While it is well documented that resistance training induces increases in both skeletal muscle and fat free mass, the relative changes in each are not as well understood. While fat free mass has been reported to increase by about 2 kilograms after 10-16 weeks of weight training, increases in skeletal mass are unknown. These results are published in the British Journal of Sports Medicine.
To date, MRI has only been used to assess regional areas of skeletal muscle mass. However, improved technology and demonstrated safety now allow its use on the entire body. In this study, male subjects were evaluated from head to toe after a period of 16 weeks of heavy weight training including squats, knee extensions, knee flexions, bench press and latissimus dorsi pull downs. Workouts included a warm up set followed by three sets of 8-12 repetitions, to failure. Weights were increased as needed to maintain the 8-12 repetition per set standard.
Total body skeletal muscle mass and distribution were measured using MRI, from the first cervical vertebra to the ankle joints. Hydrostatic weighing was used to measure body density, and those figures were used to calculate body fat percentage. Body mass minus fat mass was used as the estimate of fat free mass.
The results showed that total skeletal mass had increased by 4.2 kilograms and fat free mass by 2.6 kilograms at the end of the study. Interestingly, the proportions of increase in muscle hypertrophy were not uniform throughout each region of the body or muscle group, and varied from person to person. The main conclusion of the study was that weight training-induced increases in skeletal muscle are larger than weight training induced increases in fat free mass. The researchers write, "…our data show that muscle hypertrophy did not occur uniformly throughout each individual muscle or region - for example, trunk, arm, and leg - of the body. Therefore the distribution of muscle hypertrophy and skeletal muscle mass are important for evaluating the effects of total body resistance training because there are differences between relative changes in individual muscle cross sectional area and skeletal muscle mass."