Rapid changes in muscle size

In a previous post I wrote about the different time-course of neural and hypertrophic adaptations and how these contribute to the early changes in strength during training. What didn’t come across in that post is that while neural mechanisms may dominate, a single-bout of training activates hypertrophic signalling pathways, but it just takes more time for these adaptations to manifest in a way that can contribute to strength changes.

It’s not that the muscle isn’t starting to grow; the processes can be set in motion by a single bout. Rather, it’s likely any changes in the dimensions of the muscle in a short time are so small that they don’t contribute heavily to changes in strength (lack of effect), are below the limits of detection of the techniques we use to determine changes in cell size or muscle mass (lack of sensitivity), or are so small that most studies would never have a sample size large enough to be adequately powered (lack of statistical power). This explains, at least to an extent, the reliance in acute (single-bout) strength training studies on the phosphorylation status of various signalling pathways (mTOR), and on measures of protein synthesis (mixed, myofibrillar, sarcoplasmic) as a proxy for muscle growth to occur down the road.

However a recent study used ultrasound to assess changes in muscle thickness of select limb and trunk musculature at regular intervals and suggests that changes in muscle dimensions can occur earlier than what was originally thought.

Time course for arm and chest muscle thickness changes following bench press training

The authors recruited seven healthy young men (untrained) to complete a 24 week strength-training program. The participants trained the bench press three days per week completing three sets of ten at 75%-1RM with 2-3 minutes rest between sets. One-rep maxes were assessed at three week intervals to adjust training loads over the course of the program.

Over the course of training, muscle thickness was measured using B-mode ultrasound for the biceps and triceps brachii and pec major. Each site was marked in felt-marker to facilitate same site testing over the study, and muscle thickness was assessed one week prior to the training study, every monday during training, and three days following the final training session.

Not surprisingly, strength increased following training and while all three muscles had increased thickness, the time course of the changes differed between them. Pec major increased in size following week one of training and triceps brachii after week five. Despite the effect of time (training) on biceps thickness in the repeated-measures ANOVA (p=0.035), any pair-wise analysis failed to find a change in biceps size (don’t you just love stats!), and after viewing the graph you can see any change in biceps thickness is fairly unimpressive.

Nevertheless this study demonstrates that changes in muscle thickness can occur relatively early in the training period. It would have been nice to see another measurement technique for hypertrophy, and if I had my way I’d love to have some myofibre specific areas by fibre type alongside this. But it’s hard enough to convince people to have their vastus lateralis biopsied, let alone their pec, biceps and triceps, so ultrasound will have to suffice for now. I also think that the inclusion of other experimental groups, relying only on single-joint, isolation exercises for each respective muscle group would have strengthened any conclusions on differential growth rates between limb and trunk muscles, assuming it presented the same temporal pattern. An argument could be made that differential rates of growth could have more to do with the selection of a compound exercise (bench press) that emphasizes one muscle group over another, and that the differences in the paper have more to do with muscle activation during the exercise than actual intrinsic muscle growth rates (this is acknowledged in the paper).

If you don’t think I say this enough, it’s not an either/or answer

Early adaptations to strength training do not reside exclusively in neural or muscle tissue, but rather represent a blend of the two. While neural adaptations may dominate early changes in strength, this paper suggests that hypertrophic adaptations may occur more rapidly than originally thought.