A wider lens would likely work better than a 50mm on APS-C for this kind of shoot. Your 50mm gives a fairly narrow view and can make the scene feel flatter, while a wider lens can add energy and make the viewer feel more “in” the action.

For lighting, adding flash is the main way to improve the look in a dim gym. An off-camera flash placed to the side or slightly behind the subject can create more dramatic, powerful shape and separation. If you can’t use an off-camera setup, a hot-shoe flash with a swivel head bounced off a nearby wall is a practical alternative and can still give a much nicer result than the existing gym light alone.

So the basic approach is:

• use a wider lens if possible for a more dynamic perspective
• add flash rather than relying on the gym’s ambient light
• place the light to the side/back for drama, or bounce it for softer light

Since the images are instructional, keep the pose and body position clearly visible even when aiming for a dramatic look.