The Nasdaq Composite (^IXIC 0.33%) has seen its share of big drops followed by big rebounds, and Thursday was merely another instance of that phenomenon. After dropping considerably on Wednesday on fears about the COVID-19 pandemic and uncertainty regarding the upcoming election, the market managed to get its optimism back today. As of just after 2:30 p.m. EDT, the Nasdaq was up nearly 2.5%.

Conspicuously absent from the rally, however, were a couple of stocks that have done extremely well so far in 2020. Both of these companies have risen because of what they offer people who have had to stay home rather than returning to work or leisure activities. Even with COVID-19 concerns on the rise, Peloton Interactive (PTON 3.59%) and Zoom Video Communications (ZM -0.24%) saw some considerable declines on the Nasdaq on Thursday.

Peloton pedals backwards

Shares of Peloton Interactive were down more than 5%. The connected treadmill and stationary bike manufacturer has had some big victories because of the closures of gyms and fitness centers and the need for people to find ways to exercise. However, the company got a vote of no confidence from a prominent investment research company.

Person holding weight and kneeling next to Peloton treadmill.

Image source: Peloton Interactive.

Citron Research posted a tweet about Peloton on Thursday, suggesting that the company could have stronger competition than many investors believe. Citron and researcher Andrew Left pointed instead to Apple (AAPL -0.65%), which is working on a fitness-oriented service that would provide much of the same interactive exercise workout programming that Peloton currently provides.

It's certainly true that any company could engage well-known fitness professionals to do video workouts, and as we've seen in the movie and television streaming business, content tends to come at a premium. However, Peloton has built up a considerable first-mover advantage, and its large and growing network effect guarantees a built-in audience that attracts top fitness professionals who want to reach the biggest audiences possible.

Citron has been bearish on Peloton since the first part of 2020, and so far, it's been on the wrong side of the trade. If the pandemic results in renewed gym closures, it'll only help to bolster what's already been a strong year for the fitness equipment specialist.

Zoom zooms lower

Zoom Video Communications also suffered a downturn. The stock was lower by 6% in late-afternoon trading on Thursday.

Increasingly, institutional investors have suggested that the market for young tech stocks had entered bubble territory. For instance, Greenlight Capital's David Einhorn has taken short positions in technology stocks, and while he didn't name specific positions, many would argue that Zoom meets some of his criteria, including a "remarkable valuation" and a "parabolic ascent" in the stock price.

That said, Zoom is still moving full speed ahead to make sure that its service is as good as it can be. The company announced earlier this week that it would integrate end-to-end encryption capabilities into the Zoom platform, making it available not just to paying subscribers but also on its free platform. The move addresses some concerns that some users had earlier this year, with unauthorized people tapping into conversations that they weren't invited to attend.

At this point, it's too early to say whether the bull market in COVID-19 stay-at-home plays is over. But today's moves lower in Zoom and Peloton should prompt shareholders to make sure they know exactly why they've chosen to invest in these stocks -- and whether they plan to stay the course even if declines in the share price continue.