The role of body segment movements on the control of centre of mass during balance corrections

List of abbreviations:
COM: centre of mass; CNS: central nervous system; SCA: Spinal Cerebellar Ataxia; APA:
anticipatory postural activity.
Human stance is an instable bipedal posture characterized by a high centre of mass (COM)
located near the hips. The COM (projected onto ground level) needs to be held within the small
area of support (defined by the two feet) to maintain equilibrium.
Elderly people and those with neurological deficits have problems with balance. About 30% of
community-dwelling adults aged 65 and older fall at least once each year. Falls and fall-related
injuries have been shown to be independent determinants of functional decline. Falls occur in
different directions and at different speeds depending on the direction and intensity of the
perturbation to balance.
The way the central nervous system (CNS) responds to an impending fall depends on many
factors, direction and velocity of falling are the two most crucial. Thus, when human stance is
perturbed, the CNS must utilize and integrate the available sensory and environmental
information to select an appropriate response strategy, especially for fast backward falls.
In this thesis standing balance was perturbed using servo-controlled multi-directional rotations of
the support surface. Balance perturbations consisted of combined pitch and roll rotations (7.5°
and 60°/s) presented randomly in different directions. Thus, in a sideways rotation of the support
surface to the right the subject’s COM moved to the right side and needed to be corrected to
avoid a fall. A visual feedback of COM position based on surface reaction forces was presented
prior to stimulus onset in order to standardise stance position. Outcome measures were
biomechanical responses (kinematics and kinetics) and surface EMG activity of several muscles.
The action of the CNS can be investigated by studying patient groups with clearly defined
balance deficits. Thus, patients with spinal cerebellar ataxia (SCA) were the focus of the first
study in this thesis. The goals of this study were to investigate the correlations between body
segment movements and COM velocity during pathological balance corrections of SCA patients
compared to controls, and to relate correlations indicating instability to EMG activity differences. Therefore, activation patterns of several leg and trunk muscles, kinematics and
kinetics were compared between a group of SCA patients and age-matched controls. The results
showed that, for lateral perturbations, peaks in COM lateral velocity were larger in SCA patients
than controls. These peaks were correlated with increased (“hypermetric”) trunk roll downhill
and reduced uphill knee flexion velocity. Subsequent arm abduction partially corrected the
lateral instability. Excessive posterior COM velocity coincided with marked trunk hypermetric
flexion forwards. Early balance correcting responses in knee and paraspinal muscles have
reduced amplitudes compared to normal responses, not increased response amplitudes as
expected. Later responses were consistent with compensation mechanisms for the lateral
instability created by the stiffened knee and pelvis.
It was concluded that truncal hypermetria coupled with insufficient uphill knee flexion are the
primary causes of lateral instability in SCA patients. Holding the knees and pelvis more rigid
possibly permits a reduction in the controlled degrees of freedom and concentration on arm
abduction improves lateral instability. For backwards perturbations excessive posterior COM
velocity coincided with marked trunk hypermetric flexion forwards. A further conclusion was
that this flexion and the ensuing backwards shift of the pelvis results from rigidity which jeopardizes posterior stability. Timing considerations and the lack of confirmatory changes in
amplitudes of EMG activity suggest that both lateral and posterior instability in SCA is primarily
a biomechanical response to pelvis and knee rigidity resulting from increased muscle
background activity rather than changed evoked responses.
It has been shown that balance corrections depend on the impending fall direction. Thus
direction is crucial in programming muscle activity to recover balance or damping a fall. Muscle
activity is controlled by the CNS. Directional sensitivity of the sensory inputs and ensuing
responses were shown by Allum et al. (2008) and Carpenter et al., 1999, 2001). The question is
whether the CNS independently controls roll and pitch movements of the human body during
balance corrections. To help provide an answer to this question, the balance of 16 young healthy
subjects using multi-directional rotations of the support surface was perturbed. All rotations had
pitch and roll components, for which either the roll or the pitch component were delayed by 150
ms or not at all.
Across all perturbation directions, delayed roll caused equally delayed shifts (150 ms) in peak
lateral COM velocity. Across directions, delayed pitch did not cause equally delayed shifts in
anterior-posterior COM velocity. After 300 ms however, the vector direction of COM velocity
was similar to the directions seen in the no delay condition. Trunk, arm and knee joint rotations
followed this roll compared to pitch pattern but were different from the no delay rotation
synergies after 300 ms, suggesting inter-segmental compensation for the delay effects. Balance
correcting responses of muscles demonstrated both roll and pitch directed components regardless
of axial alignment. Muscles were categorised into three groups: pitch oriented, roll oriented and
mixed. Lower leg muscles were pitch oriented, trunk muscles roll oriented, and knee and arm
muscles mixed. The results of this study suggest that roll, but not pitch components, of balance
correcting movement strategies and muscle synergies are separately programmed by the CNS.
Reliance on differentially activated arm and knee muscles to correct roll perturbations reveals a
dependence of the pitch response on that of roll, possibly due to biomechanical constraints, and
accounts for the failure of delayed pitch to be transmitted equally in time across all limbs
segments. Thus it appears the CNS preferentially programs the roll response of the body and
then adjusts the pitch response accordingly.
During an impending fall some body segments may be preferentially used to recover balance. As
shown in the study of SCA patients, the knees play a critical role for correcting fall in lateral
directions - stiff knees impair balance recovery. Thus, training adequate knee flexion would help
to recover balance.
To determine whether voluntary movements can be effectively incorporated into balance
corrections two studies with voluntary body movements were performed. “Knee flexion” and
“trunk bending” young healthy subjects had to execute unilateral knee flexion and lateral trunk
bending, respectively, simultaneously with support surface tilts. Unilateral uphill knee flexion
benefited balance recovery. Subjects rotated their pelvis uphill more than predicted. Downhill
knee bending also reduced COM motion. This because of a greater than predicted simultaneous
lateral shift of the pelvis uphill. Leg muscle activity of voluntary knee bending showed
anticipatory postural activity (APA) with similar profiles to early balance correcting responses.
EMG response amplitudes for combined voluntary and compensatory responses were generally
not different from just compensatory responses and therefore smaller than predicted. These
results suggest that because EMG patterns of APA of voluntary motion and early balance
corrections have similar profiles, the CNS is able to incorporate voluntary activation associated with unilateral knee flexion or lateral trunk bending into automatic postural responses. The effect
on movement strategies appears to be non-linear.
In conclusion, these studies provide crucial insights into central programming of balance
reactions useful for developing rehabilitation programs to improve balance.
References
1. Allum JHJ, Oude Nijhuis LB, Carpenter MG. Differences in coding provided by proprioceptive and
vestibular sensory signals may contribute to lateral instability in vestibular loss subjects. Exp Brain Res
2008; 184:391-410.
2. Carpenter MG, Allum JHJ, Honegger F. Directional sensitivity of stretch reflexes and balance corrections
for normal subjects in the roll and pitch planes. Exp Brain Res. 1999; 129:93-113.
3. Carpenter MG, Allum JHJ, Honegger F. Vestibular influences on human postural control in combinations
of pitch and roll planes reveal differences in spatiotemporal processing. Exp Brain Res. 2001; 140:95-111.