Benefits of resistance training not limited to muscle hypertrophy

The scientific literature concerning mechanisms underlying strength increases through resistance training has focused largely on the role of muscle mass. However, it is increasingly recognised that the gains in strength are due to a combination of both morphological and neurological factors.4 ,5 The primary morphological adaptations are an increase in the cross-sectional area of the whole muscle and individual muscle fibres. Neurological adaptations are essentially changes in coordination and learning that facilitate improved recruitment and activation of the involved muscles during a specific strength task.

Strength gains following resistance training do not parallel muscular hypertrophy, especially in the early phase of training.6 The early gain in strength is often attributed to adaptations in the neural activation of muscles, with modifications occurring in both intramuscular and intermuscular coordination. Such adaptations may include decreased antagonist coactivation,7 improved coordination of synergist muscles8 and increased neural drive to agonist muscles resulting in the recruitment of additional motor units.9 There is also growing evidence of more central neurological adaptations as a result of resistance training. The specificity of strength gains to the performed resistance training indicates a process of motor learning, which has been shown to be related to cortical reorganisation.5 In addition, there is evidence that training of one limb causes strength increases in the contralateral untrained limb.10 This ‘crossover’ effect is often considered to be a result of central adaptations to resistance training. In some muscles, imagined contractions also appear to increase strength by inducing purely central nervous system adaptations.5

The loss of muscle mass with progressing age may limit the benefits from resistance training that can be achieved by muscle hypertrophy. It is possible that the primary means by which benefits to functional tasks will be derived is through neuromuscular adaptations to resistance training.6 The specific neural adaptations experienced by older adults and the mechanisms by which these changes occur will determine to what extent such changes benefit the performance of movements other than the training exercises. For exercise to have an impact on falls we require effects on movements other than the training exercises.

Several exercise intervention programmes shown to prevent falls11 ,12 state that they contain ‘strengthening’ exercise yet do not explicitly use the principles of muscle overload traditionally associated with resistance training interventions and appear to use weights that would be too light to have an overload effect for many participants. The impact of these approaches to exercise on falls despite the lack of focus on muscle overload may suggest a greater role of central and peripheral neurological factors rather than morphological muscle adaptations.

‘Balance’ is really motor control

Many exercise interventions designed to prevent falls target balance. The term ‘balance’ means different things to different people but most researchers now favour a broad definition of balance. For example, the Cochrane review of exercise interventions to improve balance13 uses a definition based on the one proposed by Winter that ‘balance is the ability to stay upright and steady when stationary and during movement … the ability to maintain the projection of the body's centre of mass (CoM) within manageable limits of the base of support, as in standing or sitting, or in transit to a new base of support, as in walking’. Therefore, balance is a key aspect of the performance of all daily tasks and we cannot separate the measurement and training of balance from the measurement and training of daily task performance. Balance requires the appropriate timing and activation (ie, coordination) of many muscles, so relies on central and peripheral neurological function. Improved balance with exercise is probably as a result of central and peripheral neurological adaptation.

Task specificity provides greater benefits

As it seems that both strength and balance are closely linked to task performance, strategies to improve them should involve practice of tasks relevant to daily activities. Greater benefits are possible for people with impaired mobility when exercise is relevant to daily tasks. We found standing up ability to be enhanced in rehabilitation inpatients when strength training was conducted in weight-bearing rather than seated positions14 and a greater impact on balance of standing exercise rather than seated exercise in older people who have recently been in hospital.15

If the primary aim of an exercise regimen is the prevention of falls, exercises should be relevant to fall avoidance that is, to maintaining balance for different tasks involving different body postures with a different base of support. The exercises that make up the Otago Exercise Program11 challenge balance in different positions (eg, tandem stance and walk, sideways walking). They also target muscles needed for maintenance of the upright postures (eg, extensor muscle activation by sit-to-stand practice and calf muscle activation during heel raises).

Liu-Ambrose et al 3 have highlighted the role of executive functions in falls and response to exercise. We suggest that central and peripheral neurological adaptations are a crucial part of strength and balance enhancement and should not be forgotten when trying to understand the mechanisms for the effect of exercise interventions.