Wallin, D., Bjorn, E., Grahan, R. & Nordenborg, T. (1985). Improvement of muscle flexibility: A comparison between two techniques. The American Journal of Sports Medicine, 13, 263-268.

Males (N = 47) were formed into four groups. All groups trained three times per week using a modified contract-relax (CR) method or ballistic stretching. After 30 days, one of the CR groups (N = 10) trained once a week, another (N = 10) three times a week, and the third group (N = 10) five times a week. The fourth group (N = 17) trained with traditional ballistic stretching. After 30 days (14 training sessions), the ballistic group switched to the contract-relax method. Retests were performed after 60 days since commencement.

Once a week contract-relax stretching was enough to maintain flexibility. Training three to five times per week was necessary to increase flexibility. Ballistic stretching improved flexibility, but not as effectively as the contract-relax method. When the ballistic group switched to contract-relax, flexibility improved further and that group caught up to the others who had only been performing contract-relax activities.

Implication. Contract-relax (PNF) stretching is more effective for increasing flexibility than is ballistic stretching. Only one PNF session per week is needed to maintain flexibility.

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