Relative importance and plasticity of anatomical and neuromuscular factors affecting joint torque production

The present research aimed to determine (i) the relative influence anatomical and neuromuscular variables on maximal isometric, concentric and eccentric knee extensor torque (Study 1); (ii) whether the change in strength following a 10-week strength training program is associated with changes in specific anatomical and neuromuscular variables (Study 2a); (iii) whether anatomical and neuromuscular adaptations are dependent on their pre-training magnitudes; and (iv) whether it is possible to ‘predict’ an individual’s adaptation to strength training based on their anatomical and neuromuscular pre-training magnitudes (Study 2b). The variables assessed throughout the studies include muscle cross-sectional area (CSA), fascicle length and angle from the proximal, middle and distal regions of the four quadriceps components; agonist (EMG:Mwave) and antagonist (EMG normalised to MVC) muscle activity, percent voluntary activation (%VA; interpolated twitch technique); maximum isometric and slow speed concentric and eccentric (60°/s), unpotentiated and potentiated twitch torques; and patella tendon moment arm distance. Using a cross-sectional (observational) study design (Study 1; n = 56) models incorporating CSA, fascicle angle and muscle activity and activation were found to best predict both maximum isometric and eccentric torque (R2 = 0.72 and 0.62). Maximum concentric torque was best predicted by a model incorporating CSA, fascicle angle and moment arm (R2 = 0.64) making it suitable for predicting maximal torque in clinical/rehabilitation populations. Proximal CSA was included in the strongest models rather than the traditionally used mid-muscle CSA, indicating its potential functional importance. The strong predictive ability of models incorporating both CSA and fascicle angle indicate that the quantity of contractile tissue strongly influences inter-individual differences in strength expression. Following 10 weeks of heavy lower-limb heavy strength training (Study 2a; n = 36), the change in isometric torque was best (although weakly; R2 = 0.27) predicted by models incorporating the change in proximal-region vastus lateralis CSA and fascicle angle, and changes in concentric and eccentric torque were best predicted by average quadriceps muscle activity, proximal-region CSA (either vastus lateralis or whole quadriceps) and vastus intermedius fascicle angle (R2 = 0.40 and 0.41). Changes in fascicle angle were weakly correlated with the change in strength despite its inclusion in the strongest models, highlighting the requirement to examine interactions between variables when assessing their influence on strength change. Furthermore, the weak relationships observed between the change in strength and the change in neuromuscular variables (Study 2a) indicate that the assumption that simultaneous changes observed in strength, anatomical structure and neuromuscular function following training indicate potential causal association may need to be reconsidered. While muscle activation measured pre-training during isometric contractions was moderately and negatively correlated with the strength change following training (Study 2b), there was no correlation for proximal-region CSA. This indicated limited scope for improvement in activation isometrically in individuals with greater levels of activation prior to training, but that all individuals had similar scope for hypertrophy. It was not possible to predict the strength change elicited by training from the measurements obtained before training (R2 = 0.06 to 0.27). A comparative data set presented in Study 1 provides clinicians with a tool to evaluate an individual’s maximum torque capacity, anatomical structure and neuromuscular function. While accurate prediction of strength change following training cannot be made based on pre-training testing using the current protocols (Study 2b), strength training programs targeted to improve muscle activation (Study 2a) might elicit the greatest improvements in concentric and eccentric knee extensor strength

Anatomical and neuromuscular determinants of strength change in previously untrained men following heavy strength training

