Age-dependent variations in the directional sensitivity of balance corrections and compensatory arm movements in man

• Published in: The Journal of physiology Vol. 542; no. 2; pp. 643 - 663
• Main Authors: Allum, J. H. J., Carpenter, M. G., Honegger, F., Adkin, A. L., Bloem, B. R.
• Format: Journal Article
• Language: English
• Published: Oxford, UK The Physiological Society 15-07-2002; Blackwell Publishing Ltd; Blackwell Science Inc
• Subjects: Adult; Aged; Aging - physiology; Ankle Joint - growth & development; Ankle Joint - physiology; Arm; Electromyography; Female; Humans; Male; Middle Aged; Movement - physiology; Muscle, Skeletal - growth & development; Muscle, Skeletal - physiology; Original; Posture - physiology; Rotation
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.2001.015644

We investigated the effects of ageing on balance corrections induced by sudden stance perturbations in different directions.
Effects were examined in biomechanical and electromyographic (EMG) recordings from a total of 36 healthy subjects divided
equally into three age groups (20–34, 35–55 and 60–75 years old). Perturbations consisted of six combinations of support-surface
roll (laterally) and pitch (forward-backward) each with 7.5 deg amplitude (2 pure pitch, and 4 roll and pitch) delivered randomly.
To reduce stimulus predictability further and to investigate scaling effects, perturbations were at either 30 or 60 deg s −1 . In the legs, trunk and arms we observed age-related changes in balance corrections. The changes that appeared in the lower
leg responses included smaller stretch reflexes in soleus and larger reflexes in tibialis anterior of the elderly compared
with the young. For all perturbation directions, onsets of balance correcting responses in these ankle muscles were delayed
by 20–30 ms and initially had smaller amplitudes (between 120–220 ms) in the elderly. This reduced early activity was compensated
by increased lower leg activity after 240 ms. These EMG changes were paralleled by comparable differences in ankle torque
responses, which were initially (after 160 ms) smaller in the elderly, but subsequently greater (after 280 ms). Findings in
the middle-aged group were generally intermediate between the young and the elderly groups. Comparable results were obtained
for the two different stimulus velocities. Stimulus-induced trunk roll, but not trunk pitch, changed dramatically with increasing
age. Young subjects responded with early large roll movements of the trunk in the opposite direction to platform roll. A similarly
directed but reduced amplitude of trunk roll was observed in the middle-aged. The elderly had very little initial roll modulation
and also had smaller stretch reflexes in paraspinals. Balance-correcting responses (over 120–220 ms) in gluteus medius and
paraspinals were equally well tuned to roll in the elderly, as in the young, but were reduced in amplitude. Onset latencies
were delayed with age in gluteus medius muscles. Following the onset of trunk and hip balance corrections, trunk roll was
in the same direction as support-surface motion for all age groups and resulted in overall trunk roll towards the fall side
in the elderly, but not in the young. Protective arm movements also changed with age. Initial arm roll movements were largest
in the young, smaller in the middle aged, and smallest in the elderly. Initial arm roll movements were in the same direction
as initial trunk motion in the young and middle aged. Thus initial roll arm movements in the elderly were directed oppositely
to those in the young. Initial pitch motion of the arms was similar across age groups. Subsequent arm movements were related
to the amplitude of deltoid muscle responses which commenced at 100 ms in the young and 20–30 ms later in the elderly. These
deltoid muscle responses preceded additional arm roll motion which left the arms directed ‘downhill’ (in the direction of
the fall) in the elderly, but ‘uphill’ (to counterbalance motion of the pelvis) in the young. We conclude that increased trunk
roll stiffness is a key biomechanical change with age. This interferes with early compensatory trunk movements and leads to
trunk displacements in the direction of the impending fall. The reversal of protective arm movements in the elderly may reflect
an adaptive strategy to cushion the fall. The uniform delay and amplitude reduction of balance-correcting responses across
many segments (legs, hips and arms) suggests a neurally based alteration in processing times and response modulation with
age. Interestingly, the elderly compensated for these ‘early abnormalities’ with enlarged later responses in the legs, but
no similar adaptation was noted in the arms and trunk. These changes with age provide an insight into possible mechanisms
underlying falls in the elderly.