August 2018

Neuromuscular and Kinematic Adaptation in Response to Reactive Balance Training

This article has been excerpted from “Neuromuscular and Kinematic Adaptation in Response to Reactive Balance Training – a Randomized Controlled Study Regarding Fall Prevention,” by the same authors, which appeared in Frontiers of Physiology; 2018;9:1075. References have been removed for brevity. Use is per the Creative Commons Distribution 4.0 International License. To read the full article, go to https://www.frontiersin.org/articles/10.3389/fphys.2018.01075/full

This randomized controlled study evaluated the efficacy of slip-simulating reactive balance training compared to conventional balance training in regard to falls prevention.

By Anne Krause, Kathrin Freyler, Albert Gollhofer, Thomas Stocker, Uli Brüderlin, Ralf Colin, Harald Töpfer, and Ramona Ritzmann

Introduction

Fall scenarios and related injuries among children, adults, and seniors constitute a major public health problem…. Fall prevention programs have been established to counteract postural instability through more effective compensatory muscle activation in young and old populations and to diminish consequential costs…. Reactive balance training (RBT) is a novel intervention that simulates the fall situation itself through the application of random, multi-directional displacements of the support surface. Crucial adaptive skills for resisting falls can be acquired rapidly among young and older adults through a single session of RBT. Transfer effects persist beyond the laboratory for fall situations encountered in daily living; however, evidence for the effectiveness of RBT applied over several weeks is limited and varies greatly regarding perturbation stimuli in simulated fall-risk paradigms.

In contrast, conventional balance training (CBT) has been validated to improve postural stability and elicit functional and neuromuscular adaptations beneficial for fall avoidance….

Comparing RBT to CBT from a biomechanical point of view, stabilizing torques are shifted from distal to proximal. According to the pendulum model, “punctum fixum” and “punctum mobile” are exchanged from the support surface (CBT) to the center of mass itself (RBT), challenging the subject to maintain equilibrium above an unstable moving support surface (RBT) instead of oscillating around a fixed ledger (CBT) (Figure 1). As a consequence, RBT requires an accurate repositioning of the center of mass (COM) utilizing rapid and appropriate neuromuscular responses to regain a stable body position after surface translation. Thus, RBT may challenge reactive postural stability more than CBT and may be more effective as an intervention to counteract falls.

Key Takeaways

  • Perturbation-related falls in response to slips or trips are major causes of injury over the lifespan.
  • Not age, but the overall level of movement control, seems to be the limiting factor to break a person’s fall.
  • Neuromuscular control can be acquired quickly by frequently reproducing the unexpected nature of real-life slipping situations.

Thirty-nine healthy sport students (24 females, 15 males, age 24 ± 3 years) participated in the study.… Volunteers who performed any other kind of balance training or were suffering from acute injuries or neurological irregularities were excluded.… Participants were randomly divided by “matched-pairs” into either an RBT or CBT group. Matched-pairs were determined prior to the interventions based on postural sway measures….

Two protocols were used in a random order to assess the ability to compensate for stance and gait disturbances in stumbling situations after the interventions…. In protocol 1, training-induced effects on postural reactions in a static balance setting were investigated, while, in protocol 2, a dynamic test setting during locomotion was investigated. Electromyograms (EMGs) of 4 leg muscles as well as ankle, knee, and hip joint kinematics were recorded during both settings before and after the interventions (Figure 2).

For both groups, the training interventions lasted 4 weeks and comprised 3, 20-minute sessions per week.…

  • RBT was performed on an electromagnetically driven swinging platform generating surface translations in the horizontal plane.
  • The CBT group trained with conventional balance devices, including unstable surfaces ranging from easy to more challenging postural demands.

Figure 1: Study overview and models comparing conventional versus reactive balance training. Comparison of the 2 training paradigms on the basis of pendulum models (Winter, 1995; Schmitt, 2003) with the conventional training (CBT) describing a single inverted pendulum, while participants in the reactive balance training (RBT) group oscillate around the fixation point located within the center of gravity. In this randomized controlled study, baseline data collection was followed by a random “matched-pair” assignment of 39 volunteers to either the CBT group or RBT group. Before and after 4 weeks of training, measurements were conducted to assess changes in fall-related risk factors.

