromyographic response amplitudes in the leg and thigh muscles. Loading of the subjects out of water resulted in a saturation of the response amplitude. The biomechanical signals recorded during the displacements indicated that neither vestibulospinal nor muscle proprioceptive reflex mechanisms can account for the effect observed under water. It is suggested that the EMG responses are mediated by reflexes which are activated by pressure receptors within the body in order to hold the centre of gravity over the feet.

Adult↗

PubMed1989

Compensation of translational and rotational perturbations in human posture: stabilization of the centre of gravity.

EMG responses in the leg muscles, head acceleration and joint movements induced by dorsiflexing rotation of the ankle have been analysed. Perturbations were induced while subjects were standing on a platform with the ankle joints co-linear with the axis of rotation or while standing above the axis (between 5 and 25 cm), which introduced an additional translational component to the displacement. The former condition was followed by a monosynaptic gastrocnemius reflex potential and a late tibialis anterior activation (latency about 100 ms); in the latter condition the monosynaptic reflex response became smaller and a longer latency gastrocnemius response (latency about 70 ms) appeared, the strength and duration of which increased in parallel with the translational component, while the amplitude of the tibialis anterior activity decreased. Neither vestibulospinal nor muscle proprioceptive mechanisms can solely account for this effect as the biomechanical parameters were little changed in the different conditions. It is suggested that the controlled variable in the investigated task is to hold the bodies' centre of gravity over the feet which makes the change in the pattern meaningful: pure dorsiflexion of the feet is followed by a backwards sway of the body, thus a tibialis anterior activation is functionally essential to hold the centre of gravity over the feet. A backwards translation of the feet withdraws the supporting surface, thus a gastrocnemius activation is needed to restore the centre of gravity over the feet. The inhibition of the monosynaptic reflex with increasing translation is suggested to arise from a reciprocal modulation of mono- and polysynaptic gastrocnemius reflex responses.

Acceleration↗

PubMed1988

Reproducibility and adaptation of the EMG responses of the lower leg following perturbations of upright stance.

The electromyographic (EMG) response of gastrocnemius, soleus and anterior tibialis muscles to a backwardly directed perturbation of stance was recorded in 12 normal subjects using surface electrodes and studied with regard to its reproducibility (test-retest reliability coefficients, variability coefficients) and to adaptational effects. (1) Reproducibility was shown to be uniformly high and can be interpreted as an index for the high intraindividual constancy of the results. (2) Adaptational effects have been found and should be circumvented, either by pre-adapting the subjects to the motor task, or by restriction of the period of measurement. (3) Variation of the position of the electrodes produced only small effects. The impedance of the surface electrodes was not critical if kept below 5 k omega. EMG investigations with surface electrodes during stance and perturbations of stance provide highly reliable results with respect to intraindividual changes but interindividual variability of the results clearly marks the limits of this method. The interindividual variability observed with surface electrodes is of the same order as that reported in the literature for inserted needle recording.

Adaptation, Physiological↗

PubMed1987

Fatigue during stretch-shortening cycle exercises: changes in mechanical performance of human skeletal muscle.

Stretch-shortening cycle (SSC), which is a normal contraction behavior of muscle, was used as a model to investigate muscular fatigue. Nine male volunteers were subjected to 100 repeated and exhaustive SSC contractions of the forearm extensors using a special sledge apparatus incorporating a force plate system. The fatigue contractions were performed on submaximal levels but the before-after comparison also included maximal drop-jump condition on the sledge as well as falls on to the floor. The results indicated that in the 100 submaximal SSCs the fatigue was characterized by increases in the contact times for both the eccentric and concentric phases of SSC, but the influence was more pronounced on the concentric part. The force-time curves during contact on the platform were influenced by fatigue so that the initial force peak became higher and the subsequent initial drop of force more pronounced. During submaximal and maximal drops, the angular velocities changed in the two phases of SSC. With progressing fatigue, the eccentric maximal angular velocity increased and the corresponding concentric velocities decreased. These changes were accompanied by slight changes in the elbow joint mechanism with respect to the contact, release, and maximal flexion angles. The results suggest that repeated SSC induces fatigue and the fatigue effects on the mechanical behavior of the muscle are very much similar to those induced by either isometric or concentric fatigue contractions. However, the transfer of the energy between eccentric and concentric phases was drastically reduced and this implies that SSCs can be used effectively to examine the fatiguability of the system regulating muscle stiffness during exercise.

Adult↗

PubMed1987

Interaction between man and shoe in running: considerations for a more comprehensive measurement approach.

