Muscular strength, the maximal force a muscle group can produce, is an important physiological variable for both athletic and daily tasks. In athletic populations, muscular strength has been identified as a key contributor to performance and injury prevention. In nonathletic populations, increasing maximal strength helps improve mobility (gait, stairs, etc.), prevent or reduce the progression of joint diseases, and prevent injuries. The training method most commonly employed to increase strength is resistance training. The mechanisms for strength improvement can be broken down into two categories: neural and morphological. Neural improvements in strength can be attributed to agonist activation and increased antagonist coactivation. Morphological increases in strength can be attributed to improvements in physiological cross-sectional area, pennation angle, and connective tissue. Despite all the benefits of resistance training, research has yet to precisely determine the exact mechanisms of strength improvement. Little is known about which of these mechanisms, or combination thereof provide the greatest contributions. By uncovering more knowledge of these contributions, more effective exercise prescriptions can be developed.
Results from research on neural and morphological influences on strength have greatly varied. Significant and nonsignificant results have been obtained for agonist activation: electromyography (EMG) results finding significant results and interpolated twitch technique demonstrating nonsignificant results. Similarly, magnetic resonance imaging (MRI) results have found both significant and nonsignificant findings for muscle cross-sectional area. The variation in research findings has led to confusion about which variables attribute to gains in strength and when these mechanisms take place. To further confound the issue of where strength comes from, there is a paucity research examining more than two of the possible contributions to strength gains. Lastly, pre-training strength levels have been found to influence subsequent responses to strength training. Accounting for pre-training strength levels will provide the most accurate evaluation of individual adaptations to resistance training. Therefore, the purpose of this study was to assess the individual and combined contribution of the adaptations in and morphological variables to the individual changes in strength after resistance training while accounting for pre-training strength levels.
Purpose: The purpose of this study was to assess the individual and combined contribution of the adaptations in and morphological variables to the individual changes in strength after resistance training while accounting for pre-training strength levels.
Subject Description: Forty-eight (n=48) healthy males were selected for this study. Only subjects whom had not participated in a lower body resistance training program for greater than 18 months were accepted. Subjects were randomly allocated to one of two resistance training groups (RT) or control (CON). Groups were matched for maximal voluntary torque (MVT) and body mass. Six subjects dropped out (n=42). For RT (n=28) average age was 25 ± 2 years, average height was 1.75 ± 0.07 m, average physical activity was 2067 ± 1157 MET, and average body mass was 70 ± 9 kg. For CON (n=14), average age was 25 ± 3 years, average height was 1.76 ± 0.06 m, average physical activity was 2321 ± 1614 MET, and average body mass was 72 ± 7 kg.
Procedures and Methods: Prior to the onset of the study, a familiarization session was performed to help eliminate any increases due to learning and to provide the subjects with a greater level of comfort. This session included maximum voluntary and evoked twitch knee extension contractions. Pre/post-testing included two duplicate sessions for each (total of 4) with ample time between each testing session. Force and EMG testing were completed in a rigid custom-made isometric dynamometer with knee and hip angles of 115° and 126°. Straps were used to prevent excessive movement of the pelvis and shoulders. A calibrated S-beam strain gauge was placed above the medial malleolus to measure force. EMG of the quadriceps and hamstrings were measured using a wireless EMG system (Delsys). EMG sensors were placed at fixed 1 cm intervals (6 total) along the quadriceps with two sites on the rectus, two on the vastus lateralis, and two on the vastus medialis. Two sensors were also placed on the hamstrings, one on the biceps femoris and the other on the semitendinosis. Testing required subjects to push as hard as possible (pull as hard as possible for hamstring testing) into the stationary dynamometer. This was repeated 3-4 times for 3-5 seconds. Knee extensor maximal voluntary torque (MVT) is defined as the greatest instantaneous torque achieved during the session. Total quadriceps (QEMGMVT) and hamstring EMG (HEMGANTAG) were also recorded at this time. To determine muscle volume (QUADSVOL), a 1.5 telsa T1 MRI was performed on the dominant leg with a knee angle of approximately 163 degrees. Repeated analysis demonstrated inter and intra-rater reliability of MRI of 1.2% and 0.4%. The quadriceps pennation angle (QUADSθ) was evaluated via ultrasound. Each quadriceps muscle was assessed and the pennation angle was defined as the angle of insertion of the fascicles onto the deep aponeurosis. The ultrasound was performed by the same person, and repeated analysis found an intrarater reliability of 1.6%.
