It is not known what level of intensity is required to achieve maximal increases in muscular size and strength. Muscular fatigue or volitional fatigue may not be prerequisite for recruitment of all motor units and that would assume that maximal recruitment of motor units is required for optimal increases in strength.
Volitional fatigue denotes a conscious, deliberate decision to terminate an exercise.
Muscular fatigue implies failure to complete the concentric phase of the repetition despite a maximal effort not terminated by volition.
Generally, training programs of 2 days/week have been shown to provide the same increases in muscular strength as compared to 3 days/week (Carroll et al., 1998).
However, some studies have shown that 3 days/week training is superior for strength development compared to 1 day/week and 2 days/week, but the effect size is often small (80% of the derived strength benefits can be obtained with 2 days/week compared to 3 days/week) (Braith et al., 1989). Studies done on some muscle groups(lumbar extensors, cervical extensors and rotators) show that training one, two, or three times per week are equally effective.
The benefits derived from participation in strength training appears to be maintained with a training frequency of 1 day/week. Thus, having practical implications for those individuals on minimal time schedules or athletes desiring to maintain strength during the in-season.
The more is better principal does not apply to many aspects of the strength training realm (sets, frequency). Benefits can be derived from strength training by exercising 2-3x/week. Additional training beyond this threshold range does not provide additional effects for development of muscular strength (Rozier & Schafer, 1981).
In addition, evidence provided by MacDougall et al. (1995) suggests that the rate of protein synthesis after resistance training peaks at approximately 24 hours and returns to baseline by 36 hours post training. Therefore, adequate rest (perhaps up to 48 hours) between training sessions may be necessary for optimal muscle growth (hypertrophy) to occur during prolonged training (greater than 6 weeks; after neurological adaptations).
However, the effect of performing resistance training on different groups of muscles each session (split training) compared to whole body training in which greater than 24 hours recovery between sessions is provided, has yet to be determined.