Sebastien Foucan (freerunning's inventor) says that training alone is the best for the individual. Why? Because the freerunner can explore by him/her self and be creative and just channel that parkour vision better. Whereas it is easier to copy a friend and not be creative with your body.
I personally think that a little of both is very effective. Alone is good because of Sebastien's points, but with a group or with a friend is good because you have someone to push your abilities and to kind of inspire your movement. You also have someone to encourage you and to spot new, creative moves. Well, whatever you guys want is cool.
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