BODY CONFIGURATION AND BALANCE RECOVERY
M43
METHODS
Subjects
Twenty-six community-dwelling elderly adults (12 women
and 14 men) with a mean age of 75 Ϯ 4 (SD) years, body
height of 1.66 Ϯ 0.11 m, and body mass of 72 Ϯ 15 kg were
recruited through advertisements in local newspapers and
seniors’ centers in the San Francisco Bay area. Potential
subjects were excluded if they met any of the following crite-
ria that would either prevent them from being able to perform
the experiment or would represent potentially confounding
(and, given our small sample size, undesirable) influences on
performance: (i) inability to stand independently and walk a
distance of 5 m; (ii) impairment of neuromuscular function
secondary to previously diagnosed neurological disease (e.g.,
stroke, Parkinson’s disease, peripheral neuropathy); (iii) am-
putations, severe arthritis, or other debilitating orthopedic
problems; (iv) severely impaired vision (e.g., inability to read
newsprint at arm’s length with corrective lenses); (v) use of
medications known to affect balance (e.g., sedatives, antiar-
rhythmics); and (vi) cognitive impairment (Folstein Mini-
Mental State Exam score Ͻ27). Informed written consent
was obtained from each subject, and the experiment was ap-
proved by both the Committee on Human Research at the
University of California, San Francisco, and the Committee
for the Protection of Human Subjects at the University of
California, Berkeley.
Figure 1. Balance recovery by stepping experiment. A, A horizon-
tal tether and electromagnet were used to release subjects from a
backward inclination. Body segment motions were determined based
on the positions of 20 skin-surface markers located at the crown of
the head and L5-sacral junction, and the right and left acromion, lat-
eral humeral epicondyle, distal intersection of the radius and ulna,
junction of the second and third metatarsal, lateral malleolus, mid-
shin, lateral femoral epicondyle, midthigh, and anterior superior iliac
spines; B, in each trial, a six-camera, 60-Hz motion analysis system re-
corded body segment positions and a force plate measured foot con-
tact forces (R). The stepping angle (␣ ) and body lean angle ( ) were
c
c
calculated based on body configuration at toe contact. Joint torques
due to muscle contraction at the ankle, knee, and hip (T , T , T )
A
K
H
during step contact were calculated from inverse dynamics. Joint ro-
tation was defined positive for dorsiflexion at the ankle and for flex-
ion at the knee and hip. Joint torque was defined positive if plantar
flexor at the ankle and extensor at the knee and hip. Ankle and knee
joint centers were estimated from markers overlying the lateral mal-
leolus and femoral epicondyle, respectively. The hip joint center was
estimated from the anterior superior iliac spine and sacral markers,
using a routine developed by Vaughan and colleagues (19).
Experimental Protocol
During the experiment, subjects were released suddenly
from a backward inclined position by means of a horizontal
tether, which attached at one end to a chest harness and at
the other end to an electromagnet (Figure 1A). This protocol
is similar to that used by other researchers to study balance
recovery by forward stepping (13,15–18). At the initiation
of each trial, the subject stood barefoot on a walkway that
had a force plate of surface area 60 ϫ 90 cm (model 6090H,
Bertec Corp, Worthington, OH) mounted flush to its sur-
face. A linoleum cover concealed the location of the force
plate. We then instructed the subject to lean backward into
the tether, the length of which was adjusted to provide an
latter, synchronized recordings were acquired via the force
plate and a six-camera, 60-Hz motion capture system (Mac-
Reflex, Qualisys Inc, Glastonbury, CT) of the contact force
generated between the foot and the ground and the three-
dimensional positions of 20 surface markers placed bilater-
ally (Figure 1A). The MacReflex system has a measurement
accuracy of approximately 1.0 mm, which was more than
adequate for the large motions associated with the experi-
ment. Marker position data were filtered with a recursive,
fourth-order low-pass Butterworth filter, with a 6-Hz cutoff
frequency. From each trial, we determined the step contact
time, the body lean angle at step contact, the stepping angle
at step contact, and the ratio of stepping angle divided by
body lean angle (16). Only the first step of each trial was an-
alyzed. The step contact time (tstep) was calculated as the
time interval, in milliseconds, between tether release and
initial contact of the stepping foot with the force plate. The
initial lean angle of 7Њ from the vertical (which we deter-
o
mined from pilot studies to exceed sway-based recovery
abilities). We then informed the subject that, in the event of
tether release, he or she should “try to recover balance with
a single step.” To increase the unexpectedness of the re-
lease, we randomized between 10 and 60 seconds the time
delay between the instant the subject assumed the leaning
position and the instant of tether release; we played music to
mask equipment noise. We provided no instructions regard-
ing whether the subject should step with the right or left leg.
The subject placed his or her arms initially to the side and
directed his or her gaze forward. For safety, the subject was
secured to a second fall restraint harness (which was slack
during stepping), and members of the research team were
positioned nearby as “spotters.”
body lean angle at step contact ( ) was determined as the
c
sagittal plane projection of the angle, in degrees, between
the vertical and the body lean axis, which we defined by the
line connecting the ankle of the stance (or pivot) foot to the
midshoulder position (Figure 1B). The stepping angle (␣c)
was determined as the sagittal-plane projection of the angle,
in degrees, between the body lean axis and the stepping leg
axis, where the latter was defined by the line connecting the
toe of the stepping leg and midpoint of the pelvis.
Following three “practice” trials (which familiarized the
subject with the experimental protocol and allowed us to
discreetly position the subject so the first step landed on the
force plate), five “actual” trials were acquired. During the