  We analyze the nonequilibrium dynamics of single inextensible semiflexible
biopolymers as stretching forces are applied at the ends. Based on different
(contradicting) heuristic arguments, various scaling laws have been proposed
for the propagation speed of the backbone tension which is induced in response
to stretching. Here, we employ a newly developed unified theory to
systematically substantiate, restrict, and extend these approaches. Introducing
the practically relevant scenario of a chain equilibrated under some
prestretching force $f_\text{pre}$ that is suddenly exposed to a different
external force $f_\text{ext}$ at the ends, we give a concise physical
explanation of the underlying relaxation processes by means of an intuitive
blob picture. We discuss the corresponding intermediate asymptotics, derive
results for experimentally relevant observables, and support our conclusions by
numerical solutions of the coarse-grained equations of motion for the tension.
