The mechanisms that cause or promote fatigue in
patients with cancer still are understood poorly and
have been the subject of several theories.14 One of the
major obstacles to basic and preclinical research has
been the problematic development of animal models
of CRF due to the inherent subjectivity of the symptom
and the difficulty of establishing objective, behavioral
correlates of fatigue. Voluntary, motivated activity
has been proposed as one such correlate in
animals