Here is my sentence なんか俺 体力低下してんなぁ meaning "I feel like I am getting weaker." 低下 means to fall/decline and it is a noun and can be used as a suru verb. It seems suru is in it's te form here, my guess is because it's describing an ongoing state and I believe the ん in してんなぁ is the "explanatory" ~ んです. If that is the case, what purpose does the なぁ have at the end of this statement and what effect does having this explanatory statement provide instead of just saying なんか俺 体力低下して. The following sentence is 手がふるえる and this is from Yotsuba (1st chapter).