Conformational elasticity theory recently developed has been used to explore the internal energy contribution to the elastic force fe/f as a function of strain for poly(cis-1,4-isoprene) and poly(trans-1,4-isoprene). Calculated fe/f values are in good agreement with those obtained experimentally. Results show that behavior of fe/f is mainly contributed by chemical structure, or intramolecular interaction, supporting the experimental observations, and that the internal energy contribution is strain dependent.``