european college of sport science

IS-BM08 Laterality and assymetries in sports
MECHANISMS OF CHANGES IN EXCITABILITY OF THE PRIMARY MOTOR CORTEX DURING UNILATERAL MUSCLE CON-
TRACTIONS: IMPLICATIONS FOR CROSS EDUCATION
HORTOBÁGYI, T., HOWATSON, G., RIDER, P., SOLNIK, S., DEVITA, P.
EAST CAROLINA UNIVERSITY
There is strong evidence suggesting that unilateral exercise increases motor function not only in the muscles of the exercised limb but
also in the homologous unexercised muscles of the contralateral limb, producing the phenomenon of cross education (10, 13). Such
adaptations occur under a variety of conditions, including exercise with voluntary isometric, concentric, eccentric, and electrical stimulation-evoked
contractions (2, 4, 8, 11). The mechanisms of these adaptations are unknown but most likely reside in the nervous system and
not in the contralateral muscle itself (1, 3). The purpose of this presentation is to examine candidate mechanisms of cross education in the
motor cortex and spinal cord. One possible mechanism is that neural drive in the “uninvolved” motor cortex increases due to its repeated
activation during unilateral muscle contractions (7, 12, 14). The magnitude and specificity of cross education are probably related to the
magnitude of activation of the ipsilateral motor cortex, which was shown to increase with intensity (7, 9, 12) and vary by type (shortening
vs lengthening) (5) of contralateral muscle contraction as determined by magnetic brain stimulation. Another possibility is that chronic
exercise reduces interhemispheric inhibition and the increased cross-hemispheric flow in turn increases the excitability of the uninvolved
motor cortex (6). Finally, spinal mechanisms cannot be excluded but evidence for cross-segmental effects appears weak or our methods
are inadequate to detect such effects (7, 9). In conclusion one of several independent mechanisms or perhaps a combination of these
may mediate increased motor output in the unexercised muscles of the contralateral limb after chronic unilateral exercise. After identifying
the exact mechanisms of cross education the next step is to determine if it is an effective method for the rehabilitation of unilateral
movement impairments in patients with a stroke or unilateral orthopedic injuries.
Supported in part by NIH AG024161
1. Carroll TJ et al. J Appl Physiol 101: 1514-1522, 2006
2. Farthing JP et al. Brain Topogr 20: 77-88, 2007
3. Hortobágyi T IEEE Eng Med Biol Mag 24: 22-28, 2005
4. Hortobágyi T et al. Med. Sci. Sports Exerc. 29: 107-112, 1997
5. Hortobágyi T et al. ACSM. Seattle, WA, May 27-30, 2009
6. Hortobágyi T et al. J Neurosci, Submitted, 2009
7. Hortobágyi T et al. J Neurophysiol 90: 2451-2459, 2003
8. Howatson G et al. Eur J Appl Physiol 101: 207-214, 2007
9. Muellbacher W et al. Clin Neurophysiol 111: 344-349, 2000
10. Munn J et al. J Appl Physiol 96: 1861-1866, 2004
11. Munn J et al. J Appl Physiol 99: 1880-1884, 2005
12. Perez MA et al. J Neurosci 28: 5631-5640, 2008
13. Zhou S. Exerc Sport Sci Rev 28: 177-184, 2000
14. Zijdewind I et al. J Neurophysiol 85: 1907-1913, 2001
NEW PERSPECTIVES ON CROSS EDUCATION: IS THERE A POTENTIAL BENEFIT OF UNILATERAL TRAINING DURING RE-
COVERY FROM UNILATERAL SPORTS INJURIES?
FARTHING, J.P.
UNIVERSITY OF SASKATCHEWAN
Cross education is the strength increase observed in the untrained contralateral limb after a period of unilateral strength training. Although
cross education was first documented over a century ago, there is still much to learn regarding the precise neural mechanisms
that control the effect and the potential utility of it during recovery from unilateral injury (1,5). Cross education is evident after a variety of
unilateral training regimens with an average magnitude of 8% or about half of the strength gain observed in the trained limb (1,8). However
the effect varies immensely, ranging from no significant effect up to 77% after voluntary training (6,11). There is now evidence that
cross education is asymmetrical, where dominant arm training elicits much greater cross education than non-dominant training in righthanded
individuals (2). This suggests that there is a preferential dominant to non-dominant direction of strength transfer in the upper
limbs, which might explain some of the variation in the literature regarding the magnitude of effect. This asymmetrical transfer of strength
is similar to what has been shown for cross education of skills and suggests involvement of higher order motor learning mechanisms
(1,2,7). A follow-up functional MRI study provided initial evidence of changes in brain activation patterns associated with the untrained left
limb, particularly in sensorimotor cortex and temporal lobe (3). While several studies have aimed to reveal mechanisms, there are few
attempts to apply cross education in a rehabilitation setting (9, 10). The purpose of this presentation is to discuss the potential for cross
education to be used as a strategy during rehabilitation from unilateral injuries (particularly those requiring immobilization) and to discuss
the implications of asymmetry of cross education in this context. A recent study demonstrated that unilateral strength training was effective
for preserving strength in an opposite immobilized limb (4). These findings suggest that there might be a therapeutic benefit of cross
education during recovery from unilateral sport injuries or in other clinical settings that involve unilateral immobilization.
Supported by the Saskatchewan Health Research Foundation (SHRF)
1. Carroll TJ et al. J Appl Physiol 101: 1514-1522, 2006 2. Farthing JP et al. Med Sci Sports Exerc 37: 1594-1600, 2005 3. Farthing JP et al.
Brain Topogr 20: 77-88, 2007 4. Farthing JP et al. J Appl Physiol In Press, 2009 5. Hortobágyi T IEEE Eng Med Biol Mag 24: 22-28, 2005 6.
Hortobágyi T et al. Med Sci Sports Exerc 29: 107-112, 1997 7. Lee M and Carroll TJ. Sport Med 37: 1-14, 2007 8. Munn J et al. J Appl Physiol
96: 1861-1866, 2004 9. Stromberg B. Am J Phys Med 65: 135-143, 1986 10. Stromberg B. South Med J 81: 989-991, 1988 11. Zhou S. Exerc
Sport Sci Rev 28: 177-184, 2000.
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ANNUAL CONGRESS OF THE EUROPEAN COLLEGE OF SPORT SCIENCE