There are two factors that determine if an exercise is effective when it comes to loading a muscle:
The direction of EFFORT
vs
The direction of RESISTANCE
When these two forces oppose each other, the muscles responsible for performing the "effort" have to do the most amount of work to overcome the resistance. The less these two forces directly oppose each other, the less the muscle responsible for producing the "effort" has to do to overcome the resistance - therefore performing the exercise that way is no longer an effective use of your time.
