We can strengthen one side of our body without lifting a single weight.
All you need to do is lift weights on the other side.
Cross-education training is using unilateral (one side of the body) resistance training to strengthen the contralateral limb (the opposite side of the body).
Why would you only train one side of the body? In short, for rehabilitation.
Use cross-education to overcome exercise restrictions
As a physical therapist, I regularly use cross-education training. Cross-education is valuable in physical therapy as it allows for strengthening — or limiting atrophy (muscle wasting) — when a limb is restricted following injury or surgery.
Whether restriction in movement is absolute (casted), minimal (brace or sling), or partially restricted (crutches), reduction in activity leads to atrophy and reduced strength. Through cross-education training, physical therapists can help patients rehabilitate a restricted limb without threatening the tissues in need of healing.
Casts and slings are designed to protect a healing body part. Depending on the severity of the injury and frailty of the area, complete or partial mobilization will be used. Unfortunately, muscles quickly diminish when they are not used.
Strength deteriorates within days of injury or surgery. Resistance training is not an option as the injured area cannot withstand the demanded strain.
Like a student’s GPA, it is harder and takes longer to build muscle than it does to reduce it. Following an injury or surgery, the bulk of the rehabilitation time is not waiting for the body part to heal, rather is recovering muscle mass and strength. Cross-education training can be used to speed up the timeline.
Cross-education works on all regions of the body
A recent study pooled 31 randomized control trials assessing the effects of cross-education training. The study dates ranged from 1967 to 2016.
Eleven trials assessed the knee extensors (quadriceps), eight assessed the wrists and hands, six assessed the elbow flexors (biceps), four assessed the ankle muscles, and two assessed single-leg squat assessments. The pooled data calculated the effect in upper and lower limbs after using isometric (no movement, like planks), concentric (pushing or pulling phase), eccentric (returning to start point, such as lowering a biceps curl), or isotonic-dynamic exercises (a machine keeping a constant resistance).
The researchers found cross-education training increased strength in the contralateral limb by 11.9% (upper limb: + 9.4%; lower limb: + 16.4%). Significant effects were induced by isometric (8.2%), concentric (11.3%), eccentric (17.7%), and isotonic–dynamic training (15.9%).
This data suggests all types of exercise, regardless of the region of the body, can be used for cross-education training.
Bear in mind, these studies are assessing strength — the maximum amount of force developed — not muscle size. Cross-education training can reduce atrophy, but it will not restore lost muscle.
Implementing cross-training in rehabilitation
The goal of cross-education training is to expedite the rehab process by reducing muscle loss and increasing strength while still allowing an injured area of the body to heal. The healing timeline should not change, but the overall rehabilitation timeline will.
There are many strategies physical therapists can use to maximize rehabilitation efforts and help people recovery from injuries and surgeries. Cross-education training is only one of them.
It is important to note that any exercise in the contralateral limb will not suffice. The intensity needs to match an exercise designed to improve strength. That means the intensity needs to be high.
Precautions should be taken when exercising at a high intensity while one limb is immobilized. Machines, simple movements, and controlled environments should be prioritized. In addition, diet and sleep should be prioritized to maximize recovery. A physical therapist can guide you through appropriate exercises to minimize the risk of over-exerting and potentially using the immobilized limb.
The human body is fascinating and we are learning more about its resilience and adaptability daily.