Protocol 1: Stance perturbation

To determine compensatory reactive responses to sudden perturbations as they occur during stumbling, posterior translations of the support surface were induced randomly to the left leg during static monopedal stance.… Fifteen displacements were induced randomly. Participants were asked to sustain balance on 1 leg (right) and to stabilize equilibrium as quickly as possible.

Protocol 2: Marching in place perturbation

For functional testing, participants marched in place with the left foot stepping on the movable platform. Fifteen displacements were induced randomly after trespassing a light barrier so that the support surface was translated during full-weighted foot contact unexpected in time. Gait pace was controlled and standardized by a metronome.… Participants were asked to sustain balance after perturbation and to stabilize equilibrium as quickly as possible.

For both protocols, trials were repeated in case of balance loss.

  • To obtain neuromuscular outcome data, surface EMGs of the left soleus (SOL), tibialis anterior (TA), biceps femoris (BF), and rectus femoris (RF) were recorded.
  • To assess kinematic outcomes, ankle, knee, and hip joint angles were recorded with monoaxial electrogoniometers.
  • To determine statistical differences within the independent variable groups (RBT group versus CBT group) and time (pre versus post), a repeated-measures analysis of variance (rmANOVA) was conducted. Dependent variables were iEMG data (SOL, TA, BF, RF), latencies (SOL, BF, RF) and angular excursion, and velocity (ankle, knee, and hip joint).

Results

Protocol 1 – Stance perturbation

Neuromuscular Activity

  • Significant time effects for both groups could be shown for RF in SLR and MLR, indicating higher activation amplitude after the training intervention.
  • For the RBT group, reflexive BF muscle activity showed a tendency toward augmentation in SLR and LLR.
  • Interaction effects (time x group) were observed for SOL, TA and RF in long latency responses (LLR).

Muscle Onset Latencies

  • Onset diminished significantly for the stance perturbations in RF, BF, and SOL in the RBT group and in SOL for the CBT group.

Kinematics

  • The rmANOVA revealed significant time effects in response to training:
  • Ankle angular velocity decreased for both groups.
  • Knee joint amplitude increased for both groups in response to training.
  • Hip angular velocity was reduced after RBT only.

Significant percentage changes in neuromuscular and kinematic data were more pronounced after RBT compared to CBT, which is illustrated in larger effect sizes ranging between medium to large sizes for RBT and small to medium sizes for CBT.

Protocol 2 – Marching in place perturbation

Neuromuscular Activity

  • The rmANOVA revealed significant time effects for both groups
  • SOL iEMG activity increased in SLR only
  • RF iEMG activity increased in SLR and MLR.
  • For RBT, muscle activity of RF, BF, and SOL was enhanced in LLR120–150 and LLR150–210 significantly.
  • For CBT, shank muscle activity increased significantly in LLR120–150 and showed a tendency in LLR150–210. In the thigh, muscle responses increased in RF only.
  • Larger effect sizes were reached for CBT versus RBT in the SLR (large versus medium effect sizes). In the MLR, greater effect sizes were evident for RBT (medium to large effect sizes). In LLR120–150, in almost all muscles, effect sizes reached large effects; in LLR 150–210 this is still true for SOL for RBT.

Muscle Onset Latencies

  • Onset latencies diminished significantly in RF and BF in the RBT group.
  • No significant changes were manifested for the CBT group.

Kinematics

  • The rmANOVA revealed a significant time effect illustrated in reduced ankle angular velocity for both groups.
  • For the RBT group, hip angular velocity also decreased.
  • Effect sizes of significant percentage changes ranged from medium to large for RBT and from small to large for CBT.

Figure 2: Outcome measures during posterior perturbation of a representative participant. Recording of neuromuscular data (bottom) and kinematic data (middle) during posterior translation of the support surface (top) were carried out. For muscular activation, the soleus (SOL), tibialis anterior (TA), biceps femoris (BF), and rectus femoris (RF) muscles were measured during short- (SLR), medium- (MLR), and long-latency responses (LLR) after perturbation onset. Simultaneously, joint excursions of the ankle, knee, and hip joint were assessed.