This study examined whether changes in strength following a moderate-duration strength training program were associated with changes in specific combinations of anatomical and neuromuscular variables. 36 men (18–40 y) completed 10 weeks of lower-limb heavy resistance (6-RM) strength training. Measurements included cross-sectional area (CSA), fascicle length (lf) and fascicle angle (θf) from proximal, middle and distal regions of the four quadriceps components; agonist (EMG:M), antagonist (EMG) muscle activities and percent voluntary quadriceps activation (%VA; interpolated twitch technique); patellar tendon moment arm distance; and maximal isometric, concentric and eccentric (60° s–1) torque. Multiple regression models were developed to quantify the relationship between the change in maximum torque and the changes in combinations of anatomical and neuromuscular variables. The best model for each contraction mode was determined using Akaike’s Information Criterion (AICc), an information-theoretic approach for model selection. Strength increased significantly following training (mean range = 12.5–17.2%), and moderate relationships were observed between modeled data (using best-fit prediction models) and the change in torque for each contraction mode. The change in isometric torque was best (although weakly) predicted by the linear combination of the change in proximal-region vastus lateralis (VL) CSA and fascicle angle (R2 = 0.27, p \u3c 0.05; AICcwi = 0.52, i.e., the probability the model would be selected as the “best model”). The models best predicting the change in concentric and eccentric torque both included the combination of the change in quadriceps (i.e., mean of all muscles) EMG:M and the change in vastus intermedius fascicle angle combined with either a change in proximal-region VL (R2 = 0.40, p \u3c 0.001; AICcwi = 0.15) or whole quadriceps (R2 = 0.41, p \u3c 0.001; AICcwi = 0.30) CSA (concentric and eccentric, respectively). Models incorporating the change in proximal CSA typically received substantial support (AICC \u3c 2) for concentric torque prediction models, and the change in % VA and pre-training moment arm distance had substantial support for use in eccentric torque prediction models. In conclusion, adaptations varied between individuals, however strength training programs targeted to improve a group of variables that particularly includes agonist muscle activation might yield the greatest improvements in concentric and eccentric knee extension strength, whereas proximal muscle size and fascicle angle appear most important for isometric torque improvements

Increased fascicle length but not patellar tendon stiffness after accentuated eccentric-load strength training in already-trained men

Purpose This study examined whether additional external load during the eccentric phase of lower limb strength training exercises led to greater adaptations in knee extensor strength, muscle architecture, and patellar tendon properties than traditional concentric–eccentric training in already-trained men. Methods Twenty-eight men accustomed to strength training were randomized to undertake 10 weeks of supervised traditional (TRAD) or accentuated eccentric loading (AEL) or continue their habitual unsupervised (CON) strength training. TRAD and AEL trained 2∙week−1 with a six-repetition maximum (RM) session and a ten-RM session. TRAD used the same external load in both concentric and eccentric phases, while AEL used 40% greater load during the eccentric than concentric phase. Tests were performed at pre- and post-training, including: maximum unilateral isokinetic (30°·s−1) concentric, eccentric and isometric torques by isokinetic dynamometry, unilateral isometric ramp contractions with muscle–tendon ultrasound imaging to measure tendon stiffness and hysteresis, and resting vastus lateralis and medialis fascicle angle and length measured by extended-field-of-view ultrasound. Results After training, both TRAD and AEL significantly increased maximum concentric and isometric torque (p < 0.05), but only AEL increased eccentric torque (AEL: + 10 ± 9%, TRAD: + 4 ± 9%) and vastus lateralis (AEL: + 14 ± 14%, TRAD: + 1 ± 10%) and medialis (AEL: + 19 ± 8%, TRAD: + 5 ± 11%) fascicle length. Conclusion Both TRAD and AEL increased maximum knee extensor strength but only AEL increased VL and VM fascicle length. Neither training program promoted changes in fascicle angle or changes in patellar tendon properties in our already-trained men.peerReviewe

Neuromuscular adapatations to traditional versus eccentric-overload resistance training

Eccentric actions are thought to be particularly important to increase muscle strength and size but are often performed at submaximal intensities during traditional resistance training. The aim of the present study was to compare neuromuscular adaptations from eccentric-overload resistance training (EO) to traditional resistance training (TRT)

CTCF mediates insulator function at the CFTR locus

Regulatory elements that lie outside the basal promoter of a gene may be revealed by local changes in chromatin structure and histone modifications. The promoter of the CFTR (cystic fibrosis transmembrane conductance regulator) gene is not responsible for its complex pattern of expression. To identify important regulatory elements for CFTR we have previously mapped DHS (DNase I-hypersensitive sites) across 400 kb spanning the locus. Of particular interest were two DHS that flank the CFTR gene, upstream at −20.9 kb with respect to the translational start site, and downstream at +15.6 kb. In the present study we show that these two DHS possess enhancer-blocking activity and bind proteins that are characteristic of known insulator elements. The DHS core at −20.9 kb binds CTCF (CCCTC-binding factor) both in vitro and in vivo; however, the +15.6 kb core appears to bind other factors. Histone-modification analysis across the CFTR locus highlights structural differences between the −20.9 kb and +15.6 kb DHS, further suggesting that these two insulator elements may operate by distinct mechanisms. We propose that these two DHS mark the boundaries of the CFTR gene functional unit and establish a chromatin domain within which the complex profile of CFTR expression is maintained