What is the 70% rule and why is the 'give 100%' attitude dangerous?

Bruce Frantzis teaches the 70% rule as a fundamental principle for all Taoist exercises. In any tai chi or chi gung exercise, first estimate what 100% of your capacity is in terms of range of movement or duration of practice; that is, how far your body can actually stretch and how much your body can endure before it collapses. Once you determine this, you then only move or practice to about 70% of your capacity. Over time this will allow your absolute level of performance to increase beyond what the "give 100%, no pain, no gain" philosophy will give you. In addition, you will not build up a habit of strain in your nerves as you always remain in your comfort zone.

The principle upon which the 70% rule is based is that development must begin by considering your weakest link. Do not seek maximum performance, as that quest may both damage the weak link and cause the whole system to contract and tense up. Commonly, when people try to give 100%, they inadvertently go to 110% or 120% of their body's maximum capacity, which results in injury, sometimes slight and sometimes severe. For example, many athletes will overtrain to win, resulting in permanent damage to their bodies. The 70% rule applies to all aspects of Taoist practice, including working with chi and your breathing. This rule allows you to make your body and mind work in a more relaxed, efficient, and healthy manner for the rest of your life.