BIOMECHANICAL MOVEMENT SYNERGIES - Biomechanics and Motor Control of Human Movement - page 282

responded in anticipation with appropriate polarities to the polarity of
the shoulder moment. A flexor shoulder moment resulted in a posterior
postural response (hip extensors, knee flexors, and ankle plantarflex-
ors), while an extensor shoulder moment resulted in an anterior postural
response (hip flexors, knee extensors, and ankle dorsiflexors).
3. Rietdyk et al. (1999) identified the total body balance recovery mech-
anisms from medio-lateral external perturbations of the upper body
during standing.

Human gait is a complex bipedal movement with many subtasks that
must be simultaneously satisfied and that are continuously changing over
the stride period. These tasks may be complementary or competitive (Win-
ter, 1991): muscles generate and absorb energy at the same time as they are
also responsible for the control of balance and vertical collapse of the body.
This chapter is presented to detail major examples of synergistic motor pat-
terns and demonstrate how kinetic and EMG profiles aid in identifying these
movement synergies.
The term synergy is defined here as muscles collaborating towards a com-
mon goal. and therefore to identify such synergies, it is critical to clarify the
goal and over what period of time the muscle groups collaborate.

11.1 THE SUPPORT MOMENT SYNERGY

In Section 5.2.6, a detailed description of the three lower limb moments
over stance were presented. Also introduced was the concept of a total limb
extensor pattern called the support moment M s = M k − M a − M h (Winter,
1980) using the moment convention presented in Figure 5.14. Considerably
more information about this synergy becomes evident when we analyze repeat
trials on the same subject and analyze the considerable variability at each of
the joints (Winter, 1984, 1991). Figure 11.1 presents the ensemble averaged
profiles for the same subject over 9 days, where the subject was instructed
to walk with her natural cadence. Note that the convention for this plot
has changed, with extensor moments at each joint being plotted

+

ve, thus

M s = M h + M k + M a .
The joint angle kinematics over these nine trials was very consistent: the
rms standard deviation over the stride period was 1.5 ◦ at the ankle, 1.9 ◦
at the knee, and 1.8 ◦ at the hip. The cadence remained within 2%. As can
be seen, M h and M k show negligible variability over swing but considerable
variability over the stance period ( CV h = 68%, CV k = 60%), while M h + M k

shows considerably reduced variability ( CV h + k = 21%). Also M s , which is

the sum of all three moments, has CV s = 20%. Thus, on a trial-to-trial basis,

there is some “trading off” between the hip and knee; on a given day, the
hip becomes more extensor, while the knee becomes more flexor, and vice
versa on subsequent days. We can quantify these day-to-day interactions by