single-handedly

yes, it's correct...

Single-handedly does not mean what you appear to think it does. It adds nothing to the sentence and does nothing to convey the intended meaning. We use the phrase 'single-handedly' to describe solitary or unaided actions by people, e.g. in the woods, Joe single-handedly killed three bears that threatened his family.

Thanks to both of you.

Contrex, what word could I use to express the idea that when each variable was analysed individually, being male best explained... as opposed to the combined effect of two or more variables?

Of all of the variables, being male best explained lean body mass during adolescence.