Ramona Ritzmann

The effect of whole body vibration on the H-reflex, the stretch reflex, and the short-latency response during hopping

The effect of whole body vibration (WBV) on reflex responses is controversially discussed in the literature. In this study, three different modalities of reflex activation with increased motor complexity have been selected to clarify the effects of acute WBV on reflex activation: (1) the electrically evoked H‐reflex, (2) the mechanically elicited stretch reflex, and (3) the short‐latency response (SLR) during hopping. WBV‐induced changes of the H‐reflex, the stretch reflex, and the SLR during hopping were recorded in the soleus and gastrocnemius muscles and were analyzed before, during (only the H‐reflex), immediately after, 5 min and 10 min after WBV. The main findings were that (1) the H‐reflexes were significantly reduced during and at least up to 5 min after WBV, (2) the stretch reflex amplitudes were also significantly reduced immediately after WBV but recovered to their initial amplitudes within 5 min, and (3) the SLR during hopping showed no vibration‐induced modulation. With regard to the modalities with low motor complexities, the decreased H‐ and stretch reflex responses are assumed to point toward a reduced Ia afferent transmission during and after WBV. However, it is assumed that during hopping, the suppression of reflex sensitivity is compensated by facilitatory mechanisms in this complex motor task.

A new sledge jump system that allows almost natural reactive jumps

AIM Sledge jump systems (SJS) are often employed to examine the underlying mechanical and neuromuscular mechanisms of the stretch-shortening cycle (SSC) as they allow the systematic variation of impact velocity and energy. However, in existing SJS the jumps are not very comparable to natural jumps because of the long contact times (∼200%), which prevent the storage of kinetic energy. The aim of the present study was to evaluate if an ultra-light sledge, built in a way that joint movement is barely restricted, allows jumps that are comparable to natural jumps. METHODS Ground reaction forces, kinematic and electromyographic (EMG) data of 21 healthy subjects were compared between normal hoppings (NH) on the ground and hoppings in a custom-built SJS (sledge hoppings, SH). RESULTS Normalized to NH, the ground contact times for the SH were prolonged (+22%), while the peak forces (-21%) and the preactivity of the soleus and gastrocnemius medialis muscles were reduced (-20% and -22%, respectively). No significant changes were observed for the iEMG of the short latency response of those muscles (+1% and +8%) and the ranges of motion in the ankle, knee and hip joint (differences of 1, 1 and 2 degrees). The reduced peak forces were associated with a reduced leg stiffness (-21%). CONCLUSION The new system allows reactive jumps that are rather comparable to natural jumps. Therefore, the new SJS seems to be an adequate system in order to examine the SSC under controlled and almost natural conditions.

Load Dependency of Postural Control--Kinematic and Neuromuscular Changes in Response to over and under Load Conditions.

Introduction Load variation is associated with changes in joint torque and compensatory reflex activation and thus, has a considerable impact on balance control. Previous studies dealing with over (OL) and under loading (UL) used water buoyancy or additional weight with the side effects of increased friction and inertia, resulting in substantially modified test paradigms. The purpose of this study was to identify gravity-induced load dependency of postural control in comparable experimental conditions and to determine the underlying neuromuscular mechanisms. Methods Balance performance was recorded under normal loading (NL, 1g), UL (0.16g; 0.38g) and OL (1.8g) in monopedal stance. Center of pressure (COP) displacement and frequency distribution (low 0.15-0.5Hz (LF), medium 0.5-2Hz (MF), high 2-6Hz (HF)) as well as ankle, knee and hip joint kinematics were assessed. Soleus spinal excitability was determined by H/M-recruitment curves (H/M-ratios). Results Compared to NL, OL caused an increase in ankle joint excursion, COP HF domain and H/M-ratio. Concomitantly, hip joint excursion and COP LF decreased. Compared to NL, UL caused modulations in the opposite direction: UL decreased ankle joint excursions, COP HF and H/M-ratio. Collaterally, hip joint excursion and COP LF increased. COP was augmented both in UL and in OL compared to NL. Conclusion Subjects achieved postural stability in OL and UL with greater difficulty compared to NL. Reduced postural control was accompanied by modified balance strategies and compensatory reflex activation. With increasing load, a shift from hip to ankle strategy was observed. Accompanying, COP frequency distribution shifted from LF to HF and spinal excitability was enhanced. It is suggested that in OL, augmented ankle joint torques are compensated by quick reflex-induced postural reactions in distal muscles. Contrarily, UL is associated with diminished joint torques and thus, postural equilibrium may be controlled by the proximal segments to adjust the center of gravity above the base of support.

