Objective: To examine the absolute and relative changes in skeletal muscle (SM) size using whole body magnetic resonance imaging (MRI) in response to heavy resistance training (RT).
Method: Three young men trained three days a week for 16 weeks.
Results: MRI measured total SM mass and fat free mass (FFM) had increased by 4.2 kg and 2.6 kg respectively after resistance training.
Conclusions: RT induces larger increases in SM mass than in FFM. RT induced muscle hypertrophy does not occur uniformly throughout each individual muscle or region of the body. Therefore the distribution of muscle hypertrophy and total SM mass are important for evaluating the effects of total body RT on muscle size.
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- CSA, cross sectional area
- MRI, magnetic resonance imaging
- RT, resistance training
- SM, skeletal muscle
- FFM, fat free mass
Accurate measurements of skeletal muscle (SM) mass and distribution in humans are important for studies of SM hypertrophy response to heavy resistance training (RT). Currently, the most accurate in vivo methods of measuring SM mass are multiscan magnetic resonance imaging (MRI) and computed tomography.1 Despite its safety, most MRI studies have only evaluated regional—for example, arms, trunk, and legs—SM mass.1 We recently reported whole body MRI using a contiguous slice by slice (no interslice gap) method to evaluate total SM mass and its distribution.2 Using this approach, the distribution of RT induced whole body SM hypertrophy can be investigated.
To date, most studies3,4 have only evaluated limb muscle hypertrophy, and very few have reported RT induced muscle hypertrophy in the trunk region.5 More importantly, the distribution of the relative increases in RT induced muscle hypertrophy has not been reported. Thus the purpose of this pilot study was to examine the absolute and relative changes in SM size using contiguous whole body MRI scans in response to RT.
Three healthy young men (age 20–21 years) volunteered for the study. All were physically active, but none had participated in RT before the start of the programme. All subjects signed informed consent documents. The department’s ethical commission approved the study.
RT was carried out three days a week for 16 weeks. Three lower body (squat, knee extension, and knee flexion) and two upper body (bench press and latissimus dorsi pull down) exercises were performed. Workouts consisted of a warm up set followed by three sets to failure of 8–12 repetitions for each of the five exercises. The loads were progressively increased to maintain this range of repetitions per set. One repetition maximum (1RM) strength was determined by progressively increasing the weight lifted until the subject failed to lift the weight through a full rage of motion. Strength of the squat was assessed using the 3RM test.
Total body SM distribution and mass were measured using an MRI 1.5-T scanner (GE Signa, Milwaukee, Wisconsin, USA) with spin echo sequence (TR, 1500 milliseconds; TE, 17 milliseconds).2 Contiguous transverse images with 1.0 cm slice thickness (no interslice gap) were obtained from the first cervical vertebra to the ankle joints for each subject. Four sets extended from the first cervical vertebra to the femoral head during breath holding (about 20 seconds). The other three sets of acquisitions were obtained from the femoral head to the ankle joints during normal breathing. In each slice, the cross sectional area (CSA) was digitised, and the muscle tissue volume (cm3) per slice was calculated by multiplying the CSA (cm2) by slice thickness (cm). SM volume units (litres) were converted into mass units (kg) by multiplying the volumes by the assumed constant density for SM (1.041 kg/l).6
Body density was measured by hydrostatic weighing with simultaneous measurement of residual lung volume by oxygen dilution. Body fat percentage was calculated from body density using the equation of Brozek et al.7 Fat free mass (FFM) was estimated as body mass minus fat mass.
Mean relative increases in upper body and lower body strength (1RM or 3RM) after RT were 30% and 16% respectively. Body fat decreased by 0.6% on average, and FFM increased by 2.6 kg after RT. The mean increase in total SM mass after RT was 4.2 kg (table 1).
The greatest absolute increases in muscle CSA were seen at the level of the shoulder, chest, upper thigh, and upper portion of the upper arm (fig 1A; subject A). Relative changes in muscle hypertrophy were greater at the level of the shoulder, chest, and upper portion of the upper arm (+25–40%) compared with the waist, hip, forearm, thigh, and lower leg (+10–20%) (fig 1B). The relative increase in muscle CSA of all three subjects was 26% at the shoulder (peak CSA level) and 18% and 9% at the mid-thigh and lower leg respectively.
It has been reported that FFM increases by about 2.0 kg after 10–16 weeks of total body RT.3,4 However, very little is known about the degree of SM increase after RT. The mean increase in SM in this study was 4.2 kg. Nelson and coworkers8 reported a 1.4 kg (24 hour urinary creatinine) and 1.6 kg (in vivo neutron activation) increase in total SM in postmenopausal women after 52 weeks of randomised controlled RT. Although the SM gain in our study was threefold higher than in other reports,8 the relative increases in limb muscle CSA were consistent with the literature (5–10% increase in lower body and 15–30% increase in upper body muscle CSA after 12–16 weeks of RT).3,4 Nelson et al,8 on the other hand, only reported a 6–8% increase in arm and thigh muscle CSA. The differences in RT induced SM gain between our data and other reports are probably due to differences in the training programmes—for example, training frequency.
The novel finding of this study was that the RT induced increase in total SM mass measured by MRI was larger than the increase in FFM. Another study8 showed decreases in non-SM lean tissue (as measured by in vivo neutron activation) and increases8 or decreases9 in total body water after RT. In our study, there were no differences in total body water (bioelectrical impedance analysis method) after training (63.6–69.3% before v 63.4–67.5% after). One possible explanation is that non-SM lean tissue may decrease after RT. Clearly, more work is needed to determine if there are changes in organ or non-SM lean tissue after RT.
Take home message
Resistance training induces larger increases in skeletal muscle mass than in fat free mass. Muscle hypertrophy does not occur uniformly throughout each individual muscle or region of the body.
If changes in muscle hypertrophy were constant across every muscle, then a single anatomical CSA would reflect changes in SM mass. However, 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 SM mass are important for evaluating the effects of total body RT because there are differences between relative changes in individual muscle CSA and SM mass.
We extend our gratitude to the subjects who participated is this study and to Ms Sumie Komuro for her assistance. This study was supported in part by The Ministry of Education, Saencl, Sports and Culture of Japan (grant No 15300221).
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