Changes in vastus lateralis fibre cross-sectional area, pennation angle and fascicle length do not predict changes in muscle cross-sectional area

Abstract

New findings: What is the central question of this study? Do changes in myofibre cross-sectional area, pennation angle and fascicle length predict vastus lateralis whole-muscle cross-sectional area changes following resistance training? What is the main finding and its importance? Changes in vastus lateralis mean myofibre cross-sectional area, fascicle length and pennation angle following a period of resistance training did not collectively predict changes in whole-muscle cross-sectional area. Despite the limited sample size in this study, these data reiterate that it remains difficult to generalize the morphological adaptations that predominantly drive tissue-level vastus lateralis muscle hypertrophy.

Abstract: Myofibre hypertrophy during resistance training (RT) poorly associates with tissue-level surrogates of hypertrophy. However, it is underappreciated that, in pennate muscle, changes in myofibre cross-sectional area (fCSA), fascicle length (L_f ) and pennation angle (PA) likely coordinate changes in whole-muscle cross-sectional area (mCSA). Therefore, we determined if changes in fCSA, PA and L_f predicted vastus lateralis (VL) mCSA changes following RT. Thirteen untrained college-aged males (23 ± 4 years old, 25.4 ± 5.2 kg/m² ) completed 7 weeks of full-body RT (twice weekly). Right leg VL ultrasound images and biopsies were obtained prior to (PRE) and 72 h following (POST) the last training bout. Regression was used to assess if training-induced changes in mean fCSA, PA and L_f predicted VL mCSA changes. Correlations were also performed between PRE-to-POST changes in obtained variables. Mean fCSA (+18%), PA (+8%) and mCSA (+22%) increased following RT (P < 0.05), but not L_f (0.1%, P = 0.772). Changes in fCSA, L_f and PA did not collectively predict changes in mCSA (R² = 0.282, adjusted R² = 0.013, F_3,8 = 1.050, P = 0.422). Moderate negative correlations existed for percentage changes in PA and L_f (r = -0.548, P = 0.052) and changes in fCSA and L_f (r = -0.649, P = 0.022), and all other associations were weak (|r| < 0.500). Although increases in mean fCSA, PA and VL mCSA were observed, inter-individual responses for each variable and limitations for each technique make it difficult to generalize the morphological adaptations that predominantly drive tissue-level VL muscle hypertrophy. However, the small subject pool is a significant limitation, and more research in this area is needed.

Keywords: histology; muscle; resistance training; ultrasound.

Figure 1.. Morphological adaptations to resistance training.

PRE and POST values for (a) mean fiber cross-sectional area (fCSA), (b) pennation angle (PA), (c) fascicle length (L_f), and (d) muscle cross-sectional area (mCSA). Data are presented as mean ± (standard deviation) values, and individual participant values are plotted as well. Panel e contains a schematic showing estimated locations where PRE biopsies were obtained (open circle), VL mCSA panoramic images were obtained (dashed line), and PA and L_f panoramic images were obtained; notably, the PRE biopsy scar was a landmark to obtain POST VL mCSA/PA/ L_f panoramic images. Not pictured in panel e is the POST biopsy site which was ~2 cm proximal to the PRE biopsy site. Panel f contains a representative 20x image for fCSA determination (white scale bar = 100 μm). Panel g contains a representative mid-thigh panoramic ultrasound image used for pennation angle and fascicle length assessments. Panel h contains a representative mid-thigh panoramic ultrasound image for VL mCSA assessments. Panel i contains individual response data for each participant where data points represent PRE-to-POST percentage changes and gray bars indicate the typical error (TE), which was expressed as the coefficient of variation (CV). Abbreviations: L_f, fascicle length determined by ultrasound; fCSA, fiber cross-sectional area determined by muscle biopsy and histology; mCSA, whole-muscle cross-sectional area. Other note: *, indicates statistical significance (p<0.05).

Figure 2.. Observed versus predicted change in VL mCSA with a regression model.

These data represent the multiple regression model used to predict VL mCSA changes to resistance training. As noted in the results, the model did not significantly predict mCSA changes when using changes in mean fiber cross-sectional area, fascicle length, or pennation angle.