Improved postural control in response to a 4-week balance training with partially unloaded bodyweight

Balance training (BT) is successfully implemented in therapy as a countermeasure against postural dysfunctions. However, patients suffering from motor impairments may not be able to perform balance rehabilitation with full body load. The purpose of this study was to investigate whether partial unloading leads to the same functional and neuromuscular adaptations. The impact on postural control of a 4-week BT intervention has been compared between full and partial body load. 32 subjects were randomly assigned to a CON (conventional BT) or a PART group (partially unloaded BT). BT comprised balance exercises addressing dynamic stabilization in mono- and bipedal stance. Before and after training, centre of pressure (COP) displacement and electromyographic activity of selected muscles were monitored during different balance tasks. Co-contraction index (CCI) of soleus (SOL)/tibialis (TA) was calculated. SOL H-reflexes were elicited to evaluate changes in the excitability of the spinal reflex circuitry. Adaptations in response to the training were in a similar extent for both groups: (i) after the intervention, the COP displacement was reduced (P<0.05). This reduction was accompanied by (ii) a decreased CCI of SOL/TA (P<0.05) and (iii) a decrease in H-reflex amplitude (P<0.05). BT under partial unloading led to reduced COP displacements comparable to conventional BT indicating improved balance control. Moreover, decreased co-contraction of antagonistic muscles and reduced spinal excitability of the SOL motoneuron pool point towards changed postural control strategies generally observed after full body load training. Thus, BT considering partial unloading is an appropriate alternative for patients unable to conduct BT under full body load.

Balance impairments and neuromuscular changes in breast cancer patients with chemotherapy-induced peripheral neuropathy

OBJECTIVE Chemotherapy-induced peripheral neuropathy (CIPN) is a common side effect of cancer treatment. Resulting sensory and motor dysfunctions often lead to functional impairments like gait or balance disorders. As the underlying neuromuscular mechanisms are not fully understood, we compared balance performance of CIPN patients with healthy controls (CON) to specify differences responsible for postural instability. METHODS 20 breast cancer patients with CIPN (PAT) and 16 matched CONs were monitored regarding centre of pressure displacement (COP) and electromyographic activity of M. soleus, gastrocnemius, tibialis anterior, rectus femoris and biceps femoris. We calculated antagonistic co-contraction indices (CCI) and elicited soleus H-reflexes to evaluate changes in the elicitability and sensitivity of spinal reflex circuitry. RESULTS PATs COP displacement was greater than CONs (p=.013) and correlated significantly with the level of CCIs and self-reported CIPN symptoms. PAT revealed prolonged H-wave latency (p=.021), decreased H-reflex elicitability (p=.001), and increased H-reflex sensitivity from bi- to monopedal stance (p=.004). CONCLUSIONS We summarise that CIPN causes balance impairments and leads to changes in elicitability and sensitivity of spinal reflex circuitry associated with postural instability. We assume that increased simultaneous antagonistic muscle activation may be used as a safety strategy for joint stiffness to compensate for neuromuscular degradation. SIGNIFICANCE Sensorimotor training has the potential to influence neuromuscular mechanisms in order to improve balance performance. Therefore, this training modality should be evaluated as a possible treatment strategy for CIPN.

Four weeks of training in a sledge jump system improved the jump pattern to almost natural reactive jumps

In spite of extensive training regimens during long-term space missions with existing training devices, astronauts suffer from muscle and bone loss. It has been suggested that reactive jumps inducing high forces in the muscles—consequently exposing the bones to high strains—help to counteract these degradations. In a previous study, a new sledge jump system (SJS) was found to allow fairly natural reactive jumps. The aim of the present study was to evaluate if training in the SJS would further reduce the differences between jumps in the SJS and normal jumps, particularly with respect to ground reaction forces (GRF) and rate of force development (RFD). Sixteen participants in a training group (TG) and 16 in a control group (CON) were tested before and after the TGs four-week hopping training in the SJS. During the tests, kinetic, kinematic and electromyographic data were compared between hops on the ground and in the SJS. After the training period, the GRF, the RFD and the leg stiffness in the SJS significantly increased for the TG (but not for CON) by 10, 35 and 38%, respectively. The kinematic and electromyographic data showed no significant changes. A short training regimen in the SJS reduced the differences between jumps in the SJS and normal jumps. Considering that a natural movement that exposes the muscles and thus also the bones to high loads is regarded as important for the preservation of muscle and bone, the SJS seems to be a promising countermeasure.

Whole Body Vibration Training - Improving Balance Control and Muscle Endurance

Exercise combined with whole body vibration (WBV) is becoming increasingly popular, although additional effects of WBV in comparison to conventional exercises are still discussed controversially in literature. Heterogeneous findings are attributed to large differences in the training designs between WBV and “control” groups in regard to training volume, load and type. In order to separate the additional effects of WBV from the overall adaptations due to the intervention, in this study, a four-week WBV training setup was compared to a matched intervention program with identical training parameters in both training settings except for the exposure to WBV. In a repeated-measures matched-subject design, 38 participants were assigned to either the WBV group (VIB) or the equivalent training group (CON). Training duration, number of sets, rest periods and task-specific instructions were matched between the groups. Balance, jump height and local static muscle endurance were assessed before and after the training period. The statistical analysis revealed significant interaction effects of group×time for balance and local static muscle endurance (p<0.05). Hence, WBV caused an additional effect on balance control (pre vs. post VIB +13%, p<0.05 and CON +6%, p = 0.33) and local static muscle endurance (pre vs. post VIB +36%, p<0.05 and CON +11%, p = 0.49). The effect on jump height remained insignificant (pre vs. post VIB +3%, p = 0.25 and CON ±0%, p = 0.82). This study provides evidence for the additional effects of WBV above conventional exercise alone. As far as balance and muscle endurance of the lower leg are concerned, a training program that includes WBV can provide supplementary benefits in young and well-trained adults compared to an equivalent program that does not include WBV.

