If you train in a fatigued state, you won't learn or improve movement skills. Since agility drills consist of high-intensity efforts through rapid changes of direction and complex movements that require precise coordination, your energy stores and central nervous system need to be at full capacity.
When we do our agility work, we use 1:12 and 1:20 work-to-rest ratios, depending on the drill. These ratios are the only way to maintain proper mechanics and body position, and reduce force in a controlled, coordinated manner. Once you lower the ratio, you begin training your conditioning level, and your body will be too fatigued to achieve proper positions and movement angles. If I wanted to work the girls' conditioning during agility drills, I'd use a 1:1 or a 1:3 ratio.
You don't want the mindset of the faster you go, the more burn you feel, and the more out of breath you are, the harder you are working. If all that were true, you wouldn't be gaining the elasticity and explosiveness that true agility drills offer.
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