Title

Adaptations underpinning the development, retention and loss of muscular strength in response to unilateral strength training and contralateral limb detraining

Author Identifiers

Grant Rowe
ORCID: 0000-0002-6980-0511

Date of Award

2020

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Medical & Health Sciences

First Advisor

Professor G. Gregory Haff

Second Advisor

Professor Tony Blazevich

Abstract

Although adaptations to strength training should logically be confined to the trained muscles, a stimulus is also directed to the homologous, contralateral muscles that can improve strength of the contralateral limb even though it has not been exercised. This response is referred to as the cross-transfer effect or cross-education. There has been increasing research interest in the cross-transfer effect, and particularly its potential clinical use. However, several issues remain to be addressed with regards to its effectiveness in different populations. Pertinent to athletic and otherwise-active populations, no research has determined whether the cross-transfer effect occurs in previously-trained individuals. With recent research demonstrating that previously untrained individuals can prevent disuse-induce muscle loss in the contralateral limb by performing unilateral strength training, it is unclear whether the same outcome can be achieved in previously-trained participants. Understanding this response also has important implications for clinical practitioners who prescribe pre- or post-operative exercise programmes to patients. Therefore, the present research examined the cross-transfer effect in a homogenous group of previously-untrained participants after first completing 4 weeks of unilateral elbow flexion strength training of both arms (Study 2). This ensured that both limbs had a similar training status before phase two of the study and allowed for a detailed comparison to be made of adaptations to training in the force-dominant (stronger) versus force-non-dominant (weaker) arms. Subsequently, one group of subjects discontinued training completely while another group ceased training of only one arm (Study 3). This allowed an examination of whether strength loss occurs after a short period of strength training, and whether unilateral training could evoke a cross-transfer effect sufficient to attenuate the potential loss of strength in the non-training arm. To ensure that muscle size measurements could be accurately obtained in both exercise training phases, muscle size estimate errors following a single unilateral strength training session were determined on several consecutive days (Study 1). This information would provide clarity on the number of days the muscle should be rested before measurements were made. Although only peripheral quantitative computed tomography (pQCT) scanning was used to measure muscle size changes in Studies 2 and 3, ultrasonography was used in parallel in Study 1 to provide evidence as to whether the scanning techniques might be affected by common factors. Hence, the purposes of the present PhD research were threefold: (1) to investigate the muscle size estimate errors following a single unilateral strength training session using pQCT scanning and ultrasound imaging; 2) to quantify the adaptive responses in both the stronger and weaker elbow flexors to a short (4-week) period of unilateral strength training of both arms; and 3) to determine whether cross-transfer effects evoked by the continued training of one arm influence neuromuscular and strength changes in a contralateral limb that ceases training.

In Study 1, pQCT- and ultrasound-derived estimates of muscle size increased markedly (5.4% and 6.9%, respectively) up to 72 h post-exercise and were significantly correlated. The similarity in change in these measures as well as the significant correlation between them indicates that changes measured by pQCT scanning reflect the same changes measured by ultrasound imaging. Also, as both measures of muscle size showed no signs of returning to baseline by 72 h post-exercise, muscle size measurements in the training studies (2 and 3) were conducted at least six days following the final training session. In Study 2, increases in 1-RM strength were observed in the stronger (+2.1 kg) and weaker (+1.8 kg) elbow flexors after 4 weeks of unilateral training of both arms, with no differences in strength or hypertrophic adaptations being detected between arms. Thus, pre-training between-limb asymmetries did not affect neuromuscular or strength adaptations to training, and between-limb strength differences were not reduced by the training; thus, reduction of between-limb differences might require different training stimuli to be applied to each arm. In Study 3, maximal isoinertial (1-RM) strength of the non-trained (weaker) arm increased statistically, although minimally (+0.8 kg), after continued training of the stronger, contralateral arm (+2.0 kg) while strength was maintained in both arms in the detraining group. Thus, cross-transfer effects were sufficient to improve contralateral limb strength in previously-trained muscles. These changes occurred despite a decrease in elbow flexor CSA (CSAFlexor) of 3.4% in the non-training arm, while continued training of the contralateral arm led to a further, 2.5%, increase in CSAFlexor. The findings provide evidence that cross-transfer effects allow for the retention of (or small improvement in) muscular strength in a previously-trained limb during a period of disuse, at least when the stronger arm is selected for continued training. The findings are consistent with recent data showing that disuse-induced strength loss can be prevented when untrained individuals are subjected to limb immobilisation. Nonetheless, as no strength loss was detected in either arm in the detraining group, the retention of strength in the non-trained arm of the training group may have also partly resulted from other causes. The improvement in contralateral limb strength without identification of any neuromuscular adaptations suggests that alterations in muscle activation patterns (i.e. rather than increased agonist activation per se) likely contributed to the observed changes. This series of studies provides valuable new information relating to the adaptations underpinning the development, retention, and loss of muscular strength in response to unilateral strength training and contralateral limb detraining.

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