BIOMECHANICAL MOVEMENT SYNERGIES - Biomechanics and Motor Control of Human Movement - page 288

11.2.2 Medial Lateral Balance Control during Workplace Tasks

When standing on both feet human beings must maintain their balance
whether they are standing quietly or whether they are performing manual
tasks with their upper extremities. Anterior/posterior (A/P) balance is con-
trolled by the plantarflexors/dorsiflexors (Horak and Nashner, 1986), whereas
medial/lateral (M/L) balance is controlled by the hip abductors/adductors
(Winter et al. 1996). M/L balance was identified as a “load/unload”
mechanism, where increased abductor forces on one side are accompanied
by decreased abductor forces on the contralateral side. Such a pattern
attempts to lift the pelvis/HAT mass, thus loading the ipsilateral side, while
simultaneously unloading the contralateral side. This loading/unloading
moves the center of pressure (COP) toward the ipsilateral foot, thus causing
an acceleration of the center of mass (COM) of the body toward the

contralateral side.
This synergistic pattern was examined in an ergonomics task, where sub-
jects stood in front of a table carrying out a variety of manual tasks for
a period of 2 hours (Nelson-Wong et al., 2008). Surface EMG records of
right and left gluteus medius muscles were recorded to quantify this syner-
gistic pattern during this fatiguing event. A cross-correlation analysis (see
Section 2.1) of left and right gluteal activity quantified this synergy: a neg-
ative correlation resulted when the right activity increased at the same time
as the left activity decreased, and vice versa (for a typical subject, see
Figure.11.5). For this subject R xy ( τ ) had a peak value of - 0.677 during
a 15-minute block of the 2-hour trial. The R xy ( τ ) is plotted for τ 1sec

and is fairly flat over that range because the fairly long duration of the left
and right gluteal bursts of activity. Thus, the peak of R xy at τ =

=±

0 . 16 sec
reflects what we see from these bursts of activity are virtually 180 ◦ out of
phase.
Conversely, when the load/unload synergy was not present, there was a
positive correlation [ R xy ( τ ) =

0 . 766 during the 15-minute block]; both right
and left activity were increasing and decreasing together, indicating an inef-
ficient cocontraction (see Figure 11.6). Again, the peak of the R xy ( τ ) was at
τ 0 . 06 sec, indicating on this 15-minute record that the left and right gluteal

activity was virtually in phase, and therefore the right and left abductors were
fighting each other to maintain M/L balance.
In this 2-hour workplace study (Nelson-Wong et al. 2008), 23 subjects
reported their pain levels for each 15-minute work task on a visual ana-
log scale, scored from 0 to 40. Fifteen of the subjects who exhibited this
inefficient cocontraction pattern reported a pain score increasing from an
average of 4 in the first work task to 32 in the last task. The remaining
eight subjects exhibited the efficient load/unload pattern and reported an
average pain score of 1 in the first task, increasing slowly to 8 in the last
task. There was a significant main effect between these two groups with a
p < 0 . 0005.

=