The raucous home crowd goes quiet as they watch the reigning Most Valuable Player step up to the free-throw line. For those watching at home, a familiar chyron displays relevant statistics—how many free throws he’s taken, how many he’s made, how much he’s scored so far tonight. But on the sidelines, his coaches and the team’s training staff are looking at an entirely different array of information: the player’s heart rate, skin temperature, and his pulse, all surreptitiously measured by a small, watch-like device on his wrist. This data, they believe, will help them glean crucial insights into how their star’s body operates, which they just might be able to parlay into advantages that could matter come playoff time.

In leagues around the world, wearable biometric trackers that provide these statistics are the norm; in some countries, they can even display a player’s heart rate in real time on the Jumbotron or home broadcast. So why isn’t this a common sight in American professional basketball?

The answer seems simple: the players’ union, which is far stronger in the National Basketball Association than in comparable leagues in other countries. Through the collective-bargaining process, the union has carved out concessions that are keeping so-called “wearables” off the court for the time being, and keeping the data out of the hands of coaches and managers. That’s because, as much as teams try to assure players that the technology will only be used to help them—and even though many players, including stars, have taken to using them in their personal training regimens—the players know the data very well could end up costing them millions when they sign their next contracts. But all that could change in the next few years, not just in basketball but in other leagues as well.

After all, it’s not just the NBA that’s grappling with the idea of tracking biometric data; wearables have been creeping around the edges of other American professional sports as well. They’ve been in use in a limited capacity in Major League Baseball since 2016, when the league approved two devices, one a special sleeve fitted with sensors aimed at understanding stress on pitchers’ arms and another tracking a player’s heart rate, skin temperature, and sleep cycle; some teams in the National Football League, meanwhile, are using microchips in players’ jerseys to gather similar information in practice but not during games. The rollout’s been a bit slower in the NHL, which has only recently introduced the kind of motion-tracking hardware that’s been commonplace in other sports for years, but advocates of the technology see the sport as a natural fit.

A similar conversation is also happening outside of professional sports. Already, many employers have begun to embrace “big data,” often comparing it to the early-2000s statistical revolution in sports immortalized in Michael Lewis’s 2003 book Moneyball. Some startups have even taken aim at introducing wearables into conventional offices in order to track things like the physical whereabouts of employees.

But it’s in basketball, which has already seen multiple dustups over wearables, and where the league has taken to actively pitching the devices at its annual tech summit, that the conversation’s been the most instructive when it comes to understanding where the controversy lies.

For now, the right to opt out of using biometric trackers is enshrined in the NBA’s new collective-bargaining agreement, the 600-page document outlining relations between the league, the teams, and the players’ union that takes effect in the fall. According to a three-page section on wearables, the devices are prohibited in games, and use in practice is strictly voluntary and constrained to one of six brands. Any team requesting a player wear one must explain, in writing, what’s being tracked and how the team will use the information, not only to the player himself but also to a six-person panel comprising three representatives for the players’ union and three for the league. Most importantly, the agreement says that “data collected from a Wearable worn at the request of a team may be used for player health and performance purposes and Team on-court tactical and strategic purposes only. The data may not be considered, used, discussed or referenced for any other purpose such as in negotiations regarding a future Player Contract or other Player Contract transaction,” under penalty of a $250,000 fine.

This is, at least nominally, a significant exemption: It means that a player doesn’t have to worry about his coaches, managers, and trainers basing the value of his next contract on whether his heart rate fluctuates enough during a game or whether he gets dehydrated too quickly. That’s not to say that teams won’t find other ways to use the data. Alan Milstein, an adjunct professor at the University of New Hampshire School of Law who specializes in sports and bioethics, suggests that the biggest potential value will come from understanding how players rest and recover. “For the teams, they would like to know how their aging players are doing on and off the field—how are they recovering from games, how are they recovering from workouts,” he says. By studying these variables, teams can use wearables to help players fight fatigue and stave off injury: If the tracker suggests that a player’s body is overtaxed, whether in the middle of a game or during practice, the team can use the data to justify giving him time off, helping make sure that everybody’s ready come playoff time.

