Heterogeneity in resistance training induced muscle strength and mass responses in the upper and lower extremities

Abstract

It has been reported that some muscle groups may demonstrate higher strength and hypertrophic gains in response to resistance training than others. In addition to this, the gains in strength and muscle size are highly individual. The purpose of this master’s thesis is to investigate how the strength and hypertrophic adaptations compare between the elbow flexors and knee extensors in response to similar resistance training regimens. A further purpose is to study the interindividual variability in the strength and hypertrophic responses and to investigate the connection of the responses between the muscle groups.

The subjects consisted of previously untrained men and women (n = 17, 28 ± 4.9 years). The training intervention lasted seven weeks which was followed by a five-week de-training period. Resistance training was performed twice a week. The training load was individually tailored for each subject based on weekly maximum repetition tests to ensure equal loading for the knee extensors and elbow flexors. One-repetition maximum muscle strength of the elbow flexors and knee extensors was measured. Ultrasound was used to study changes in muscle architecture in m. biceps brachii (BB) and m. vastus lateralis (VL).

Maximal strength and muscle size increased significantly in both the elbow flexors and knee extensors at 3.5 weeks and further developed until the end of the seven-week training period. The total increase in VL cross sectional area (CSA) was 14.2 ± 5.9% (p<0.05) which was greater than in BB CSA 11.0 ± 5.7% (p<0.05). Larger strength gains were observed in the elbow flexors 17.0 ± 11.6% (p<0.05) than in the knee extensors 14.2 ± 6.5% (p<0.05). All variables decreased significantly in response to the five-week detraining period. Individual variability was observed in the strength and hypertrophic responses, however no correlations were observed in the responses between the upper and lower extremities.

The results suggest that the time course of muscle hypertrophy and strength adaptations are similar in the upper and lower extremities in previously untrained individuals. However, individual muscle response may vary in upper and lower body muscles.