Whether you’re looking to muscle up, build endurance or improve all-round fitness, the basic formula is the same: stress + rest = gains. There’s no progress without the stressor, AKA training. But it’s during rest and recovery where the growth actually happens. It’s a balancing act. Train too hard, rest too little and you’re on a fast track to injury and burnout. Train too little, rest too hard and you know what happens. Now, a new breed of fitness trackers wants to help you tread this fine line with ‘readiness’ scores to guide your effort. So how do they work and what’s the best? We strap in for a closer look.