Inter‐ and intramuscular differences in training‐induced hypertrophy of the quadriceps femoris: association with muscle activation during the first training session

Abstract

The purpose of this study was to examine whether inter- and intramuscular differences in hypertrophy induced by resistance training correspond to differences in muscle activation during the first training session. Eleven young men completed 12 weeks of training intervention for knee extension. Before and after the intervention, T1-weighted magnetic resonance (MR) images were recorded to determine the volume and anatomical cross-sectional area (CSA) along the length of the individual muscles of the quadriceps femoris. The T2-weighted MR images were also acquired before and immediately after the first training session. The T2 was calculated for each pixel within the quadriceps femoris, from which the muscle activation was evaluated as %activated volume and area. The results showed that the %activated volume after the first training session was significantly higher in the vastus intermedius than the vastus medialis. However, the relative change in muscle volume after the training intervention was significantly greater in the rectus femoris than the vasti muscles (vastus lateralis, intermedius and medialis). Within the rectus femoris, both the %activated area and relative increase in CSA were significantly greater in the distal region than the proximal region. In contrast, the %activated area and relative increase in CSA of the vasti were nearly uniform along each muscle. These results suggest that the muscle activation during the first training session is associated with the intramuscular difference in hypertrophy induced by training intervention, but not with the intermuscular difference.

Regional and muscle-specific adaptations in knee extensor hypertrophy using flywheel versus conventional weight-stack resistance exercise

“…Regarding the region-specific analysis, there was a tendency for a greater hypertrophic effect at the distal compared with the proximal site. This is similar to previous research indicating that the increase in individual muscle sizes is somewhat greater at the distal site (Narici et al 1996;Seynnes et al 2007;Wakahara et al 2017). The justification for assessing potential differences in the hypertrophic response across different anatomical sites along the muscle stems from the inherent differences in the characteristics of the training devices.…”

Single-joint eccentric knee extension training preferentially trains the rectus femoris within the quadriceps muscles

“…This study is the first to show different hypertrophic responses of individual QF muscle after ECC vs CON training. However, it should be noted that a similar result in terms of greater hypertrophy of RF than the vasti has been reported after single‐joint knee extension training using a conventional weight lifting/lowering model (ie, involving both CON and ECC contractions), or even after CON‐only knee extension training, but not after ISO knee extension training (ie, equivalent hypertrophy among QF muscles) . These findings and the current result together imply that (a) dynamic (ECC and/or CON) but not static (ISO) knee extension training results in pronounced hypertrophy in RF and (b) an ECC stimulus is not a requisite but promotes muscle hypertrophy.…”

Back Squat vs. Hip Thrust Resistance-training Programs in Well-trained Women

“…One important limitation that should be acknowledged is regarding the use of ultrasound measures to assess muscle hypertrophy, especially in the quadriceps muscles. Ultrasound gives only an estimated MT in a specific point, and previous studies have shown considerable inter-and intramuscular differences in training-induced hypertrophy [31]. Therefore, its results cannot precisely define hypertrophy of the whole muscle.…”