Discuss the relative contribution of muscle hypertrophy and neurological adaptations to the strength gains observed in response to a resistance training programme.

Resistance training or strength training is a type of exercise used to improve athletic performance, augment musculo-skeletal health and enhance body aesthetics. Resistance training is now adopted as an essential training component for a number of different athletes ranging from swimmers to soccer players, even endurance athletes (Wilmore and Costill, 1999). It is essential for strength and conditioning coaches to understand the mechanisms and processes involved in the development of strength for the successful implementation of appropriate and effective training programmes. Literature suggests that there are two main factors, which influence the role of strength development; hypertrophy and neurological adaptations. In general, muscle strength improves during the early phases of resistance training, with improvements in muscle size occurring later.
Muscular strength is defined as the maximal force a muscle can produce in one contraction (Wilmore and Costill, 1999). Substantial improvements (25-100%) in muscle strength have been observed after just 3 to 6 months of resistance training. However, the physiological adaptations, which occur as a result of resistance training, differ significantly over time and as a result of the stimulus provided. Traditional theories suggested that improvements in strength were as a direct result of muscle hypertrophy. Hypertrophy refers to muscular enlargement, which will occur as a result of regular high-resistance activity after a few months training. There are two types of muscle hypertrophy, which can occur as a result of resistance training. In terms of this review, chronic hypertrophy will be discussed throughout and reflects actual structural changes to the muscle in terms of an increase in the number of muscle fibres or the size of existing fibres. In contrast, transient hypertrophy refers to fluid accumulation in the muscle as a result of a single bout of training.
Increases in fibre size appear to result from an increase in myofibrils, more actin and myosin filaments and more sarcoplasm, which would result in more cross-bridges for force production. Ultimately, this results in an increase in the cross-sectional area of existing muscle fibres. Resistance training does not result in a uniformed response from muscle fibres and the amount of enlargement is dependent on the type of the muscle fibre and the pattern of recruitment. The degree of hypertrophy must be evaluated in terms of the quality and quantity of the contractile proteins, which adapt as a result of the strength training stimulus. The quality of the proteins refers to the type of protein found within the muscle with increases of myosin heavy chains occurring with the first few weeks of heavy resistance training (Fleck and Kraemer, 1997). As highlighted previously, there is no uniform response of muscle fibres to resistance exercise. Type 2 muscle fibres appear to make larger relative gains compared to type 1 fibres. In addition, type 2 fibres adapt by increasing the rate of protein synthesis, while type 1 fibres adapt by reducing the amount of degradation. Long term increases in muscle strength are generally assumed to result from fibre hypertrophy after neural adaptations have occurred. This could be a result of the time in takes for changes in protein synthesis to occur or indeed, for the degradation of protein to decrease.
In addition to increases in the size of existing muscle fibres, there is some evidence to suggest hypertrophy may occur as a result of an increase in the number of muscle fibres (Fleck and Kraemer). An increase in the number of muscle fibres, commonly termed fibre hyperplasia, appears to result from muscle fibre splitting and subsequent fibre hypertrophy however, there is still some debate regarding the relative importance of fibre hyperplasia in relation to fibre hypertrophy due to a limited number of studies reporting the occurrence of the hyperplasia in humans as opposed to animals. Whole muscle hypertrophy is therefore likely to be a result of individual fibre hypertrophy with a chance of a relatively small contribution of fibre hyperplasia.
There are a number of examples, which suggest improvements in strength are not entirely based on an increase in muscle size. Studies in women have shown a 100% increase in strength with no increase in muscle size (Wilmore and Costill, 1999). This suggests that there are other factors, in addition to muscle size, which ultimately determine strength. This does not mean to say hypertrophy will have no affect on strength gains as an obvious example of such a case can be seen in Olympic weight lifting classification. As weight classification increases, assuming a correlation between classification and muscle mass, so does total weight lifted. During the early phases of resistance training, muscle strength gains have been observed without muscle hypertrophy (McCardle et al., 2007). An increasing body of literature suggests motor unit recruitment is as important to strength gains as hypertrophy and in fact may account for a substantial amount in the improvements seen as a result of resistance training programmes. Such neurological adaptations to the early phase of resistance training include increased neural drive to the muscle, increased synchronization of the motor units, increased activation of the contractile apparatus and a decrease in the protective mechanisms of the muscle (Fleck and Kraemer, 1997). Hiroshi et al. (1999) investigated the effect of short periods of isokinetic training on muscle strength. Seven males performed isokinetic resistance training nine times over 13 days, consisting of 10 sets of five maximal isokinetic knee extensions. Results showed increases in muscle strength without any muscle hypertrophy, indicated by magnetic resonance imaging (MRI). It was postulated that increases in muscle strength was due to increased muscle contractile activity.
The nervous system works by the motor cortex receiving a message from the brain, while a message from a muscle is transmitted to the brain stem. From this stage, the message is transmitted to the motor neurons of the muscle in order to activate specific motor units (Moritani and DeVries, 1980). An integrated feedback system to the brain allows for intricate modification of the force produced and enables communication to other systems such as the endocrine system. Different training stimuli will result in different adaptations along this neurological pathway. Part of the training process and subsequent neurological adaptation is referred to as the size principle (Fleck and Kraemer, 1997). This refers to the motor-unit twitch force and recruitment threshold. Trained individuals are thought to be able to voluntarily recruit the highest threshold motor units, which mean they are able to maximally activate their muscles. There is also information to suggest that there is less muscle tissue activation in trained individuals. This means that unless there is a progressive increase in the training stimulus, less muscle will be activated as muscle strength increases.
Moritani et al. (1980) used electromyogram analysis the relative contribution of neural factors to increase muscle strength. Following 2 weeks of resistance exercise, the authors concluded that neurological adaptations accounted for 90% of the improvements found in muscular strength. In general, neural factors account for the majority of gains in maximal strength up until 8 weeks of training, after which, the percentage contribution of hypertrophy increases exponentially. In addition, Staron et al. (1994) used muscle biopsies to assess muscle fibre cross-sectional area following an 8-week, high intensity resistance training programme. Muscle strength improved substantially after 8 weeks of training, with the greatest gains occurring after only 2 weeks. The authors found that despite these improvements, biopsy results indicated only a minimal increase in the cross-sectional area of the muscle fibres, indicating strength gains were a result of neural adaptations and not fibre hypertrophy per se.
A substantial range of adaptations occur as a result of a resistance training programme, ultimately resulting in increased strength. An increase in the size of the active muscle, commonly termed hypertrophy, is seen as the long term adaptation to a resistance training programme, although hyperplasia may play a small part in this. There is a substantial weight of evidence to suggest a significant proportion of strength gains observed during the early phase of training is as a result of neurological adaptations. In principle, this increases the training stimulus to which the muscle is then exposed. It must be stressed that there are a number of confounding reports in the literature and although this seems like the most plausible explanation for the early development of strength when no change in muscle size is evident, the area remains ambiguous. Sports coaches and practitioners must understand the structural and functional developments of muscle as a result of strength training in order to develop specific programmes to suit each individual athlete. For example, research suggests that with the correct stimulus or strength programme, significant improvements in strength can occur without increased muscle mass. This may be particularly important for female athletes in particular who are often adverse to adding bulk or weight classification sports such as rowing or wrestling where increased strength would be beneficial but increased muscle bulk would be detrimental. This information is also important if the primary aims of the strength training programme is hypertrophy as it is apparent no increases in muscle size will be evident to begin with.