🧠 Is There a Magic Number for Concussions?

One of the most common questions we hear from athletes, parents, and coaches is: “How hard do you have to get hit to get a concussion?”

The truth? There’s no single threshold of impact force that guarantees or rules out a concussion.

Systematic reviews indicate that the threshold for concussion in adult athletes generally occurs between 70–120 g of linear acceleration and 4,500–6,000 rad/s² of rotational acceleration, though these ranges vary by age, sport, and individual tolerance (1,2).

In other words, it’s not just about how hard you get hit — it’s about who gets hit, how, and where.

Note: This is a quick take on the topic. Read the full deep-dive by Mike Bradford here. 

 

Linear vs. Rotational Acceleration

Concussions aren’t just caused by big hits — they’re caused by acceleration of the brain inside the skull. This acceleration can happen in two key ways (1,2):

  • Linear acceleration: A straight-on hit that causes the brain to move forward/backward
  • Rotational acceleration: A glancing blow that causes the brain to twist or spin

Studies suggest that rotational forces may play a bigger role in concussion symptoms than linear forces alone, due to the shearing forces on neural tissue (2).

 

The Brain Is Not a Helmet

Think of your brain like a bowl of gelatin — it’s soft, floating, and suspended in fluid. When the skull stops suddenly (from a hit or fall), the brain keeps moving, sloshing and stretching tissue.

The result? Stretching and deformation of:

  • Axons (nerve fibers)
  • Blood vessels
  • Chemical and metabolic systems that regulate brain function

Concussions are associated with neurometabolic disruption caused by acceleration forces sufficient to alter brain physiology (3).

 

Other Risk Factors That Influence Concussion Susceptibility

Force alone doesn’t determine whether a concussion occurs. Other important factors include:

  • Neck strength and stability (weaker necks = greater head movement)
  • Previous concussion history
  • Genetic predispositions (e.g., ApoE ε4 allele)
  • Point of impact (hits to the side of the head or jaw produce more rotational force)
  • Age and sex (females and younger athletes are more susceptible on average)

Pediatric data suggests lower mean thresholds for injury, with youth athletes sustaining concussions at 62–85 g linear and 2,600–4,500 rad/s² rotational acceleration (2). This is why two athletes can experience the exact same hit — and only one ends up concussed.

 

What This Means for Return-to-Play Decisions

There is no safe “impact threshold” we can use to rule out concussion. That’s why we need (3):

  • Clinical exams
  • Symptom checklists
  • Vestibular, visual, and exertion testing

And most importantly, access to trained concussion professionals.

📍 Find a Certified Concussion Clinic near you for evidence-based testing and return-to-play planning.

 

📚 Related Resources

Key Takeaways

  • There is no specific g-force threshold that causes concussion
  • Rotational acceleration is more damaging than linear
  • Individual risk factors play a massive role
  • Every concussion requires a comprehensive evaluation

‌References
  1. Tierney GJ, Falvey ÉC. Establishing biomechanical concussion injury thresholds in sport: a systematic review and meta-analysis. Br J Sports Med. 2021;55(10):538–46.
  2. Sundaram A, Schneider DK, Meehan WP III, et al. Biomechanical characteristics of concussive and sub-concussive impacts in youth sports: a systematic review and meta-analysis. J Sports Sci. 2023;41(3):265–74.
  3. Eliason P, Galarneau JM, Kolstad AT, et al. Prevention strategies and modifiable risk factors for sport-related concussions and head impacts: a systematic review and meta-analysis. Br J Sports Med. 2023;57(12):789–801.