The researchers from Texas A&M and the University of Pittsburgh say they have identified a potential new factor that may be independently associated with the characteristics and variability of muscle responses to a controlled resistance exercise training program.
They say the factors that explain the differences in the magnitude of the responses to resistance exercise training between individuals are largely unknown.
Researcher Chang-Woock Lee says their study, which involved 73 healthy women between 18 and 31 in whole-body resistance exercises three times per week for 10 weeks, is meaningful - the women were divided into two groups - 34 who used oral contraceptives and 39 who did not use the contraceptive hormones - the women were encouraged to eat enough protein to promote muscle growth.
The researchers say there were marked differences in lean muscle mass gains between the two groups - it increased by just 2.1% in pill users compared with 3.5% in the others - the magnitude of differences in muscle gains between the two groups was a surprise, with the non-OC women gaining more than 60% greater muscle mass than the pill users and other muscle responses, such as strength gains and arm/leg circumferences, showed similar differences.
Tests also showed that blood levels of three muscle-building hormones were significantly lower and one muscle-breaking hormone was significantly higher in OC users than non-OC users.
Lee says the findings could help explain why OC users showed diminished muscle gains from resistance exercise training and as numerous health and performance benefits including improved exercise/athletic performance, body composition, aesthetic beauty, and self-image can be attained from the increased muscle mass and strength associated with resistance exercise training - OC users may not be able to fully enjoy those benefits and may also experience impaired exercise performance and difficulties achieving athletic goals due to their diminished muscle response.
The scientists say more research is needed to explain the reasons behind the results.
The team presented their findings at the American Physiological Society meeting, part of the Experimental Biology 2009 scientific conference currently underway in New Orleans.
