Question

The handle-bars also assist in maintaining the seat at first.

Answer 1

The handlebars keep the rider from falling off at first.
Most people cannot ride a motorcycle or a bicycle without holding the handlebars.

Answer 2

The handlebars keep the rider from falling off at first.

The original sentence describes one of the purposes of the handlebars as they "assist in maintaining the seat". This means that the handlebars help the rider stay on the seat of the bicycle. Bicycle riders usually have to lean forward while on the seat. The handlebars help the rider support his/her weight while leaning forwards so that the rider does not slide or fall of the seat. They also help the rider to steer the bicycle in the correct direction.