Prince William, Kate Middleton Struggle on Bikes During Spin Class to Promote Mental Health

William and Kate are in serious need of some Lululemon ... because they look ridiculous in their business attire as they peddle furiously at a fitness center.

The Prince and Princess hit up the Aberavon Leisure & Fitness Centre for a worthy purpose -- showing how exercise can benefit mental health.

That said, they both hopped on bikes during a spinning class and showed what they got ... for a grand total of 45 seconds. They both looked like they were going to pass out.

Kate was dressed in a jumper and skirt, choosing heels over tennis shoes.  William was in his go-to suit and tie.

William looked exhausted at the end of the ride, as others in the class ignored the obvious and gave them a hearty round of applause.

The couple competed against each other, and in the end, Kate was the victor ... winning a tiny gold trophy. 🏆