There are three primary reasons why a muscle fiber stops contracting when trained to failure. It is important to understand the characteristics of each in order to apply them to a resistance training program.

The following events are should be taken as an overview in order to understand the principles involved, and where appilcable, put them to use.

Myofibril Failure

Myofibril failure occurs if the resistance is so great that the contractile components fail as immediate energy stores are depleted rapidly and input from the nerves prevents any further contraction. This results in optimized recruitment and growth of existing myofibrils, especially in the white fast twitch motor units. To achieve this effect, the 4 to 6 rep range is recommended. Please note, however, that frequent uses of sets using fewer repetitions to failure can result in acute, or accumulative damage to soft tissues.

For exampel, if someone were to train to failure in the 1 to 3 rep range over a long period of time, he or she may not feel pain from accumulative muscle tissue injury. However, microscopic tears that require repair can still occur in these tissues. The greater the damage, the larger the scar tissue; the larger the scar tissue, the more the scar will inhibit proper musculoskeletal function. This, in turn, could lead to acute muscle injury.

Soft tissue damage is also possible when someone routinely locks out joints at the top of pushing movements, or fully extends at the start of pulling movements. This is because in these positions, passive structures such as the bones and ligaments - and not the muscle - are supporting the weight.

Intermediate Failure

This type of failure occurs when the contractile components fail at or about the same time short term energy stores are depleted. This, in theory, will result in adaptation through the formation of new myofibrils and mitochondria, especially in the red fast twitch motor units.

The optimal rep range to use for this affect is 12 to 15. Sets of about 7 to 11 reps call on varying participation of both the white fast twitch motor units and the red fast twitch motor units. Although it might seem to be the best of both worlds, this range should best be avoided because neither type of motor unit will be stressed optimally.

Mitochondrial Failure

This type of failure occurs due to depleted energy stores and a subsequent accumulation of contraction inhibiting substances. This, in theory, will result in adaptation through the increased storage of energy, and the building of new mitochondria, especially in the red slow twitch motor units.

To achieve this effect, the optimal range is 20 to 25 reps. Performing sets of 16 to 19 reps will once again call on the varying involvement of both the red fast twitch motor units, and the red slow twitch motor units. As in the other forms of contractile failure, this lower rep range will not provide the best involvement of either of the two fiber type motor units individually and so is not recommended.

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