But once these trackers go out into the real world in the fall, that line between using biometric data for strategic or health-related purposes and factoring it into contracts will be hard to discern. After all, arguably the most important single factor in a team’s strategy is who they have on their roster. It’s why teams are willing to spend hundreds of millions of dollars to sign top-tier free agents to multi-year contracts but only a few million to lock up a coach for the same duration: Even the best strategist can’t do a whole lot if his players don’t have the raw physical ability and skill to contribute to a winning team. Any player who can’t stay on the floor, whether due to actual injury or because he needs to rest frequently to avoid getting hurt, is a distinctly less valuable use of a team’s limited money and even more limited roster space than if he could play hard every game. So when a piece of information that could provide insight into a player’s health becomes available, a team’s management has every reason to find ways to work around the prohibition on using biometric data in negotiations—especially considering that a $250,000 penalty is minuscule compared to the millions doing so could save them on salary.

Consider, for example, perhaps the most pernicious injury problem in professional sports, and the biometric data that’s being used in attempt to solve it. Every season, baseball teams waste millions of dollars on pitchers who spend most of the year on the bench with torn ulnar collateral ligaments, or UCLs. Nowadays, it’s not always a career-ender; since 1974, doctors have become increasingly adroit at replacing the strap of elastic tissue that holds the elbow together as it snaps forward and hurls a ball 100 miles per hour, although some pitchers never return to their pre-surgery form. Still, the injury frequently sidelines pitchers, from little league to the upper echelons of the majors, for as long as 18 months—and nobody knows how to prevent it, or even how to predict it.

Biometric data could change that. Since 2016, the MLB has been partnering with a wearable company that manufactures a special sleeve whose sensors measure the tension pitching places on the UCL; in 2017, the league’s newly-ratified collective-bargaining agreement introduced language providing for voluntary MRIs for pitching prospects entering the draft. With this information, teams hope that they’ll better understand how the elbow works and be able to stem the tide of UCL tears that costs teams so much money and players valuable years of their career.

But where does strategy end and negotiation begin? It would be entirely reasonable for teams to argue that using a bad MRI or worrisome data from a sensor sleeve at the bargaining table is necessary for tactical or strategic purposes, or that they’re well within their rights to “shut down” a pitcher and rest him for the final month of a season because the data predicts he’s about to hurt himself.

Try telling that to the player, though. Those several weeks that he sits out will almost certainly affect his next contract negotiations. No matter how well he pitched the remainder of the season, no matter whether his arm was actually hurt, no matter whether the data was accurate in assessing the warning signs, the notion that he may not be able to play a whole season will loom over his next contract negotiations. This is especially true if only his own team knows what exactly it was about the data that led to him resting, in which case other teams can only speculate—and with so many teams facing similar pressures to maximize every dollar and every roster spot, many may assume the worst. “It seems inherently geared to advantage the team,” says Michael LeRoy, a professor of law at the University of Illinois. “When it’s not linked to performance and not actually linked to injury, just correlation, and these correlates really are going to affect [a team’s] offer, it’s hard to see where that data can be used to the advantage of a player.”

It’s even worse for prospects, those high-school and college students who often come into the draft having played baseball year-round for at least a decade without the training and medical staff—or union backing—afforded to professional players. The upside of voluntary MRIs is self-evident for teams, who might be able to avoid burning a valuable draft pick on a player whose future is in serious jeopardy because his elbow is a ticking time bomb. Even if the damage to the player’s arm isn’t enough to rule out drafting him, it very well might cause a team to reduce the bonus they offer upon signing. That’s obviously good for the team, which gets to save money to spend on future draftees, but clearly bad for the player, especially considering he’ll likely be making a pittance in the minors for years as he waits for his chance in the majors.

Theoretically, a pre-draft MRI could raise a player’s value: “If an MRI is clean,” says Jeff Passan, whose book The Arm details the history of UCL tears and the surgeries that fix them, “what’s preventing the player from going back to the team and saying, ‘Your doctor tells me I have a beautiful elbow, I want $300,000 more’?” But there’s no way for a player to know until he gets his results, at which point the team’s leadership will always know better what to look for, giving them all the leverage they need to use whatever they find against the player.

