Title

Neuromuscular adaptations of joint angle-specific force change after isometric training

Date of Award

2013

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Exercise and Health Sciences

Faculty

Computing, Health and Science

First Advisor

Associate Professor Anthony Blazevich

Second Advisor

Professor Ken Nosaka

Abstract

Increases in force production in response to isometric training typically occur at or around the joint angles adopted during the training, but the mechanisms underpinning this have not yet been fully elucidated. This PhD thesis project investigated the mechanisms underpinning joint angle-specific strength changes after isometric training, focussing on muscle region-specific cross-sectional area (CSA), muscle fascicle length (Lf) and muscle activation adaptations. For this, the validity and reliability of a two-dimensional extended-field-of-view ultrasonography (EFOV) method for measuring muscle CSA (Study 1) and Lf (Study 2) were examined. Small standard errors of measurement (SEM) and high intra-class correlations (ICCs) were found for CSA measurements (0.6-1.2% and 0.95-0.99, respectively) at proximal and mid-thigh (30, 40 and 50% of the distance from the superior border of the patella to the medial aspect of anterior superior iliac spine) but not distal sections and CSA measurements were very similar to those obtained using computed tomography scanning. Small SEMs and high ICCs were also obtained for Lf measurements (0.8% and 0.95, respectively), and they were accurate when compared to directly-measured swine muscle fascicles. Nonetheless, because of the time required for EFOV CSA scanning and its unreliability for the distal quadriceps (despite a high ICC, the 95% CI of ICC at 20% section = -0.04-0.99), MRI was used for CSA measurement in the subsequent study.

The third study aimed to examine joint angle-specific neuromuscular adaptations in response to isometric knee extension training at short (SL; !knee = 38.1 ± 3.7°) versus long (LL; !knee = 87.5 ± 6.0°) muscle lengths. Sixteen men trained three times a week for six weeks at a knee angle at which peak muscle force (i.e. quadriceps torque/moment arm) was 80% of the peak force obtained at the optimum joint angle. Clear joint angle specificity was seen in SL (force increased 13.4 ± 2.4% at 40°), which was associated with an increase in VL EMG around the training (40°; 26.4 ± 15.5%) and adjacent (50°; 22.5 ± 14.9%) angles, without a shift in the electrically evoked force-angle relationship or changes in muscle size. In contrast, increases in force in LL occurred at angles further from the training angle and varied between subjects. Also, muscle volume and CSA increased significantly and the changes in CSA of specific muscle regions were correlated with the changes in peak force produced at both 30° and at 100°. This occurred with small changes in vastus lateralis (VL) and rectus femoris (RF) muscle EMG activity and no detectable change in coactivation, thus selective regional muscle hypertrophy was most associated with the direction of shift in the force-length relationship. A small (5.4 ± 1.4%) and similar increase in Lf was found in both groups, which was not associated with angle-specific force changes. The effect of isometric training on the concentric torque-velocity relationship was examined in Study 4 to determine whether the isometric training influenced dynamic force production. Isokinetic torque at seven velocities (30, 60, 90, 120, 180, 240 and 300°"s-1) was assessed at weeks 0, 3 and 6. Torque increased only in LL, and only at slow angular velocities (30 - 120°"s-1). The change in torque correlated well with changes in VL, VM and RF CSA, although there was little relationship with Lf. There was no change in angle of peak isokinetic torque.

These results reveal a different mechanism of joint angle–specific adaptation between training at short versus long muscle lengths; neural adaptations underpinned changes after training at short quadriceps lengths but muscular (hypertrophic) changes predominated after training at long lengths. Importantly, clear angle specificity was only observed after training at the short length, although muscle mass acquisition and improvements in dynamic muscle force production were elicited only after training at longer lengths. Thus, although specificity is reduced, greater functional benefit appears to be derived after training at longer lengths. Further research is required to determine why some individuals improved force production at shorter muscle lengths after training only at longer muscle lengths and whether such ‘nonspecificity’ can be predicted before training.

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