  The renormalization of quasiparticle (QP) dispersion in bilayer high-$T_{c}$
cuprates is investigated theoretically by examining respectively the
interactions of the QP with spin fluctuations (SF) and phonons. It is
illustrated that both interactions are able to give rise to a kink in the
dispersion around the antinodes (near $(\pi,0)$). However, remarkable
differences between the two cases are found for the peak/dip/hump structure in
the lineshape, the QP weight, and the interlayer coupling effect on the kink,
which are suggested to serve as a discriminance to single out the dominant
interaction in the superconducting state. A comparison to recent photoemission
experiments shows clearly that the coupling to the spin resonance is dominant
for the QP around antinodes in bilayer systems.
