Protein Supplementation Does Not Further Increase Latissimus Dorsi Muscle Fiber Hypertrophy after Eight Weeks of Resistance Training in Novice Subjects, but Partially Counteracts the Fast-to-Slow Muscle Fiber Transition

Abstract

The response to resistance training and protein supplementation in the latissimus dorsi muscle (LDM) has never been investigated. We investigated the effects of resistance training (RT) and protein supplementation on muscle mass, strength, and fiber characteristics of the LDM. Eighteen healthy young subjects were randomly assigned to a progressive eight-week RT program with a normal protein diet (NP) or high protein diet (HP) (NP 0.85 vs. HP 1.8 g of protein·kg(-1)·day(-1)). One repetition maximum tests, magnetic resonance imaging for cross-sectional muscle area (CSA), body composition, and single muscle fibers mechanical and phenotype characteristics were measured. RT induced a significant gain in strength (+17%, p < 0.0001), whole muscle CSA (p = 0.024), and single muscle fibers CSA (p < 0.05) of LDM in all subjects. Fiber isometric force increased in proportion to CSA (+22%, p < 0.005) and thus no change in specific tension occurred. A significant transition from 2X to 2A myosin expression was induced by training. The protein supplementation showed no significant effects on all measured outcomes except for a smaller reduction of 2X myosin expression. Our results suggest that in LDM protein supplementation does not further enhance RT-induced muscle fiber hypertrophy nor influence mechanic muscle fiber characteristics but partially counteracts the fast-to-slow fiber shift.

Keywords: myosin isoform; nutrition; single muscle fiber mechanics; strength training; supplementation; whey protein.

Figure 1

Experimental design. LDM = latissimus dorsi muscle; RT = resistance training; NP = normal protein diet; HP = high protein diet.

Figure 2

Effects of training and protein amount in diet on 1 RM at latissimus pull down. (a) All subjects (n = 18); (b) subjects were divided into high protein (HP, n = 9) and normal protein groups (NP, n = 9). A paired Student’s t-test (merged data) and a mixed model ANOVA (with one between-subjects factor and one within-subjects factor) were used. MEANS and Standard Errors. *** p < 0.001.

Figure 3

Effects of RT and protein supply (NP and HP) on Cross-Sectional Area (CSA), Isometric Force (Fo), and Isometric tension (Po). Merged NP + HP data are shown in (a–c), while individual data for the NP and HP groups are shown in (d–f). A paired Student’s t-test was used for merged data (a,d), while mixed-model ANOVA (with one between-subjects factor and one within-subjects factor) was used for HP and NP analysis (b,c,e,f). Means and Standard Errors. * p < 0.05; ** p < 0.005 (pre- vs. post-).

Figure 4

MyHC isoform distribution in biopsy samples collected pre- and post-training. (a) All subjects pooled together (n = 18); (b) only subjects with lower protein intake (NP, n = 9); (c) only subjects with higher protein intake (HP, n = 9). Means and Standard Errors. * p < 0.05, ** p < 0.005.

Figure 5

Response of individual fiber types, classified on the basis of their MyHC isoform composition, to RT training. Fibers from NP and HP subjects are pooled together. Means and Standard Errors. Significant difference between post- and pre-training: * p < 0.05.

Figure 6

Response of individual fiber types, classified on the basis of their MyHC isoform composition and treatment (HP vs. NP) to RT training. Means and Standard Errors. Significant difference between post- and pre-training * p < 0.05.

Figure 7

Increase in nuclear density after eight weeks of RT. * p < 0.05. (a) Average values pre- and post-training from six subjects each with eight fibers analyzed in each condition; (b) correlation between the average myonuclei density post- (ordinate) and average density pre- (abscissa) training calculated for each subject. Two subjects showed no change or a slight decrease, while four subjects showed an increase. The regression line is Y = 1.10 ± 0.08X, not significantly different from the perfect line with slope 1; (c) Single muscle fibers stained with anti-actinin antibody (red) and with Hoechst nuclear staining (blue).