Researchers from the Centre de Recherche et d'Innovation sur le Sport at the Universite Claude Bernard-Lyon in Cedex, France, were interested in investigating whether imagery can improve stretching and flexibility the way it has been found to enhance learning and motor performance.
They compared flexibility scores in 21 synchronized swimmers before and after a 5-week mental practice program that included five stretching exercises in active and passive conditions.

The imagery training program resulted in selective increased flexibility, regardless of the stretching method. Overall, the improvement in flexibility was greater in the imagery group than in the control group for the front split (F(1,18) = 4.9, P = 0.04), the hamstrings (F(1,18) = 5.2, P = 0.035), and the ankle stretching exercises (F(1,18) = 5.6, P = 0.03).
There was no difference in shoulders and side-split flexibility (F(1,18) = 0.1, P = 0.73 and F(1,18) = 3.3, P = 0.08 respectively).

Finally, there was no correlation between individual imagery ability and improvement in flexibility.
The researchers conclude that psychological and physiological effects of motor imagery may increase range of motion, suggesting that imagery enhances joint flexibility during both active and passive stretching.

Citation:  Guillot A, Tolleron C, Collet C.  Does motor imagery enhance stretching and flexibility? Journal of Sports Science. 2010 Feb; 28 (3):pages 291-8. [email protected]