Question

Why our shoulders will pain a
When hill a mountain

Answer 1

genrally oxygen levels is low at middle of mountain
our shoulders muscles are our use oxygen usual so we have increase rate of respiration in order to increase rate of oxygen

reason

when we hill mountain at curtain height o2 is less,that time our muscles use the anaerobic respiration
due to that anaerobic reaction ,the released Lactic acid ,so we have some pain