有一名年轻壮实男孩的母亲

大力妈妈(da4 li4 mama)
大力 = great strength. It could be a nickname of a big boy.
妈妈 = mom

力量之母(li4 liang4 zhi mu3)
力量 = strength
之母 = mother of

If the phrase "mother of strength" is a description, or even a nickname, of the mother who has a strong son, I would like to put it as "大力妈妈”. This nickname is casual and a bit intimate.

If you want to generalize the meaning of the mother of a big boy to mean someone, or even something, who is so important as being the origin of strength, then “力量之母”would be a better choice. Sometimes we use it in refering our land or our country.