Hey and welcome to post 41 of Strength and Conditioning for Therapists. Today we’re looking at the cross-education effect; I first wrote about this way back and presented on the topic at Therapy Expo conference in 2017. Since that time it’s become more familiar to therapists and seems now to be talked about as one of the ‘next big things’. The phenomenon has actually been know about since the 1800s would you believe, but only recently has it been applied to rehabilitation settings. Let me tell you a little more about the cross-education effect and how it can be used to accelerate rehabilitation outcomes.
What is the Cross-Education Effect?
The cross-education (CE) phenomenon describes the strength gain in the opposite (contralateral), untrained limb following unilateral contralateral resistance training. That’s right, train the right limb and get strength gains in the left, a well as the right! Crazy huh? Well, it’s not universally the case, we have to make sure that the resistance training intervention is designed correctly to achieve this, but more of that later.
As I mentioned above, the cross-education effect has been know about for some time and is particularly well-documented in the sports performance / exercise physiology literature. Get this, in healthy and athletic populations, recent meta analyses have reported the magnitude of the cross-education effect to represent an absolute gain of 8%–12%, or ~50% relative to the improvement in the trained limb (Carrol et al. 2006; Manca et al. 2017). That’s quite a lot, wouldn’t you agree? Despite this, only recently has this phenomenon received significant scrutiny from clinical researchers. That is apart from those involved stroke research; they’re way ahead of us in MSK.
Cross-Education Effect In Clinical Settings
I’m sure you can clearly see the opportunity for using the cross-education effect in MSK clinical settings. Typical orthopaedic and MSK populations represent patients with single limb injuries / immobilisations and, or, surgeries. We know the consequences of these can mean significant muscle deconditioning and atrophy, so wouldn’t it be great if we could limit the deconditioning in the injured/operative limb?
Perhaps the first well-controlled study to explore this phenomenon with an orthopaedic application was by Farthing et al. in 2009. They casted one arm of health of healthy participants and split the group randomly into a resistance training of the opposite arm, or nothing. As predicted, once the cast was removed, muscle strength changes were observed. There was a 14.7% decrease in strength of the casted arm of the group who received no training (Cast group), but no significant change in the strength of the casted arm in the trained group (Cast-Train group) – i.e. those who received training on the opposite side.
There was a protective effect you might say, or what’s called currently ‘strength sparing‘. And so it sparked the interest in clinical populations… well, at least just a little more.
It’s clear to me that if we amalgamate the evidence in healthy populations, the findings of immobilisation studies and the studies now emerging in clinical populations, the cross-education effect has definite potential for clinical utility. However, the protocol needs to be potent and specific enough to elicit these effects.
Cross-Education Training; What’s The Formula?
So, what works an what doesn’t? Well I think the most important thing is to get the intensity of the intervention right, which is strength-training in focus: high load, low repetitions; then provide adequate rest between sets (1.5 – 2mins) and then; provide an adequate dose, potentially 2 times per week.
Why high intensity? Cross-education effects are neurally-driven; higher intensity muscle contractions maximise the stimulus of the corticomotor pathway that activates the resting / unexercised limb. So, strength-training (proper strength training!) protocols are far more likely to generate an effect by comparison to interventions to optimise hypertrophic and muscle endurance responses (lower intensity contractions). Indeed, studies using eccentric contractions, which typically recruit greater proportions of fast twitch motor units and that are frequently of higher intensities show a little more promise. That said, high-intensity isometric contractions have also been shown to work.
Duration? Whilst a recent study demonstrated that you might see strength and rate of force development changes in the non-exercised limb in just 2 weeks (Carr et al, 2019), few studies have investigated such a short duration of training and the population studied were young, healthy resistance-trained males, so not immediately translatable to patients who are often inhibited, in pain and even unsure about /un-habituated to high-intensity resistance training.
Cross-Education Effect and Rehabilitation
I recently finished a clinical trial that experimented with applying the cross-education effect in an ACL population. At the time of conception of our trial, there was only one study that had investigated this, however, since our completion, there are now 3 clinical trials that have investigated the cross-education effect in an an ACL population, albeit with mixed results.
The mixed results, meaning that some showed a positive effect of CE, whilst others didn’t, in my opinion are due to the protocol design. As I mentioned above, we need to get the prescription right to elicit CE effects. The clinical trials that have seen an effect have used high-intensity muscle contractions of a strength-training prescription. Those that haven’t have prescribed a hypertrophy-focussed protocol involving 8-12 RM and limited inter-set rest intervals.
I’m almost done writing the full paper for publication, but here is a sneak peek at some of our results. So we recruited 44 patients who’d elected to undergo unilateral ACL reconstruction surgery and we randomly allocated them to either a control (CON) group [confusingly labelled below as Placebo] or a cross-education (CE) group [labelled Contralateral below]. All patients followed the normal, standardised ACL rehabilitation programme and in addition, the CE group performed 8-weeks of strength training on the non-operative limb: 3 sets of 3-5 repetitions maximum (RM), of: knee extensions; hamstrings curls and leg press, 3 times per week and the CON group, a time-matched upper limb flexibility training programme (as a placebo), both commenced 2 weeks following surgery.
We assessed a range of things, like strength, rate of force development, functional and patient-reported outcomes.
Here’s a draft figure of the peak force (strength) of the quadriceps, measured pre-surgery, 10-weeks post, and 24-weeks/6-months post of the injured and non-injured limb
The statistics revealed that first, the strength training intervention was successful in increasing strength of the trained limb: 7% vs a 7% loss in the control group [figure on the right above], measured at 10-weeks post surgery. Scores remained superior at 6-months post surgery too.
in the ACL-Reconstructed limb, there was a 30% loss in quadriceps strength [figure on the left above], BUT only a 16% loss in the CE resistance trained group’s reconstructed limb. Remember, no additional training was performed on the reconstructed limb! So, that means the CE intervention was successful in almost halving the loss of strength in the quadriceps following surgery.
At 6-months post, whilst the data looks different i.e. the CE group look stronger, statistically they weren’t. There could be a few reasons for this, one of which being that unfortunately we suffered a few drop outs at the final measurement point, which rendered the analyses underpowered to detect this ‘difference’.
Return To Play
What’s also interesting to note is the strength performance compared to baseline. Whilst I’d argue that we’d want to restore performance to beyond that of pre-surgery measures before discharge and return to play (RTP), many people opt to use pre-operative measures as a goal. Note how the CON group failed to reach pre-operative levels at 6-months post-surgery – and incidentally this is very commonly reported in the literature, however, the CE group had achieved it.
These data are for me interesting to consider alongside the literature that discusses criteria to determine RTP and the discussion of time frames. There’s now a move away from advocating a first RTP at 6-months post ACLR in favour of maybe 9-months or event 12-months rehabilitation, which is partly based on risk of re-injury. IF lasting performance deficits are a factor for re-injury, perhaps cross-education training embedded within rehabilitation programmes may offer a strategy to accelerate rehabilitation outcomes and mitigate the risk of re-injury …?
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- Carrol et al. (2006). Contralateral effects of unilateral strength training: evidence and possible mechanisms. J. Appl. Physiol. 101(5): 1514–1522
- Manca et al. (2017). Cross-education of muscular strength following unilateral resistance training: a meta-analysis. Eur. J. Appl. Physiol. 117(11): 2335–2354
- Farthing et al (2009). Strength training the free limb attenuates strength loss during unilateral immobilization. J Appl Physiol 106: 830–836,