When I was a teenage athlete, concussion recovery meant retreating to a dark room, avoiding screens and waiting for the headache to disappear. At 15, boredom felt like the real medical emergency.
Fortunately, our understanding of concussion treatment has changed considerably since then. Today, international guidelines recommend only 24–48 hours of complete rest, followed by a gradual return to physical and cognitive activity as tolerated. Light exercise, once discouraged, is now considered an important part of recovery.
But there’s an important distinction between returning to activity and returning to sport. And getting that distinction wrong can have devastating consequences.
The tragedy behind Rowan's Law
In May 2013, 17-year-old Ontario rugby player Rowan Stringer sustained multiple head injuries over five days. She experienced headaches, fatigue and ringing in her ears. In text messages to friends, she described her symptoms and admitted she probably had a concussion. But Rowan kept playing. On May 8, she sustained another head injury during a rugby game. She never regained consciousness and died four days later. Her death inspired Ontario's Rowan's Law, legislation intended to improve concussion awareness and prevention in sport.
Rowan's case is frequently discussed in connection with second impact syndrome (SIS), a rare and potentially fatal condition in which a second head injury occurs before the brain has recovered from an earlier one. In fact, SIS is so rare that its very existence as a distinct clinical syndrome remains debated. Some researchers argue that reported cases may instead represent catastrophic brain swelling caused by a single injury.
Nevertheless, the underlying concern is real: a recently concussed brain may be particularly vulnerable to another impact. Exactly why remains an active area of research. What follows is our best current understanding of what may be happening inside the brain in the hours and days after a concussion, and where the science remains incomplete.
An energy crisis inside your skull
One leading model suggests that when the head experiences a sudden impact, neurons stretch and their membranes become disrupted, setting off a series of cellular changes known as the neurometabolic cascade of concussion.
Essentially, the brain’s electrical and chemical housekeeping goes haywire. Potassium leaks out of neurons while calcium rushes in, disrupting the carefully maintained balance that allows brain cells to function. Restoring that balance takes energy, so neurons begin burning through glucose at an accelerated rate. The problem is that, just as the brain’s energy demands increase, its ability to meet them can falter. Excess calcium interferes with mitochondria, the cellular machinery responsible for producing energy, while prolonged calcium accumulation can begin activating pathways that damage or kill cells. Thus, the brain is left in a precarious position: demanding more energy while becoming less able to produce it. It is during this metabolic crisis that another injury may be especially dangerous.
Experimental research suggests that a second injury during this vulnerable period may push already strained neurons beyond their capacity to recover. It is a compelling explanation for second impact syndrome, but not a complete one: much of the evidence comes from animal studies, and exactly how these cellular changes translate into catastrophic brain swelling in humans remains unclear.
So, when is someone actually recovered?
Currently, concussion recovery is assessed through symptoms, cognitive testing, balance examinations and progressively challenging physical activity. These tools are useful, but they cannot necessarily tell us whether the brain has fully recovered at a biological level.
Australian cricketer Will Pucovski illustrates how difficult that distinction can become. Once considered among the country’s most promising young players, Pucovski retired in 2025 at just 27 after repeated concussions and persistent symptoms including headaches, dizziness and fatigue. An expert medical panel ultimately recommended that he retire. His experience represents an extreme version of a question faced after every concussion: how do we know when an athlete’s brain is actually ready to return?
The stakes also extend well beyond the next game. The NFL has faced growing scrutiny over the relationship between repeated head impacts and chronic traumatic encephalopathy (CTE), a degenerative brain disease currently diagnosable only after death. A 2026 study found that at least one-quarter of former NFL players who died between 2016 and 2021 had CTE. Importantly, CTE is associated with cumulative head impacts, not necessarily repeated diagnosed concussions. Although the study cannot tell us how many current players will develop the disease, it raises troubling questions about the long-term consequences of injuries sustained throughout an athletic career.
So where does that leave clinicians deciding whether an athlete is ready to return? Research suggests that changes in balance and movement control can persist even after concussion symptoms resolve, highlighting concerns about whether clinical recovery always reflects a full return to normal function. Biomarkers, brain imaging and movement assessments may eventually provide a more complete picture, but none can currently tell us precisely when the brain is ready to withstand another collision. For now, return-to-play decisions remain clinical judgments without a definitive measure of biological recovery.
What we know, and what we don't
Much of my MSc research is focused on studying concussion and biomechanics, and throughout my degree one thing has become increasingly apparent: there is still a gap between what we can measure clinically and what we would like to know before sending an athlete back onto the field. Return-to-play protocols rely on symptoms and clinical assessments that are useful, but imperfect. Symptoms are subjective, athletes may minimize or conceal them, and the absence of symptoms does not necessarily mean that every aspect of brain function has returned to normal.
The science has come a long way since my teenage days of dark rooms, screen bans and waiting out the symptoms. The next challenge is figuring out how to capture what symptoms and clinical tests miss. More objective measures of recovery could help clinicians make better return-to-play decisions, reduce the burden on athletes to accurately judge their own recovery and, ultimately, give us a better chance of protecting their brains long after they leave the field.
Sophie Tseng Pellar is a Masters Student in the Surgical and Interventional Sciences program at McGill. Her research interests include exercise physiology, biomechanics and sports nutrition.
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