Rep ranges need to determined by movement type lever arm length, mental status, glycogen depletion, so many variables. The most important thing is safety. Leave your ego out of weight choice. Unless u r tapering lifts during training may be related indirectly to true gains. Focus on learning the movement correctly and enjoying what u r doing. Generally flexion will require more reps than extensions due to activation of accessory muscles rather than prime movers w/very heavy weight.
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