Expressing "strong" with 结实 and 壮实
The adjectives 结实 and 壮实 have the same meaning, but differ slightly in usage.
The adjective 结实
The adjective 结实 
is used when talking about something that is difficult to break.
is used when talking about something that is difficult to break.Examples
这些链子看着挺结实
These chains look quite strong
zhèxiē liànzi kànzhe tǐng jiēshi
他穿着结实的鞋
He wears sturdy shoes
tā chuānzhe jiēshi de xié
结实 can also be used figuratively when talking about human health.
Examples
大家都希望自己孩子的身体结实，少生病
Everyone wants their children to be healthy and less sick
dàjiā dōu xīwàng zìjǐ háizi de shēntǐ jiēshi, shǎo shēngbìng
The adjective 壮实
The adjective 壮实 is only used to emphasize someone's health or strength.
is only used to emphasize someone's health or strength. Pay attention
It cannot be used to describe objects.
Examples
他是个壮实的孩子，长的很健康
He's a strong and healthy boy
tā shì ge zhuàngshi de háizi, zhǎng de hěn jiànkāng
他中等个头，身体壮实
He is of medium height and hasa strong stature
tā zhōngděng gètóu, shēntǐ zhuàngshi