Further, research indicates that the magnitude improvement in VO2 max in response to exercise training is also genetically determined (14). For example following a standardized training program, there is wide variation in the level of VO2 max improvemen among individuals (12, 53). It is currently believe that the heritability of training gains in VO2 max is approximately 47% (14). Evidence points to difference in mitochondrial DNA as being important in both the individual differences in initial (i.e., untrained) VO2 ma and the training-induced improvements in VO2 max (14). Genetically gifted individuals that show large improvements (e.g., 40%-50% increase) in VO2 max following endurance training are often referred to a "high responders to exercise training. See A Close Look 13.1 for more information on this topic.