Biomedical subjects

Birgit Larsen

Publications and source records attributed to Birgit Larsen.

5 recordsLinked to original sources

PubMed2006

Quadriceps H-reflex modulation during pedaling.

The main aims of this study were 1) to investigate possible phase-, speed-, and task-dependent changes in the quadriceps H-reflex during pedaling, and to achieve this, 2) to develop an optimized H-reflex recording and processing procedure for recording of quadriceps H-reflexes during movement. It was hypothesized that the behavior of the quadriceps H-reflex concerning phase, speed, and task dependency corresponds to the behavior of the soleus H-reflex during rhythmical leg movements. The applied H-reflex procedure appeared to be reliable for obtaining the quadriceps H-reflex modulation during leg movement. The vastus lateralis (VL) and rectus femoris (RF) H-reflexes showed a phase-dependent modulation during pedaling at a frequency of 80 rpm with almost parallel changes in the reflex amplitude and motor recruitment level. However, when the speed of movement was reduced from 80 to 40 revolutions per minute (rpm) and crank load simultaneously increased (i.e., a halving of the movement speed with a constant motor recruitment level), the quadriceps H-reflex modulation pattern changed significantly in relation to the pattern of motor recruitment, i.e., at 40 rpm, the reflex excitability remained high during a gradual derecruitment during power generation in downstroke. Comparison of the "operationally defined H-reflex gain function" obtained during 1) pedaling at 80 rpm and 2) isometric quadriceps contractions in sitting position showed no significant task-dependent changes in the quadriceps H-reflex. Consequently, the hypothesis was only partly corroborated, and the findings indicate differences in the neural control of the soleus and the quadriceps muscle during rhythmical movements.

Adult↗

PubMed2006

Changes in the soleus stretch reflex at different pedaling frequencies and crank loads during pedaling.

The influence of pedaling frequency and crank load on the sensitivity of the soleus short latency stretch reflex (SLR) was examined in nine healthy subjects during pedaling by the use of a custom-built robotic actuator. The SLR decreased successively in downstroke when pedaling frequency increased from 20 to 40 and 60 revolutions per minute at a constant crank load (p = .005). The SLR was unchanged at crank load increases of 2.6 or 5.1 Nm at a constant pedaling frequency (p > .05). Accordingly, it was shown that increased muscle activation level as a consequence of added crank load and increased movement speed does not increase the sensitivity of the soleus SLR.

Adult↗

PubMed2005

The amplitude modulation of the quadriceps H-reflex in relation to the knee joint action during walking.

Previously the modulation of the quadriceps H-reflex has only been investigated in the initial part of the gait cycle, and it was suggested that the quadriceps H-reflex modulates with relative high reflex gain at heel contact and decreases during the subsequent part of stance (Dietz et al. 1990b). The objectives of the present study was to elaborate on the previous results by increasing the measurement resolution around heel contact and include additional measures in order to relate the H-reflex modulation to the mechanical function of the knee extensors throughout the gait cycle. EMG profiles were measured in quadriceps and the antagonistic hamstring muscles simultaneously with the knee joint kinematics in ten subjects during treadmill walking at preferred speed. H-reflex excitability was measured in vastus lateralis (VL) and rectus femoris (RF) at 11 selected positions during the gait cycle. The resulting excitability curves showed a significant modulation of the quadriceps H-reflex during the gait cycle. The H-reflex amplitude increases shortly after heel contact and reflex inhibition is present in the remaining part of stance and most of the swing phase. The modulation of the quadriceps H-reflex during walking does not follow the classical pattern of reciprocal inhibition between antagonistic muscles. It is suggested that at least during the stance phase the modulation of the quadriceps H-reflex is controlled by presynaptic inhibition. The present results confirm the idea that the excitability of the quadriceps H-reflex is controlled to comply with the different mechanical demands on the muscle during the gait cycle in humans.

Adult↗

PubMed2004

Changes in the gain of the soleus H-reflex with changes in the motor recruitment level and/or movement speed.

The behaviour of the soleus H-reflex is considered to be motor task-dependent. However, the speed of movements as well as the motor recruitment level is altered when motor tasks are changed. Therefore it is ambiguous to what extent the motor task-dependent changes found between walking and running, for example, are simply due to changes in these two parameters. The purpose of this study was to investigate how movement speed and motor recruitment level separately influence the soleus H-reflex behaviour when the motor task is unchanged. Soleus H-reflexes were elicited during pedalling at different cadences and crank loads, by which movement speed and muscle recruitment level were modified separately. The H-reflex gain was expressed as the slope of the linear relation between the reflex amplitudes and the background electromyelogram (EMG), and the reflex threshold was expressed by the intercept. The results showed a decrease in reflex gain by 54% ( P=0.001) when the speed of movement was doubled from 40 to 80 rpm (repetitions per minute) without changes in the level of soleus EMG activity. Reflex gain decreased 40% ( P=0.002) when the soleus EMG level was increased by 47% without changing the speed of movement. No significant changes were found in the reflex threshold. We conclude that as speed of movement and motor recruitment level influence the gain of the soleus H-reflex, it is significant that these two parameters are comparable before changes in H-reflexes are stated to be task-dependent.

Adaptation, Physiological↗

PubMed2002

A task dependent change in the medium latency component of the soleus stretch reflex.

In comparison to the H-reflex, the task dependency of the human stretch reflex during locomotive and postural tasks has not received a great deal of attention in the literature. The few studies on reflex task dependency that have been performed to date have concentrated on either the group Ia mediated H-reflex or the short latency stretch reflex. In the present study the medium latency component of a mechanically evoked stretch reflex is investigated during walking, pedalling, and sitting. Stretch reflexes were evoked in the soleus muscle using dorsiflexion perturbations generated with a portable stretching device. Perturbations of equal amplitude and velocity (8 deg, 300 deg/s) were presented to 16 healthy subjects while they walked on a treadmill and pedalled a cycle ergometer. For eight of these subjects, an additional set of data was collected as they sat on the ergometer holding a steady posture. Perturbations were presented in the early to mid stance phase of walking and the downstroke of the pedal cycle. During all three conditions, the background soleus muscle activity was matched. The short (SLR) and medium (MLR) components of the soleus reflex responses were quantified by calculating the area of each burst in a 15-ms window centred on the peak of the respective burst. In addition, the stretch velocity-stretch reflex input-output curve was examined for the two locomotion tasks over a range of velocities from 100 to 400 deg/s. Peak latencies for the two reflex responses were observed at 52+/-5/77+/-6 ms (SLR/MLR) for walking, 51+/-3/76+/-6 ms (SLR/MLR) for pedalling, and 50+/-3/76+/-7 ms for sitting. A statistically significant increase in the magnitude of the MLR was observed during walking compared with pedalling and sitting ( P=0.007), whereas no difference in magnitude was observed between the three tasks for the SLR ( P=0.616). Furthermore, no difference was observed in the stretch velocity-stretch reflex input-output relationship between walking and pedalling. It is suggested that the medium component of the stretch reflex response is modulated to provide increased control for the postural demands of walking.

Adult↗