I find it's usually better to start with everything in linear mode, and then
switch to spline only where "roundness" is needed.

Another alternative is to hit Animation: Quaternion Interpolation on the
main menu.  This sounds like science fiction weirdness, but it's
actually a more restrained type of spline that doesn't go wild nearly
as often.

You can set Poser to use the Quaternion choice by default:

set

QUATERNION_ON 1

in Poser.ini.

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