Man-shoe-surface interaction in running is a complex phenomenon, and its investigation gives specific requirements for the measuring system. This study was designed to make an effort to develop a methodology for measuring the interaction between the first two components (man and shoe) under normal heel running conditions both on the force plate and on an asphalt road. The force plate system consisted of a series of 1.5-m long plates with a total length of 12 m. This allowed recordings of several natural ground contact phases in one run. By repeating the runs several times at constant velocity (3 m X s-1 and 5 m X s-1), altogether 10-30 Fz and Fx force curves could be obtained for further computerized averaging. The running shoes were equipped with special heel and toe contact sensors, which were used for recordings of even more cycles at constant velocity on the road running conditions. Telemetered EMG technique was employed to examine the response of the selected lower extremity muscles on the varying shoe and running velocity conditions. The results indicated preliminarily that the changes in ground reaction forces were more velocity than shoe (hard/soft) dependent and that EMG activation patterns were muscle specific with regard to preinnervation and impact and push-off phases for the gastrocnemius, rectus femoris, vastus lateralis, and tibialis anterior muscles. Although the actual measurements were not yet designed for comprehensive recording of each parameter, the results obtained suggest that the major leg extensor muscles change their activation patterns with the varying impact load conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1987

Fatigue during stretch-shortening cycle exercises. II. Changes in neuromuscular activation patterns of human skeletal muscle.

Detailed electromyographic (EMG) analysis of primarily triceps brachii muscle was carried out on subjects who performed 100 repeated and exhaustive stretch-shortening cycle (SSC) exercises on a special sledge apparatus incorporating a force plate. The fatigue contractions were performed on submaximal levels but the before-after comparison included also maximal "drop jumps" on the sledge as well as falls on to the floor. SSC contractions caused clear reduction in the mechanical performance capacity of the muscles. EMG analysis revealed that integrated EMG (IEMG) increased during the course of fatigue both during eccentric and concentric phases of the cycle. However, when IEMG was time-normalized, the changes were nonsignificant. The IEMG/force ratio increased especially during the eccentric phase both in submaximal and maximal drops. In maximal drops, there was also a great reduction in concentric IEMG. Rectified EMG curves were also averaged in blocks of ten successive fatigue contractions. This analysis revealed clear appearance of short (SLC), medium (MLC), and long (LLC) latency reflex components in most of the subjects during the early contact on the platform. The fatigue effects on these components were individual and also condition dependent so that in maximal drops there was a trend for reflex potentiation. In maximal drops on to the floor, which represented the highest stretch loads on the muscles, there was a tendency for reduction of amplitudes of the reflex components. The results suggest that in exhaustive SSC exercise muscular activation processes are reduced and primarily in the eccentric phase. Muscle spindles are suggested to become differentially affected by fatigue substances depending on the load intensity during eccentric contraction.

Biomechanical Phenomena↗

PubMed1986

Compensatory movements following gait perturbations: changes in cinematic and muscular activation patterns.

The present study was designed to investigate the neuronal control and biomechanical effects of stumbling reactions. To induce these perturbations, a specially prepared treadmill was used which allowed rapid and powerful changes of treadmill speed. EMG recordings of leg muscles were related to cinematic movement patterns. Following an accelerating impulse, the standing leg was displaced in a posterior direction compared with normal gait, with a premature touchdown of the contralateral, swinging leg, forward of the body axis. This was associated with simultaneous activation of the gastrocnemius and biceps femoris of the ipsilateral and the tibialis anterior muscles of the contralateral leg. Following deceleration, the standing leg was displaced in an anterior direction and the contralateral swinging leg touched the ground prematurely behind the body axis. This was associated with activation of the ipsilateral tibialis anterior and rectus femoris and contralateral tibialis anterior muscles. In both conditions, the activation of the lower leg muscles on both sides occurred with a latency of 60-70 ms, monosynaptic responses being absent. In both cases, early touchdown was associated with a forward extension of the arm. It is concluded that the EMG responses are mediated by a polysynaptic spinal pathway to keep the body center of gravity constant and to prevent falling.

Ankle Joint↗

PubMed1984

Regulation of muscle stiffness in human locomotion.

The tension development of the triceps surae muscle was analyzed during the stance phase of locomotion at different speeds with respect to its electrical activation and its lengthening and shortening behavior. The main point was the coincidence of maximal dorsiflexion, maximal Achilles tendon tension, transition from amortization to acceleration phase, and projection of the center of gravity to the supporting leg. It is concluded that the stiffness regulation needed for load compensation after impact and pushing off at the end of stance phase depends on modulation and enhancement of a centrally programmed muscle activation by a segmental stretch reflex activity.

Achilles Tendon↗

PubMedDate not supplied

Neuromuscular properties and functional aspects of taped ankles.

We used electromyographic and goniometric methods to test 40 subjects to describe the neuromuscular and biomechanical adaptation of the ankle with respect to application of two different adhesive tapes and to exercises. The neuromuscular responses to inversion injury simulation, together with the mechanical displacements of the joint complex, were analyzed before and after controlled athletic exercises. The proprioceptive amplification ratio was calculated on the basis of the integrated reflex electromyographic results and on the maximum inversion amplitude. Relevant stability gains were achieved immediately after applying tape. There was reduced tape stability after athletic exercise for one of the two tape materials tested. No further loosening was detected, even after prolonged wearing of tape (24 hours). Compared with the unprotected ankle, the taped ankle had a significant increase in the proprioceptive amplification ratio. Both fatigue and mechanical loosening may be responsible for the significant reduction in this ratio immediately after exercise. After the 24-hour interval, the ratio was increased, which could be explained by physiologic neuromuscular regeneration and mechanical restabilization of the tape itself. The sensitivity of the proprioceptive amplification ratio, both to external stabilization and to internal fatigue, supports its potential value to quantify functional joint stability.

Adolescent↗