Strength training: time for vertical thinking

As we’ve discussed in previous SPB articles, one of the key requirements for producing and effective and efficient training stimulus is training specificity. The principle of training specificity suggests that for maximum fitness and performance gains, the type of training an athlete undertakes should replicate the movements/locomotion/pattern of muscle use required in his/her actual sport as closely as possible. This explains why runners need to perform running training (and not focus on other training modes such as cycling/rowing etc) to run well in competition. Likewise, cyclists should primarily cycle and swimmers primarily swim etc.

Transfer of training effect

Although specificity is a key aspect of a training program, there’s also something else at play - the so-called ‘transfer of training’ effect. In a nutshell, although two different modes of exercise induce two different sets of specific adaptations in the working muscles, there is a degree of overlap. So for example, runners who cycle, swimmers who row, cyclists who cross-country ski etc can expect a reasonable degree of performance benefit when adding or substituting another form of training – so called cross training – into their training program.

However, this is not the only transfer of training effect at play, particularly when it comes to strength training. As Andrew Sheaff explained quite recently in his article on injury rehab and the transfer of training effect, there’s some evidence for a ‘cross-education effect’ in strength training, where training one limb results in improvements in strength in the opposite limb⁽¹⁾. This effect might be relevant for athletes unable to train an injured limb, and where training the non-injured limb could be a powerful tool for helping athletes recover from injury and return to sport safely.

Understanding the cross education effect

How is it possible that training muscles on one side of the body can produce strength gains on the other untrained side? Although the cross education effect in strength training is poorly understood (and indeed, remains a somewhat controversial theory among some researchers), the current consensus of thinking is that there are two prime reasons for this effect:

·        Neurological – when training one side of the body, powerful neural impulses are generated, which travel from the motor cortex in the brain to the motor neurons, and from there to the activated muscle fibres. However, it is likely there is some ‘spill-over’ of these neural impulses from the motor neuron pool to other muscle fibres, meaning that they also undergo a small degree of activation^(2,3). This effect may translate into longer-term strength improvements on the account of an ‘intensified central neural drive’⁽²⁾.

·        Hormonal – During vigorous training, the endocrine system secretes a number of hormones, which enter the blood stream and can then favorably stimulate different tissues such as muscle and motor neurons all over the body, regardless of whether those tissues underwent training^(4-6). Recent research suggests that these hormonal changes contribute to strength improvements of the untrained muscle groups in older adults⁽⁷⁾.

The lower-upper body cross-education effect

So far, we’ve talked about cross-education effects in terms of left/right side strength training and non-specific endurance training adaptations. However, some researchers have speculated that there could be a blend of these effects, especially in terms of upper-lower body transfer. One possibility is that upper-body strength training in runners could help develop leg strength, and help retain muscle fiber volume - ie preserving their size from decreasing after long running sessions⁽⁸⁾. This could potentially lead to improved leg power without the need for undertaking tiring leg-strength training sessions⁽⁹⁾. Likewise, resistance training of leg muscles may lead to strength improvements in upper extremities, which could benefit athletes where upper body strength and power are important for success.

To date, there’s been even less research into these upper-lower body cross-education effects than left side/right side effects. Part of the reason for this lack of data is that are just so many of exercise variations and permutations in which upper body and lower body exercises can be used. In addition, very few studies to date have looked at how upper-body and lower body strength and endurance training interventions impact each other. But now a new study has carried out a review of all the previous research on this topic, with an emphasis on upper-lower body cross-education effects (so-called ‘vertical strength transfer’)⁽¹⁰⁾.

New research

In this study by British scientists, and which was published last month in the journal ‘Sports Medicine’, the researchers set out to compile and analyze all the data from previously published research on vertical strength transfer effects, and to understand how training of one body region – for example the lower body – can affect adaptations in distant body areas (ie the upper body). In addition, they also sought to understand how different upper and lower body exercise protocols affect the induced training adaptations of either region.