hello everyone

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Hi everyone, I am new to this forum so I would just like to say hello.

I started doing weights at the gym about a year ago. I have seen a very steady rise of muscle mass gained, I tihnk it's to do with me being a natural ectomorph (a marathon runner rather than a sprinter). People have noticed my muscle gains yet I still want more!!! Lately, I have been going to the gym 4 times a week, instead of 2 previous. However, the past 2 workouts I have been seriously tired during and after my workout and my arms are severely hurting (not sore as such like before). This has seen me lose muscle mass :( people at the gym keep telling me I won't gain any weight concentrating on biceps as much as I do, and one chap even said my long bicep workout would be negative (I can see this now). I just can't really alter it, it feels like a routine. However I have now lost some decent muscle that I had last week and I notice it very badly. Should I let myself rest for a few days or a week to recover fully or keep at it and not do my workout as hard.