cross education: why to continue training the "good" limb when the other side is injured

Bottom line: If you have an injured limb and are unable to train as normal (or train at all), training the other side can still yield benefits of the injured side
 
Question time….Let’s say you injure your left knee. What is the best strategy for your training plan?
 
1 Push through pain no matter how bad it is. Don’t change your training plan. Rest is for sissies
2 Continue to train the right knee, so long as it doesn’t further injure or provoke symptoms in the left
3 Stop training both legs because you don’t want you’re right leg to become too strong relative to your left and develop an asymmetry
 
There is a “best” answer here and that answer is number 2. Answer number 1 is ridiculous, so we’ll put that one aside. Number 3 sounds compelling and while it wouldn’t necessarily be “wrong” to go that direction, there are several reasons why number 2 is better.
 
Again, this is all based on the caveat that the training you do for the uninjured limb doesn’t put the injured limb at further risk of injury or delay healing. For example, let’s say you were doing some single leg activities on the uninjured leg, but each impact sent a “shock” through the injured leg. Your best bet is to either find different activities or delay training the uninjured limb until it can be done safely without provoking the injured limb.
 
In the literature, the concept of training effects on one limb being felt on the opposite side limb is called cross education. And to the surprise of some, the evidence is quite clear that cross-education works. Unfortunately, what should be the standard of care (training the rest of the body at the highest level possible in a way that supports the healing process), is instead a novel concept to many.

There are several reasons why cross-limb education is not promoted more. One is that the medical process is often siloed and sees the world with tunnel vision. For example, most orthopedic surgeons are going to just look at the injury and not even consider. That’s not meant to be critical, it’s just the nature of their job. They’re good at going into a damaged body part and making structural corrections using surgical techniques. They aren’t paid to tell you how to train. Similarly, most PT’s in a “traditional” outpatient setting aren’t getting paid to consider the opposite leg (and in some cases are actively discouraged from looking anyplace other than the site of the injury…Not their fault, that’s just the nature of the system in which they work). uninjured limb.
 
What does the evidence say?....A meta-analysis by Green (2018) reviewed 96 studies on cross education involving healthy young adults, healthy older adults and patients, finding an average strength gain via cross education of 18% in young adults, 15% in older adults, and 29% in a patient population. Although prior meta analyses had shown conflicting results on gender differences and upper versus lower limb effects, the studies compiled by in the Green meta-analysis revealed no significant differences. Overall, the average cross body transfer ranged from 48%-77% in the compiled studies, meaning that the immobilized limb experienced strength gains of 48%-77% of the trained limb. Again, this isn’t just one study on a homogenous group of subjects…this is 96 studies in a variety of populations. (Another interesting finding was that electrically stimulation of muscles appeared to have a greater cross-education effect than voluntary stimulation).
 
WHY cross education works remains up for debate but most evidence points to neurological mechanisms as the primary factors. Per Cirer-Sastre (2017), “this owes to the fact that no significant vascular adaptations have been found, nor were any histological changes in hypertrophy levels, in enzyme concentration, in contractile protein composition alteration, in fiber type or in cross-sectional area.” In short, strength is function of the nervous system, and the effects of nervous system output are global, meaning not specific to any single body part.
 
Although the evidence has been relatively settled that cross-education does exist via single sided training, HOW to facilitate cross-education is less clear although a meta-analysis of ten studies by Cirer-Sastre, “to optimize contralateral strength improvements, cross-training sessions should involve fast eccentric sets with moderate volumes and rest intervals.
 
Finally, approaching this from a pragmatic standpoint, if you sustain an injury to a limb, you could continue training the other limbs and have three “good” limbs or you could stop training both the injured and non-injured arm/leg and have two “good” limbs. Which sounds better?
 
The concern about developing asymmetries is logical, but overall is far overblown. As we know from the research, the cross-education effect will mitigate strength losses from inactivity by an injured limb. Further, even if cross-education did not exist, you’re better off having to bring one limb up to speed rather than losing strength in both sides have having two limbs to return to normal. Simply makes no sense why you would voluntarily weaken a perfectly healthy limb. Even if an asymmetry develops during a recovery period, its not as though you were going to hop right back into peak training after you get over the injury. You might as well give yourself the highest “baseline” with a strong, healthy limb rather than let both sides regress.
 
REFERENCES
Lara A. Green & David A. Gabriel (2018) The effect of unilateral training on contralateral limb strength in young, older, and patient populations: a meta-analysis of cross education, Physical Therapy Reviews, 23:4-5, 238-249, DOI: 10.1080/10833196.2018.1499272
 
Cirer-Sastre R, Beltrán-Garrido JV, Corbi F. Contralateral Effects After Unilateral Strength Training: A Meta-Analysis Comparing Training Loads. J Sports Sci Med. 2017;16(2):180–186. Published 2017 Jun 1.