The intervention consisted of four sets of ten unilateral isometric knee extensor contractions of each leg 3 times a week for 12 weeks. The subjects altered each leg between sets. Two minutes of rest was provided between bouts. The explosive contraction were instructed to perform each rep as fast and hard as possible ≥80% MVT for 1 second and a 5 second break between repetitions. The sustained contraction group completed 3 second contractions at 75% MVT with 2 seconds of rest between contractions. Maximal voluntary contractions were measured each week to re-establish MVT’s for the week.
Results: Both RT groups increased MVT from pre to post-training. Therefore their data was pooled for the current results. The RT group ∆MVT were correlated with ∆QEMGMVT (r = 0.576, P = 0.001), ∆QUADSVOL (r = 0.461, P = 0.014), and pre-training MVT (r = −0.429, P = 0.023). No associations were found between MVT and ∆HEMGANTAG (r = 0.298, P = 0.123) or ∆QUADSθp (r = −0.207, P = 0.291). Multiple regression analysis found 59.9% of the variance in ∆MVT could be explained by ∆QEMGMVT, ∆QUADSVOL, and pre-training MVT. The single largest contributor was ∆QEMGMVT.
Overall, in MVT in the RT group increased from 234 ± 40 Nm to 283 ± 43 Nm. QEMGMVT increased from 12.3 ± 3.8 to 15.1 ± 3.5 MMAX area s−1 (paired t test P <0.001). HEMGANTAG decreased from 23.9 ± 13.0 to 19.5 ± 10.8% HEMGMAX (P = 0.046). QUADSVOL increased from 1797 ± 260 to 1897 ± 306 cm3 (paired t test P<0.001). QUADSθ increased from 14.1 ± 2.3 to 16.0 ± 2.6° (P<0.001). The control group demonstrated no significant differences in any of the measured variables.
Discussion: The results of this study indicate that three factors (∆QEMGMVT, ∆QUADSVOL, pre-training strength) explained the majority of strength gains following RT. Interestingly, neural drive appeared to have the largest impact on strength. In this study, neural drive accounted for 30.6% of the gains in strength. The results from this study are logical, as the change in quadriceps EMG mirrored the gains in strength (close to 30%). The increase in quadriceps activation can most likely be attributed to an increase in motor unit firing rate. Other possible mechanisms include an increase in motor unit recruitment and an increase in synchronization. Gains in quadriceps hypertrophy was much lower, a little greater than 5%. Despite the relatively small amount of cross-sectional improvement, these improvements accounted for 18.7% of the variance in strength gains. Results of this study indicate that approximately half of strength gains can be accounted for by two factors: neural drive and cross-sectional area. Lastly, pre-training strength levels demonstrated an inverse relationship with changes in strength. This is consistent with current research which demonstrates reduced responses to training stimuli in trained individuals. Although the study did not include anyone currently participating in RT, it is most likely that prior exposure to, or current recreational exercise explains these results. In conclusion, it appears that neural drive, increases in cross-sectional area, and pre-training strength levels can account for the majority of strength gains after resistance training.
My Thoughts: This study provided lots of useful knowledge for exercise specialists. It seems to confirm the belief that the nervous system is extremely important when it comes to strength gains. One thing to think about in future research is the status of the subjects. These individuals were essentially untrained. It will be interesting to see if these percentages change based on training status. It could be possible that changes in neural drive are only present in untrained individuals. Additionally, although this study highlighted ~60% of the gains in strength could be explained by 3 factors, it leaves an astounding 40% unknown. This is a rather large percentage of strength gains unaccounted. Future research should aim to unveil this 40%. Another future consideration for strength gains is intensity zones. Does training at a low intensity to failure, which promotes muscle growth, neglect improvements in neural drive? Current evidence suggests high intensity training results in superior muscle strength, but equal amounts of hypertrophy when compared to low intensity training to failure. It seems logical to conclude that low intensity training fails to adequately stimulate gains in neural drive.
One thing I disliked about this study was the decision to initially separate the resistance training groups into two separate groups, and then pool the data together. It almost seemed like the authors were aiming to study a different topic and then decided to say forget it. It would make better sense to just standardize the RT group.
Balshaw, T. G., Massey, G. J., Maden-Wilkinson, T. M., Morales-Artacho, A. J., McKeown, A., Appleby, C. L., & Folland, J. P. (2017). Changes in agonist neural drive, hypertrophy and pre-training strength all contribute to the individual strength gains after resistance training. European Journal of Applied Physiology, 117(4), 631-640. doi:10.1007/s00421-017-3560-x
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