There is no way to develop PRACTICAL strenght without using specificity.
Specificity is king and it is very important, thats why there are skinny guys in BJJ who can get their arm out of an armbar from a bodybuilder, while a bodybuilder cant do that to save his life.
As a lifter for 17 years and also a fellow fighter and a military hand to hand combat instructor i have enough experience to know that - without specificity, you are worth nothing.
Best long distance runners cant spar for 1 round. Best sprinters sometimes cant walk 20miles in a forest with full gear. There are BJJ masters who cant get up stairs without getting winded and there are powerlifters who cant push away a 150lb dude in a BJJ roll.
Because its all specific, and you cant train specificity without the actual specificity. Your wife has carried that kid probably much more than you have therefore she has done a specific training while you havent. Its nothing to be emberassed about.
When i was fighting and teaching full time and i had my best cardio, i could go for 10 rounds of MMA and after that i would go on dates and then fuck all night. I still got winded when getting up 9 flights of stairs. My GF uses that stair-cardio thingy and she can get up stairs easy but she cant do 20 burpees without choking on her tongue.
For overall purposes - the MORE stuff you do, the better. But to actually get better at something you must do the thing you want to get better with.
Nothing will train you to carry a baby in a specific position than carrying something around that weight in a simmilar position.
A collegue of mine, who is a powerlifter with 20 year experience and overall huge dude, once said to me : “people like us usually get injured while working with a showel or a rake”… Which is very true - you deadlifting 800lbs does not help you to dig a hole for 5 hours straight.