When you combine 强 (strong) and 壮 (robust), you get `强壮`, an adjective that powerfully evokes a sense of healthy, vigorous, physical strength. It's a complete package of power and vitality.
In Chinese culture, physical health and vitality are highly valued. Being `强壮` is seen as a great blessing and a fundamental requirement for a good life—it enables one to work hard, support a family, and withstand hardship. It's a very common and positive compliment, especially for children and young men. A key difference from the English word “strong” is its specificity. In English, “strong” is a jack-of-all-trades: strong coffee, a strong argument, a strong economy. `强壮` is almost exclusively used for the physical condition of living beings. You wouldn't use it for coffee or an argument. Comparing it to “athletic” is also useful. An athlete is certainly strong, but “athletic” might imply agility, speed, and skill in a sport. A marathon runner is athletic but might be described as lean (`精瘦 jīngshòu`) rather than `强壮`. A weightlifter or a traditional wrestler, however, is the perfect embodiment of `强壮`—powerful, solid, and robust.
`强壮` is a common, everyday word with a consistently positive connotation.
It is neutral in formality and can be used with friends, family, and in more formal written contexts like health articles.