Correlations Do Not Show Cause and Effect: Not Even for Changes in Muscle Size and Strength.

It is well known that resistance exercise results in increased muscle strength, but the cause of the improvement is not well understood. It is generally thought that initial increases in strength are caused by neurological factors, before being predominantly driven by increases in muscle size. Despite this hypothesis, there is currently no direct evidence that training-induced increases in muscle size contribute to training-induced increases in muscle strength. The evidence used to support this hypothesis is exclusively correlational analyses and these are often an afterthought using data collected to answer a different question of interest. Not only do these studies not infer causality, but they have inherent limitations associated with measurement error and limited inter-individual variability. To answer the question as to whether training-induced increases in muscle size lead to training-induced increases in strength requires a study designed to produce differential effects on muscle size based on group membership (i.e., one group increases muscle size and one does not) and observe how this impacts muscle strength. We have performed studies in our laboratory in which muscle strength increases similarly independent of whether muscle growth is or is not present, illustrating that the increases in muscle strength are not likely driven by increases in muscle size. The hypothesis that training-induced increases in muscle size contribute to training-induced increases in muscle strength requires more appropriately designed studies, and until such studies are completed, this statement should not be made as there are no data to support this hypothesis.

Time-course of muscle growth, and its relationship with muscle strength in both young and older women.

AIM: The time-course for changes in muscle size and strength is not well understood, particularly in women. In addition, contributions of muscle size to strength are commonly assessed utilizing a pre-post change score; however, a more appropriate within-subject correlational analysis has never been used. METHODS: To determine the time-course for thigh muscle size and strength in young (aged 18-25 years) and older (aged 50-65 years) women, and determine the relationship between size and strength after 8 weeks of training carried our three times per week. RESULTS: Anterior muscle thickness at the 50% site increased after 1 week of training, and exceeded the measurement error after 2 weeks (mean difference 0.23 cm, 95% CI 0.12-0.34 cm). Anterior muscle thickness at the 70% site increased and exceeded the measurement error after 2 weeks (mean difference 0.30 cm, 95% CI 0.15-0.45 cm). Posterior muscle thickness at the 50% site increased after 1 week of training, and exceeded the measurement error after 2 weeks (mean difference 0.24 cm, 95% CI 0.03-0.44 cm). Posterior muscle thickness at the 70% site increased after 1 week, and exceeded the measurement error after 3 weeks (mean difference 0.21 cm, 95% CI 0.07-0.35 cm). Muscle strength, assessed through one-repetition maximum, increased by 2 weeks in most measurements, and the within-subject variance explained by changes in muscle size ranged between 9% and 35%. CONCLUSIONS: Muscle growth appeared to occur early into a training program in both young and older women. Furthermore, although a large portion remains unexplained, the variance in muscle strength explained by muscle size is greater than previous studies not utilizing within-subject correlations. Geriatr Gerontol Int 2017; 17: 2000-2007.

The effects of upper body exercise across different levels of blood flow restriction on arterial occlusion pressure and perceptual responses.

Recent studies have investigated relative pressures that are applied during blood flow restriction exercise ranging from 40%-90% of resting arterial occlusion pressure; however, no studies have investigated relative pressures below 40% arterial occlusion pressure. The purpose of this study was to characterize the cardiovascular and perceptual responses to different levels of pressures. Twenty-six resistance trained participants performed four sets of unilateral elbow flexion exercise using 30% of their 1RM in combination with blood flow restriction inflated to one of six relative applied pressures (0%, 10%, 20%, 30%, 50%, 90% arterial occlusion pressure). Arterial occlusion pressure was measured before (pre) and immediately after the last set of exercise at the radial artery. RPE and discomfort were taken prior to (pre) and following each set of exercise. Data presented as mean (95% CI) except for perceptual responses represented as the median (25th, 75th percentile). Arterial occlusion pressure increased from pre to post (p<0.001) in all conditions but was augmented further with higher pressures [e.g. 0%: 36 (30-42) mmHg vs. 10%: 39 (34-44) mmHg vs. 90% 46 (41-52) mmHg]. For RPE and discomfort, there were significant differences across conditions for all sets of exercise (p<0.01) with the ratings of RPE [e.g. 0%: 14.5 (13, 17) vs. 10%: 13.5 (12, 17) vs. 90%: 17 (14.75, 19) during last set] and discomfort [e.g. 0%: 3.5 (1.5, 6.25) vs. 10%: 3 (1, 6) vs. 90%: 7 (4.5, 9) during last set] generally being greater at the higher restriction pressures. All of these differences at the higher restriction pressures occurred despite completing a lower total volume of exercise. Applying higher relative pressures results in the greatest cardiovascular response, higher perceptual ratings, and greater decrease in exercise volume compared to lower restriction pressures. Therefore, the perceptual responses from lower relative pressures may be more appealing and provide a safer and more tolerable stimulus for individuals.