The size of the substituents interacting at the 1 and 3 positions of an allylic group is often the largest factor contributing to the magnitude of the strain. As a rule, larger substituents will create a larger magnitude of strain.  Proximity of bulky groups causes an increase in repulsive Van der Waals forces.  This quickly increases the magnitude of the strain. The interactions between the hydrogen and methyl group in the allylic system cause a change in enthalpy equal to 3.6 kcal/mol.  The strain energy in this system was calculated to be 7.6 kcal/mol due to interactions between the two methyl groups.