I agree with the both @Axtos and @Scyzara yes we should improve ourselves when we live in the "ice age" and fighting predators. But as Scyzara said there should be penalty. In my opinion, muscle training should come with increasing basal metabolic rate, So, if the player train his/her muscle, he/she should eat more food to live.
I know, the most of TLD players and devs always say TLD is not the realistic simulation, but mini details like that make game more interesting, exciting and attractive.
we can increase fire starting chance by starting fire, we can increase sewing chance by sewing, why can not we increase carrying capacity by carrying.
In real, we have to force our muscles to improve them, maybe it can be adapted in TLD. Normally we carry 30kg, to improve that "survivor" carry above the 30kg to improve max carrying capacity. If he carry above 30 kg at least 6 hours in a day(example). Tomorrow he can carry 30.1 kg max. Also it can be limited max 40kg. and "survivor" can not improve his/her carrying capacity before the 20th day (i.e.) . according to that formula, "survivor" can carry 40kg no earlier than 120th day. while muscles are improving, basal metabolic rate increase too and survival needs more food.
Sorry for gramer