Stiftung Tierärztliche Hochschule Hannover Die ontogenetische ...

Studie II: Shift of the CoM in growing dogs
contrast, the stance time ratio between fore- and hindlimbs increased during the
course of this study despite an increase in relative velocity. Relative fore- and
hindlimb duty factors have been suggested to reflect the antero-posterior mass
distribution of trotting quadrupeds and adding mass to the forelimb led to an increase
in the fore- to hindlimb stance time ratio (Lee et al., 2004). Therefore, the increase in
the stance time ratio observed herein corroborates our conclusion that the CoM shifts
cranially when dogs grow.
Ontogenetic changes in limb loading and the position of the CoM
Between PW11 and PW13 (three dogs) or PW19 (three dogs), the time to peak
vertical force increased significantly in the hind- but not the forelimbs. Therefore,
loading rate was greater in the hindlimbs when the dogs were very young. Various
factors influence the shape of the GRF curve such as the geometric compression of
the leg spring due to the forward motion of the body as well as the limb’s angle of
attack, stiffness and anatomical design (Farley et al., 1993; Witte et al., 2004).
Spring-mass-model simulations, for example, have shown that the trajectory of the
CoM during a stride depends, among other factors, on the angle of attack (Farley et
al., 1993; Seyfarth et al., 2002). That is, steeper angles are associated with a
relatively earlier minimum in the trajectory of the CoM and accordingly an earlier
peak of the vertical force. Compared with when older, puppies appear to protract
their limbs less and hit the ground in a more vertical paw trajectory and a more
flatfooted manner; thus, they seem to have a lower angle of limb retraction before
touch down and therefore a greater rate of limb compression. One consequence of a
steeper angle of touch down is that limb muscle force must be build up more rapidly
(Seyfarth et al., 2002). However, without the according kinematic and electromyographic
data, it remains open whether the increased loading rate in the hindlimbs of
puppies is the consequence of differences in limb behavior.
Numerous studies have shown that the whole-body CoM is situated relatively
cranial in dogs (Bryant et al., 1987; Budsberg et al., 1987; Rumph et al., 1994;
DeCamp, 1997; Lee et al., 1999; ; Bertram et al., 2000; McLaughlin, 2001; Fanchon
et al., 2006; Bockstahler et al., 2007; Walter and Carrier, 2007; Katic et al., 2009;
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