Journal Article Review, October 2016

Can right-handers can cross strengthen wrist extension on either side?

Coombs TA et al: Cross-education of wrist extensor strength is not influenced by non-dominant training in right-handers. Eur J Appl Physiol. 2016 Sep;116(9):1757-69.
This study from Australia asks and answers a narrow and somewhat esoteric question, but the topic may have wide application.
Previous investigators have shown that strengthening exercises performed on the dominant right limb yielded increased strength in the unexercised nondominant left extremity. The current study expands that work by investigating if strengthening in the nondominant left upper limb in right-handed individuals would result in increased strength in the unexercised right upper extremity. In other words, does this cortical plasticity go both ways?
If so, the implications for maintaining or improving strength while one limb is immobilized are considerable. For example, after flexor tendon repair in the dominant right hand, will left hand putty exercises maintain strength in the right finger flexors without actually exercising them? Continue reading…
METHODS: The investigators studied 23 right-handed healthy adults and randomized them in to a control group or into left-hand exercise or right-hand exercise groups. They measured maximum voluntary wrist extension strength in both limbs and extensor muscle thickness (using ultrasound) both before and within a day after the test groups completed 3 weeks of wrist strengthening exercises.
The strengthening protocol was three 20 minute supervised sessions a week for 3 weeks. The test subjects performed active wrist flexion and extension exercises with a dumbbell.  Once a given weight was easily lifted, the training load was increased by 5%. The control group was instructed not to train but just to continue normal daily activities.
RESULTS: At baseline, there were no strength differences between limbs. For individuals who exercised their left wrist, extension strength increased 22% on their left wrist and 15% on their unexercised right wrist. For those who exercised their right wrist, extension strength increased 18% on the right side and 10% on their unexercised left wrist. All these differences were statistically different. The control group showed no change over time.   There were no changes in muscle thickness in any of the groups or over time.
CONCLUSION and COMMENT: The authors conclude that their findings support the bidirectional nature of cross-strengthening in right handed people. This could have implications for strength training following stroke or limb injury with or without immobilization. Whether left handed people exhibit a similar bidirectional nature of cross-strengthening is presently unknown.
In a general sense, the study highlights the plasticity of the human brain, and although there are obvious asymmetries regarding handedness and strength, exercising one side of the brain stimulates the other side as well. This is also apparent on a sensory level with mirror therapy. Whether or not you choose to use this information in your practice, awareness enhances appreciation for the complexity of our bodies.