For many sports injuries, return-to-play decisions are pretty straightforward: pain and swelling have dissipated, or an X-ray confirms that a bone has healed. For concussion, it’s not so clear. Though symptoms may resolve after a few days, the brain could still be vulnerable. Now, Cupertino, Calif.-based based startup Mindrhythm Inc. has developed a headset to assess concussion recovery using biometric measures and help doctors safely return concussed individuals to play.