The concerns are much the same in the NBA. The specter of accidentally signing a would-be star who won’t be able to provide for an entire season, let alone over an entire multi-year deal, looms over every contract negotiation. An explicit provision against explicitly bringing in data derived from wearables can’t change who’s seen it and how it may already have shaped perceptions of the party on the other side of the table. “A general manager, if he’s smart, will mask that finding, that discovery, that knowledge, in something else more nebulous,” says Tom Haberstroh, who writes about technology and analytics in the NBA for ESPN. That’s true whether the data comes during a game, in practice, or elsewhere.

The more data teams aim to collect on their players, the harder it will become to police all the ways it can creep into negotiations. One frontier teams hope to conquer using biometrics is about as far as it can get from the game itself: sleep. It almost goes without saying that a player who gets a good night’s sleep has a better chance of performing well the next day than one who doesn’t. Already, some players are embracing scientific approaches to maximizing their sleep of their own accord, generating data they can share with their own personal trainers (and, if they so choose, their teams). According to both Haberstroh and Passan, teams also hope to tap into that cache of data by having players wear their trackers at night as well as during practices and games.

Once teams have unfettered access to such data, a particularly nefarious executive may suddenly develop a new vocabulary at the negotiating table. For example, Haberstroh says, “Let’s say that he sees that this guy parties a lot and that he’s not getting much sleep. Instead of saying, ‘Okay, we’re going to offer you a $5 million contract instead of a $7 million contract because you party too much,’ they might say something like, ‘We just worry about the character, the professionalism.’”

Or, LeRoy suggests, teams may use data to extrapolate how much effort a player’s putting in. Heart rate not going up enough during shooting drills? Rumor gets out that he’s not trying hard enough in practice—and no team wants a guy who’ll be a bad example for the rookies. Oxygen intake subpar during wind sprints? The team’s suddenly got questions about his endurance, a fact they’re more than happy to let leak to keep his price down in free agency next summer. “These are all coded words that might sound good, and sound fair,” says Haberstroh, “but in reality, they’re rooted in this no-zone. You’re not supposed to use that information.”

Underlying the whole scenario is the assumption that the devices will provide information directly pertinent to player performance. In Haberstroh’s view, “information overload” may lead teams to turn biometric markers of progress into goals: “I think a lot of teams are getting a lot of data, and they’re putting this data in the hands of trainers who might not really understand what the data are saying. The power isn’t in having the data; the power is in knowing what to do with it,” he says.

As a comparison, Haberstroh points to a person who sets a daily goal of walking 10,000 steps. Conventional wisdom holds that doing so will lead to losing weight—and yet, he says, many people who set and achieve that goal instead gain weight. Why? Because, he says, “they’re confusing the process and the result.” The goal becomes reaching 10,000 steps when in fact walking 10,000 steps is just one part of the process by which a person loses weight.

Similarly, says Haberstroh, “You’re having that in the NBA too, where trainers are looking for a certain number rather than customizing what that number is” based on the specific player and the broader suite of desired results. In other words, teams may make reaching a particular heart rate or sweat level the goal of a practice rather than understanding that breaking a sweat and working oneself up to a high heart rate are indicators in the larger process of performing at a high level. It certainly wouldn’t be the first time a team acted on such a conflation: Based on the observation that players on good teams tend to high five each other frequently, some teams concluded that their players should begin giving each other more high fives rather than that high-performing teams have a lot more reason to give high fives.

As wearable tech is rolled out, this problem may detrimentally impact how players are compensated. “One of my biggest concerns,” LeRoy says, “is that the data would be decoupled from performance. You could see patterns where the data says the guy’s fatigued, but he still has a triple-double”—the coveted achievement of reaching double digits in three statistical categories in a single basketball game—“so why should his pay be discounted?”

That question is a synecdoche for the broader dilemma facing the players. They may be convinced, individually if not collectively, that biometrics can help their game, but they’re not sure that sharing that data with teams and the league is worth the risk of losing money on their next contract.

Whether the issue is handled by the six-person panel created under the new CBA or in negotiations for the next one in 2023, it will likely be addressed indirectly. “Let’s not forget that the single most important topic, and the one which governs all other conversations, is what to do with the money,” says Warren Zola, an associate professor of business law at Boston College’s Carroll School of Management. “The most important conversation,” he says, is, “‘how do we distribute the revenue pie between the owners and the players?’ All of the other pieces on the chessboard are used to play a role in getting either side more or less money.”

