I have a hard time standing up and really going for it indoors. That's especially true since coming back in September after getting hit last spring. I usually just slide forward onto the rivet and spin as fast as I can. I used to be able to actually sprint, but now my power for anything less than 1 minute is way down (like my 10s is ~200W down) from where it was a year ago before I got hit. I think some is a loss of coordination, some is a loss of fitness, and some is due to coming back and immediately trying to race right away without building up my aerobic fitness first (because racing is more fun that putting the work in and there's a race on Zwift just about whenever I want one), so I have been racing big gears/low cadence and trying to muscle my way through < 1 hour races, leaving nothing left for the sprint.
Working on getting some nice long higher-cadence Z2 rides in to get used to riding normal cadence again (for me that's only averaging ~80 rpm at the moment as I've lost my smoothness and bounce around when I go over ~90 while riding z2), but the 3 "races" I did last week I averaged 67, 64, and 64 rpm. Hopefully, I'll be able to start sprinting again once I can get the cadence up to where it needs to be.