Utilization of elasticity in walking. The fascicle length and
ATF measurements used in this study indicated that TT was
stretched slowly throughout the brake II and push I phases and
subsequently recoiled rapidly (Fig. 2). The stretched TT amplitudes
from the heel strike in both MG (4.2 1.5%) and Sol
(4.3 1.4%) muscles are consistent with similar values reported
previously in animal walking (2–6%) (8, 20). In the
present study, however, these TT behaviors during the contact
of walking do not follow the spring-like bouncing concept, in
which the tendons store energy in the first half of the step and
then return this energy in the second-half phase (2). According
to our interpretation of several studies that have examined
stretch-shortening cycle in both animals (42) and humans (34,
36, 39), the concerted fascicle action seems to refer to favoring
of the effective spring-like bouncing of TT during the ground
contact. In running and jumping, for example, the elastic
energy comes mainly from the initial negative work. But in
walking with a natural cadence, ATF is low during the brake I
and II phases (Fig. 2B). Therefore, TT cannot be stretched
under the spring-like bouncing concept during the brake I and
II phases. To support this suggestion, the TT stretching curves
in both MG and Sol did not follow the ATF development slope
during contact of walking (Fig. 2). These results clearly indicate
existence of an alternative way for the spring-like bouncing
concept to utilize elasticity.