Discussion

The outcomes of this study outline reflex phase- and segment-specific adaptations dependent on the context of the movement task: (a) a facilitation of neuromuscular activation in shank and thigh muscles was accompanied by (b) a reduced muscle onset latency and (c) a decline in angular velocity for the hip and ankle joint. Even though no interaction effects could be observed comparing RBT to CBT for the early reflex phases, effect sizes of RBT were more pronounced for recovery response adaptations with a greater emphasis on the proximal body segment. Thus, the following hypotheses are verified with the constraint that modified adaptations occur after both trainings:

Improvements induced by RBT in dynamic stabilization after perturbation ameliorate neuromuscular control for slip recovery characteristics, comprising (a) an elevated reflex activity in the shank and thigh muscles and (b) enhanced kinematic segmental stabilization to compensate for the disturbing stimulus during stance and marching in place perturbation.

Neuromuscular enhancement

Both training modalities achieved an enhancement in iEMG activity concomitant with reduced onset latencies in relevant sets of muscles that antagonize the perturbation stimulus. Thereby, marching in place perturbations were compensated by quickly delivered reflex activations (SLR) in the distal body segment.… Enhanced SOL activity without any changes in TA activity might be a result of a training-induced improved intermuscular coordination associated with a reduced antagonistic co-activation and a greater rate of force development.

Concomitant to shank muscle activation, knee extensors (RF) were activated faster and more distinctly in early reflex phases (SLR and MLR) during marching in place perturbation.

Enhanced knee extensor activation, in terms of fall prevention, could lead to segmental stabilization, a consolidation of COM vertically above the support surface, and might, therefore, reduce the risk of falling during slips and stumbles.

With an emphasis on the later reflex phases (120 ms after perturbation), next to muscle activation in SOL-RF, and TA and BF activities were raised for both groups….

Significant interaction effects and greater effect sizes for RBT versus CBT indicate intervention-dependent adaptations.

Joint kinematics and multi-segmental strategy

Studies show that falls can be significantly reduced by reducing fall risk factors…. Augmented ankle and hip joint velocities have been identified as distinctive in fallers and vice versa; reduced hip and ankle joint velocities have been identified as distinctive in non-fallers….

Participants frequently exposed to the fall situation in training—as it occurs during RBT—may learn from the risk situation, adapting their motor behavior…to counteract postural deterioration and restrict joint movement and velocity. Thereby, significantly elevated knee deflections—well pronounced after RBT and CBT—are in line with evidence during stance perturbation and slipping situations in locomotion.…

Greater effect sizes after RBT indicate that this training caused a pronounced shift in multi-segmental organization using a stiff ankle joint for immediate compensation, a deflected knee for the reacquisition of a stable COM, and reduced hip joint velocities to control COM and trunk movements to achieve fast balance recovery and safe body equilibrium.

Distinction RBT and CBT and functional relevance

With neuromuscular control being the relevant aspect to determine the quality of postural response, adaptations of both interventions might be associated with a reduced fall incidence that could be relevant for fallers across the lifespan. During stance perturbation, RBT- as well as CBT- enhanced RF activity and concomitantly reduced joint velocities in the proximal limb segment (knee and hip), but greater effects could be observed after RBT.… The musculature of the proximal segments is well known to generate compensatory forces to restore equilibrium after slips and stumbles, and participants benefit from a 2-segment strategy to restore postural equilibrium and stabilize the trunk to prevent falling. During marching in place perturbation, early activation of distal and proximal activity is achieved after both training interventions, but effect sizes are even greater after CBT.… CBT purposefully acts on the distal body segment, making use of the knee joints, which allows for the stabilization of the COM within critical trajectories and thus may reduce fall incidence.

Limitations

The methodological approach of this study was carefully chosen; however, there are 3 limiting aspects:

This investigation is limited to neuromuscular adaptation in healthy adults with excellent motor control skills. While it is an ideal population to investigate fall avoidance during stumbling situations, actual fall incidences were not observed, which is why a differentiation of kinematic and neuromuscular characteristics cannot be provided.

Simulation of stumbling executed in laboratory paradigms is always biased. Participants had to be informed about possible stumbling situations, so they knew that a fall-risk situation could occur.

Due to evidence, 2 effective training regimens for postural control were compared. The effectiveness of both trainings might be why interaction effects in fastest reflex responses were missing.

Conclusion

Both training modalities improved reactive balance recovery after perturbations with adaptations being task-specific…. Neuromuscular control can be acquired rapidly by frequently reproducing the unexpected nature of real-life slipping situations within 4 weeks.

Future investigations are needed to investigate whether high-risk fallers such as children and the elderly could benefit from RBT as a special form of CBT by counteracting age- or disease-associated degeneration of neuromuscular and kinematic strategies.

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