A study of cross-education in strength.

Cross-education has long been an area of interest. The term is credited to Scripture et al. (1894), who initiated the first series of studies in this area. These studies were mainly in the field of psychology and it was known that if a subject practised writing with one hand, skill was gained in writing with the other hand. DeLorme (1945), experimenting with adaption of weight-lifting procedures to the treatment of various disabilities of the skeletomuscular system, observed that unilateral exercise of the progressive-resistance type was associated with a concurrent development of increased strength in the symmetric muscle groups of the contralateral limb. Thus began the second series of studies, which were performed mainly by physicians and physical therapists. Hellebrandt, Parrish and Houtz (1947) suggested that neuromuscular cross-education may have a therapeutic use: where volitional control is defective, powerful isometric contraction may be evoked reflexly. Many theories have been put forward regarding the cause of cross-education of strength, none have yet been accepted to the exclusion of the others. In the opinion of this writer, it may be brought about in two ways: 1. By reflex isometric contractions occurring in the contralateral limb while the ipsilateral limb is being exercised. Hellebrandt et al. (ibid.) noticed that while subjects performed resisted knee extension exercises with one leg, the quadriceps of the other leg worked isometrically. Hansen (1961), Mathews et al. (1957) and Rarick and Larson (1962) have shown that isometric contractions will increase muscular strength. This possible cause of cross-education is referred to here as cross-training. 2. The skill learned by performing a strengthening exercise with one limb may bring about an increase in strength in the contralateral limb. Rasch and Morehouse (1957) found that subjects who trained isotonically in the standing position demonstrated a greater improvement in strength when they were tested in the familiar standing position rather than an unfamiliar lying position. They suggested that the gain recorded in the lying position was the true increase in strength, and that the difference between this score and the standing score was due to improvement in skill. This possible cause of cross-education will be referred to as cross-learning.