what about small muscle groups vs big muscle groups. for ex, biceps vs back. do you still train arms super heavy some days? there is a lot of talk out there that the smaller muscle groups don't need that heavy training??
That's total bro science. The same principles apply for small muscle groups as the physiological mechanisms that result in muscle growth are the same regardless of where the muscle cells are in a large muscle group like the back or small muscles like the biceps.
Light weights heavy reps will increase mitochondrial density and metabolic enzymes in the muscle and increases muscular work capacity. Heavy weights will increase contractile proteins in the muscle and improve neuromuscular recruitment which leads to increased strength. Both of these adaptations complement one another and result in optimal muscle growth.
The only truth I see to the argument about different muscle groups needing different training stimuli would be that some muscle groups are primarily type 1 slow twitch muscle fibers while others are primarily type 2 fast twitch muscle fibers. In which case you could do more heavy weight with the fast twitch at more light weight for the slow twitch but this is getting to the point of splitting hairs and I don't think it'd make that big of a difference as far as gains go.
Simply focus on progressive overload with volume (Increasing weight, reps and sets over time), and phasing your training by focusing on one adaptation at a time. Do heavy weight low reps only for 4-6 weeks followed by 4-6 weeks of light weight high reps and repeat with a deloading week between phases.
