The purpose of the program of research outlined in this thesis is to enhance our understanding of the consequences of resistance training for movement control in older adults. It is intended that the knowledge thus derived will add to the general knowledge about the ageing process, and be informative for clinicians and practitioners in the prescription therapeutic regimes for geriatric populations. The thesis is divided into six chapters. The first and last chapters provide, respectively a general introduction to, and summary of the research program. The remaining chapters are a series of stand-alone scientific works. Each is an integral component of the thesis.
Chapter 2 is a review of the literature concerning the interplay between the neuromuscular degeneration which accompanies the ageing process, and the adaptations to resistance training experienced by older adults. Particular consideration is given to the capacity for resistance training to enhance the performance of functional movement tasks. This chapter contains an overview of the alterations in the neuromuscular system that are responsible for the declines in strength and power, and the associated deterioration in the everyday movement capabilities of older adults. It also includes an examination of the literature concerning the neural adaptations which older adults experience in response to resistance training, and an evaluation of the degree to which these adaptations may improve the functional movement capabilities of older adults. Particular consideration is given to the marked reduction in the rate of force development that occurs in the elderly, and the principles that govern the transfer of resistance training adaptations to the performance of functional movement tasks by older adults.
Chapter 3 is a report of an experiment which focussed upon the association between the reduced rate of force development exhibited by older adults, and their ability to coordinate groups of muscles during goal directed movements. Ten young (19-29 years) and ten older adults (65-80 years) were compared with respect to their performance of a visually guided aiming task, that required the generation of isometric torque in two degrees-of-freedom about the elbow joint. While young and older adults were distinguished overall in terms of the time taken to acquire targets, the extent of the difference between the groups was influenced greatly by the particular direction in which it was required that isometric torque be applied. This indicated that the older adults were deficient in their ability to rapidly compose patterns of muscle activation which matched the demands of the specific movement task. At the neuromuscular level, differences between the young and the elderly were expressed most prominently in the bi-functional muscle biceps brachii and in certain temporal aspects of muscular coordination.
For resistance training to enhance the functional capabilities of older adults, it is essential that the benefits to muscle coordination that are derived from resistance training, transfer beyond the specific training exercise tasks to a variety of different movement contexts. The second experimental study, described in chapter 4, was conducted to assess the transfer of resistance training adaptations to a coordination task requiring the rapid production of force. Thirty adults (≥60 years) completed a visually guided aiming task prior to and following a 4 week training period. The experimental design included both groups that trained with progressively increasing loads, and groups that trained with constant-low loads. Two groups were assigned to each training load condition, but were distinguished by the direction in which isometric torque was applied during training. Both the magnitude of the training load and the nature of the training movement influenced the pattern and extent of the improvements that were accrued from the intervention. For one group that trained with progressively increasing loads, there arose a subsequent decrease in performance in one condition of the transfer task. For each group, changes in transfer task performance were accompanied by systematic changes in the coordination of muscles about the elbow joint, particularly biceps brachii.
The final experiment, discussed in Chapter 5, was conducted to determine whether older adults exhibit increases in the rate of muscle activation in response to training tailored to enhance the rate of force development. This was assessed in eight young (21-35 years) and eight older adults (60-79 years), who produced maximal rapid contractions, before and after 4 weeks of progressive resistance training for the elbow flexors. Both the young and the older adults exhibited significant training related increases in the rate of force development. These were accompanied by an increase in the electromyogram throughout the initial 100ms of muscle activation. For older adults, however, this training response was only apparent in the brachialis and brachioradialis muscles. It was not observed in surface EMG recorded from the biceps brachii muscle during rate of force development testing, or during training.
In conclusion, the program of research described in this thesis affirms that ageing results in deficiencies in the coordination of muscles, that in turn limits the ability of older adults to produce force rapidly across a wide variety of tasks. These deficiencies in coordination are not necessarily improved by undertaking training regimes that incorporate progressively increasing resistance loads. The research described in this thesis also provides evidence that young and older adults are distinguished in the specific neuromuscular adaptations that are experienced as a result of progressive resistance training.