“It’s all negotiated in the collective-bargaining agreement,” agrees Haberstroh. “The players might say, ‘This is off the table, we don’t want biometric devices, we don’t want wearables.’ And the league says, ‘Well, let’s say a company offers up $50 million [in sponsorship money] over the next 10 years. We will give you 90 percent of that money.’ Then the players are going to be excited.” That, Milstein says, is how these things have historically gone: From drug-testing provisions in baseball, to the “one-and-done” provision requiring draftees to wait a year after high school to enter the NBA draft, to the NFL commissioner’s authority to act as judge, jury, and executioner, “the owners are able to put through all kinds of onerous provisions as long as they’re willing to give up the money.”

While the leagues and their respective unions sort the situation out, wearables are already being implemented where players have the least leverage: minor-league baseball. Garrett Broshuis, a lawyer involved with the suit alleging fair-pay violations in the minors, says that, “In the major leagues, you have the MLB Players Association that’s going to be looking out for the interests of [the players]. There’s no union in the minors, so they can roll these things out without any pushback.” Broshuis, a former minor-league player himself, says that the minors are often “the guinea pigs for new, emerging technologies. When Major League Baseball wanted to institute a drug-testing policy, they rolled it out in the minor leagues first. When they rolled out the pitch clock to study pace-of-play initiatives, that of course took place in the minor leagues too.”

Both Broshuis and Passan say they’ve spoken with players who expressed concerns about teams beginning to use biometric tracking in the minors, which underscores both the importance of unions in professional sports and how vulnerable minor leaguers are to exploitation. Without a union to help secure their long-term future, Passan says, “What it comes down to is, do you want to be seen as the guy who’s not cooperating? Are you going to be punished if you don’t agree to do this testing?” Since even the lightest of pushback could cost a player his job, he says, “the players end up getting taken advantage of, and Major League Baseball looks at the minor leagues as a testing ground.” And, he says, considering that most minor leaguers make less than minimum wage, and many are recent immigrants with limited English and families they’re supporting in their home countries, “there is a moral need for minor-league players to get representation in some way or another.”

That teams are debuting wearables where players are the most vulnerable underscores one of the more troubling aspects of the devices: By gathering real-time biometric data, teams are moving one step closer to transforming the market for players’ labor into a market for players’ bodies.

In sports more than in any other professions, the boundary between the two has always been blurry. With few exceptions, it’s very rare for somebody who’s not at peak physical fitness to reach the upper echelons of professional sports, especially in basketball, where only a handful of players under six feet tall have ever truly thrived. Some of the data the leagues already collect could arguably be construed as falling in either category: When the NBA tracks how far a player runs in an average game, or a baseball team uses special slow-motion cameras to evaluate the angle of a player’s elbow as he releases a pitch, is that dissecting that player’s labor or studying his body?

“There is certainly an intrusiveness to wearable technology,” says Zola, because, unlike previous advances in analytics, biometric data provides information about the basic mechanics of a player’s body that isn’t available to the naked eye, or even the high-tech-camera-enhanced eye. For teams to finagle the right to strap on sensors, not just during games but during players’ off-time as well, Zola says, would be “a recognition that one’s body in professional athletics is a commodity. That is very much what a team is investing in. They’re investing in performance, and you get performance through one’s body.”

Broshuis worries that teams adopting such a viewpoint strip players of their humanity. “Players are not chattel,” he says. “They are not the property of these teams. It really bothers me whenever that term is used.” The argument that they use their bodies to a greater extent than people in other professions is misleading, he says: “Players sign an employment contract, just like employees in any other business. If you’re a doctor, if you’re a surgeon, you’re using your hands and your brain to perform. Well, we aren’t going to have all kinds of biometrics and genetic testing on doctors to see how they’re going to perform five years from now.”

Players’ experiences in countries where biometric tracking is already the norm paint a worrisome picture. A player who had recently returned to the NBA from a stint abroad told Haberstroh that, in Spain, “when you get to the hotel, they have a curfew, and they have people waiting in the lobby making sure you go to bed.” Another reported his concern that teams might punish him for having a glass of wine before bed the night before a game. “With these kinds of devices, your body becomes the property of the team,” says Milstein. “They may be able to determine what you’re allowed to eat, how long you’re allowed to sleep, how much sex you can have, when you’re allowed to have sex. It’s a long road that we’re just starting to walk down, and I just don’t know who’s going to put up the stop signs.”

