Changes in agonist neural drive, hypertrophy and pre-training strength all contribute to the individual strength gains after resistance training

© 2017 The Author(s)Purpose: Whilst neural and morphological adaptations following resistance training (RT) have been investigated extensively at a group level, relatively little is known about the contribution of specific physiological mechanisms, or pre-training strength, to the individual changes in strength following training. This study investigated the contribution of multiple underpinning neural [agonist EMG (QEMGMVT), antagonist EMG (HEMGANTAG)] and morphological variables [total quadriceps volume (QUADSVOL), and muscle fascicle pennation angle (QUADSθp)], as well as pre-training strength, to the individual changes in strength after 12 weeks of knee extensor RT. Methods: Twenty-eight healthy young men completed 12 weeks of isometric knee extensor RT (3/week). Isometric maximum voluntary torque (MVT) was assessed pre- and post-RT, as were simultaneous neural drive to the agonist (QEMGMVT) and antagonist (HEMGANTAG). In addition QUADSVOL was determined with MRI and QUADSθp with B-mode ultrasound. Results: Percentage changes (∆) in MVT were correlated to ∆QEMGMVT (r = 0.576, P = 0.001), ∆QUADSVOL (r = 0.461, P = 0.014), and pre-training MVT (r = −0.429, P = 0.023), but not ∆HEMGANTAG (r = 0.298, P = 0.123) or ∆QUADSθp (r = −0.207, P = 0.291). Multiple regression analysis revealed 59.9% of the total variance in ∆MVT after RT to be explained by ∆QEMGMVT (30.6%), ∆QUADSVOL (18.7%), and pre-training MVT (10.6%). Conclusions: Changes in agonist neural drive, quadriceps muscle volume and pre-training strength combined to explain the majority of the variance in strength changes after knee extensor RT (~60%) and adaptations in agonist neural drive were the most important single predictor during this short-term intervention

Physiological responses to acute cold exposure in young lean men - Fig 3

<p><b>Resting energy expenditure (A), respiratory quotient (B) and substrate metabolism (C) across study periods.</b> Values are mean ± standard deviation (n = 10). Repeated measures analysis of variance was performed, using Bonferroni post-hoc tests for pairwise comparisons. Common letters show significant differences (P≤0.05) between two specific periods. CHO: carbohydrates, REE: resting energy expenditure, RQ: respiratory quotient, ST: shivering threshold, WP: warm period, 31% and 64%: percentage of the individual’s time exposed to cold until shivering occurred.</p

Skin temperature and body gradients across study periods.

<p>Panel (<b>A)</b>: skin temperature, Panel (<b>B</b>): proxies of peripheral vasoconstriction in both arms, Panel (<b>C</b>): body and supraclavicular skin temperature gradients. Values are mean ± standard deviation. Repeated measures analysis of variance was performed, using Bonferroni post-hoc tests for pairwise comparisons. Common letters show significant differences (P<0.05) between two specific periods. Symbol <b>*</b> shows significant differences among all periods (P<0.05). ST: shivering threshold, WP: warm period, 31% and 64%: percentage of the individual’s time exposed to cold until shivering occurred.</p

Physiological responses to acute cold exposure in young lean men - Fig 2

<p><b>Mean time of the study periods (A), and room-air and cooling vest temperature (B).</b> Values are mean ± standard deviation. ST: shivering threshold, WP: warm period, 31% and 64%: percentage of the individual’s time exposed to cold until shivering occurred.</p

Electrical muscle activity (mV) and burst shivering rate (min^-1) of eight different muscles across study periods.

<p>Panel (<b>A</b>): root mean square, Panel (<b>B</b>): burst shivering rate. Values are mean ± standard deviation (n = 6). Repeated measures analysis of variance (Bonferroni post-hoc tests) and Friedman test (adjusted significance) were respectively performed for EMG RMS and EMG BSR. Common letters show significant differences between periods (P < 0.05). BSR: burst shivering rate, RMS: root mean square, ST: shivering threshold, WP: warm period, 31% and 64%: percentage of the individual’s time exposed to cold until shivering occurred.</p

Tissue saturation index (%) and relative changes in the concentration of total haemoglobin (ΔtHb), oxy-haemoglobin (ΔO₂Hb), and deoxy-haemoglobin (ΔHHb) in the abdominal and forearm regions across study periods.

<p>Panel (<b>A</b>): abdominal region, Panel (<b>B</b>): forearm region. Values are mean ± standard deviation (n = 9). Repeated measures analysis of variance was performed, using Bonferroni post-hoc tests for pairwise comparisons. Common letters show significant differences between two specific periods (P<0.05). ST: shivering threshold, WP: warm period, 31% and 64%: percentage of the individual’s time exposed to cold until shivering occurred.</p

Frequency domain parameters of heart rate variability across study periods.

<p>Panel (<b>A</b>): absolute power, Panel (<b>B</b>): normalized power, Panel (<b>C</b>): low frequency—high frequency ratio. Values are mean ± standard deviation (n = 7). Repeated measures analysis of variance was performed, using Bonferroni post-hoc tests for pairwise comparisons. No significant differences were found across study periods (P>0.05). ST: shivering threshold, WP: warm period, 31% and 64%: percentage of the individual’s time exposed to cold until shivering occurred.</p

Thermal comfort perception measured by visual analogue scales across study periods.

<p>Visual analogue scales measured thermal comfort from “no cold at all” (= 0 mm) to “maximum tolerable cold” (= 100 mm). Values are mean ± standard deviation (n = 11). Repeated measures analysis of variance was performed, using Bonferroni post-hoc tests for pairwise comparisons. Common letters show significant differences between two specific periods (P<0.05). Symbol <b>*</b> shows significant differences among all time periods (P<0.05). ST: shivering threshold, WP: warm period, 31% and 64%: percentage of the individual’s time exposed to cold until shivering occurred.</p

Study protocol.

<p>Study protocol.</p