Is it optimal to perform full or partial range of motion (ROM) reps? In a study by Gerard McMahon and colleagues, the aim was to identify the differences between training with a full ROM compared to a partial ROM. Recreationally active study participants in their late teens and early 20s completed an 8-week weight-lifting program during which muscle size, architecture, subcutaneous fat and strength were measured. Training with a full ROM resulted in significantly greater increases in strength and muscle size, changes in muscle architecture and decreases in subcutaneous fat, compared to training with a partial ROM.
If maximizing your gains in muscle size and strength is a goal, then it is absolutely necessary and highly important to emphasize proper exercise technique with a full ROM at any given intensity/load. From this technical foundation, you will be most capable of experiencing enhanced results from your lifts, which include more size and strength. In part, this is due to the higher level of mechanical stress provided by doing full range of motion training.
[McMahon, Gerard E., et al. “Impact of range of motion during ecologically valid resistance training protocols on muscle size, subcutaneous fat, and strength.” The Journal of Strength & Conditioning Research 28.1 (2014): 245-255.]