Leg stiffness can be maintained during reactive hopping despite modified acceleration conditions

AIM The aim of the present study was to evaluate reactive hops under systematically modified acceleration conditions. It was hypothesized that a high preactivity of the leg extensors and phase-specific adjustments of the leg muscle activation would compensate the alterations caused by the various acceleration levels in order to maintain a high leg stiffness, thus enabling the jumper to perform truly reactive jumps with short ground contact times despite the unaccustomed acceleration conditions. METHODS Ground reaction forces (GRF), kinematic and electromyographic data of 20 healthy subjects were recorded during reactive hopping in a special sledge jump system for seven different acceleration levels: three acceleration levels with lower than normal gravity (0.7g, 0.8g, 0.9g), one with gravitational acceleration (1g) and three with higher acceleration (1.1g, 1.2g, 1.3g). RESULTS The increase of the acceleration from 0.7g to 1.3g had no significant effect on the preactivity of the leg extensors, the leg stiffness and the rate of force development. However, it resulted in increased peak GRF (+15%), longer ground contact time (+10%) and increased angular excursion at the ankle and knee joints (+3°). DISCUSSION Throughout a wide acceleration range, the subjects were able to maintain a high leg stiffness and perform reactive hops by keeping the preactivity constantly high and adjusting the muscle activity in the later phases. In consequence, it can be concluded that the neuromuscular system can cope with different acceleration levels, at least in the acceleration range used in this study.

Reactive Balance Control in Response to Perturbation in Unilateral Stance: Interaction Effects of Direction, Displacement and Velocity on Compensatory Neuromuscular and Kinematic Responses.

Unexpected sudden perturbations challenge postural equilibrium and require reactive compensation. This study aimed to assess interaction effects of the direction, displacement and velocity of perturbations on electromyographic (EMG) activity, centre of pressure (COP) displacement and joint kinematics to detect neuromuscular characteristics (phasic and segmental) and kinematic strategies of compensatory reactions in an unilateral balance paradigm. In 20 subjects, COP displacement and velocity, ankle, knee and hip joint excursions and EMG during short (SLR), medium (MLR) and long latency response (LLR) of four shank and five thigh muscles were analysed during random surface translations varying in direction (anterior-posterior (sagittal plane), medial-lateral (frontal plane)), displacement (2 vs. 3cm) and velocity (0.11 vs. 0.18m/s) of perturbation when balancing on one leg on a movable platform. Phases: SLR and MLR were scaled to increased velocity (P<0.05); LLR was scaled to increased displacement (P<0.05). Segments: phasic interrelationships were accompanied by segmental distinctions: distal muscles were used for fast compensation in SLR (P<0.05) and proximal muscles to stabilise in LLR (P<0.05). Kinematics: ankle joints compensated for both increasing displacement and velocity in all directions (P<0.05), whereas knee joint deflections were particularly sensitive to increasing displacement in the sagittal (P<0.05) and hip joint deflections to increasing velocity in the frontal plane (P<0.05). COP measures increased with increasing perturbation velocity and displacement (P<0.05). Interaction effects indicate that compensatory responses are based on complex processes, including different postural strategies characterised by phasic and segmental specifications, precisely adjusted to the type of balance disturbance. To regain balance after surface translation, muscles of the distal segment govern the quick regain of equilibrium; the muscles of the proximal limb serve as delayed stabilisers after a balance disturbance. Further, a kinematic distinction regarding the compensation for balance disturbance indicated different plane- and segment-specific sensitivities with respect to the determinants displacement and velocity.

Acute exposure to microgravity does not influence the H-reflex with or without whole body vibration and does not cause vibration-specific changes in muscular activity

PURPOSE Many potential countermeasures for muscle and bone loss caused by exposure to microgravity require an uncompromised stretch reflex system. This is especially true for whole body vibration (WBV), as the main source of the neuromuscular activity during WBV has been attributed to stretch reflexes. A priori, it cannot be assumed that reflexes and Ia afferent transmission in particular have the same characteristics in microgravity as in normal gravity (NG). Therefore, the purpose of the study was to compare Ia afferent transmission in microgravity and NG and to assess how microgravity affects muscle activity during WBV. METHODS In 14 participants, electromyographic activity of four leg muscles as well as Hoffmann-reflexes were recorded during NG and microgravity induced by parabolic flights. RESULTS The size of the Hoffmann-reflex was reduced during WBV, but did not differ during acute exposure to microgravity compared to NG. The influence of the gravity conditions on the electromyographic activity did not change depending on the vibration condition. CONCLUSIONS As far as the electromyographic activity of the recorded leg muscles is concerned, the effect of WBV is the same in microgravity as in NG. Moreover, Ia afferent transmission does not seem to be affected by acute exposure to microgravity when subjects are loaded with body weight and postural sway is minimized.