Then there are the privacy issues. The Health Insurance Portability and Accountability Act, or HIPAA, precludes doctors from disclosing information about their patients without consent. “Let’s pretend that the technology, or the data that the technology provides, discovered something that is addressable,” says Zola. Say, for example, a team discovers an abnormality in a player’s heart rate and chooses to rest him more frequently for fear of problems down the road—which, given the history of basketball players dying from cardiac problems, some even at the peaks of their careers, is a real possibility. “What happens if it becomes public? There’s all sorts of HIPAA requirements and everything else. Even if the player and the team intend to keep things confidential, we could certainly point to leaks that have happened in professional sports,” he says.

And there’s far more potential for sensitive medical information to leak in sports than in most professions. Already, many leagues have provisions meant to reduce the likelihood that they accidentally violate HIPAA. One example is the NBA’s “Magic Johnson Rule,” which stipulates that any player with fresh blood on him, whether his own or somebody else’s, must immediately leave the area of play to prevent the spread of blood-borne diseases. The policy was rolled out in 1992 and named after Magic Johnson, whose announcement that he was HIV positive effectively ended his career and led to the rule’s introduction. But it’s also important as a means of protecting players’ privacy: If the default response to blood on the court is an abundance of caution, there’s less concern that a team may accidentally reveal a player’s secret condition in response to an injury.

But with biometric data will come new possibilities for accidental disclosure. A team may choose to release a player, or reduce their offer to him, or trade him for what looks like pennies on the dollar, because they see something in the biometric data that only they can access. Even in today’s game, in which wearables aren’t yet the norm and medical conditions that might affect a contract are typically public and visible, such a decision would immediately prompt heated speculation and inquiry until the reason behind it became public. Suddenly, provoking that frenzy may lead to a HIPAA violation. That’s not even getting into whether the leagues intend to do with biometrics what they’ve done with every other statistic they gather—that is, put all of the information in public databases for voracious fans to endlessly analyze and discuss, at which point anybody with an internet connection could take a stab at diagnosing their favorite players.

The rollout very well might be moving faster than these and other questions can be addressed. Already, there may be unexpected obstacles from the business side, where corporate interests and scientific interests are colliding. “There are six approved companies of devices that players are allowed to wear,” in the NBA, Haberstroh notes. “If there’s a Nike representative on the [wearables] panel, who’s to say Nike has an objective viewpoint? If [the NBA’s commissioner] Adam Silver is getting offered millions and millions of dollars” by a particular company that wants their product on the list, “and the players’ union is getting millions and millions of dollars, is that going to decide what they’re going to use above science?” Or, he says, many players have multimillion-dollar contracts with companies like Nike, Under Armour, and Adidas; if a team asks a player to wear another brand’s device, does he have the right to refuse?

Of course, by the time everybody’s figured out how to handle wearables, chances are technology will have moved on. “Honestly, I think this is all going to be quaint in a few years,” says Haberstroh. “In five or 10 years, I think we’re going to have embeddables, if they’re not already here. A blood tracker, a pill, a device that you get installed under your skin. What’ll we do then? If you don’t strap it on, it’s just in your body constantly. How do you police that? This is just the tip of the iceberg.”

As the technology continues to advance, so do the possibilities for teams to gather ever more data on their players. Could studying a player’s brain help understand their decision-making? Haberstroh thinks it might—and wouldn’t be surprised if teams start trying to track that as well. Could teams try to push toward genetic testing? Milstein believes there’s a possibility, considering the long history of players with genetic abnormalities that adversely affected their playing careers—and especially considering that congressional Republicans may axe regulations prohibiting employers from discriminating based on DNA testing.

Despite assurances that biometric data will be used to develop strategies that could prolong their careers, players have ample reason to believe that the information may also result in a big bite out of their future earnings. With the liberal application of money and time, and with the technology already making its way into games in a much more limited capacity, general consensus seems to be that wearables will be arriving in American professional sports in full force. With that integration, though, will come an array of problems, both logistical and ethical, for the leagues to sort out, whether they’re equipped to do so or not.