Effects of hand shape on maximal isometric grip strength and its reliability in teenagers.

Hands can be classified by their shape; width divided by length. This study examined the impact of shape of the hand (as relatively long, average, or square shaped) on maximal voluntary isometric grip strength and test-retest reliability in a group of healthy teenagers aged 13 to 17 years. When 116 boys and 112 girls were measured with a GripTrack (a computer-linked Jamar-like isometric hand grip dynamometer; JTech Medical Industries, Salt Lake City, UT), boys were significantly stronger than girls (by 11.02 kg force, p < 0.01), and dominant hands were significantly stronger than nondominant hands (by 2.53 kg force, p < 0.01). There were no significant differences due to hand shape. To examine the reliability of the grip strength test, 74 boys and 75 girls were retested, with a mean retest time interval of 15.5 days. There was a small but significant improvement in grip strength (by 0.62 kg force, p < 0.01). Regarding male subjects, the test-retest intraclass correlation coefficients (ICC 3,1) for both hands of the three hand-shape groups were excellent, with ICC values ranging from 0.954 to 0.973. For the female subjects, however, reliability values declined across the three hand-shape groups from long to square (ICC 0.920-0.476), such that compared with their male counterparts, strength test reliability examined by the 95% confidence intervals (CIs) was significantly lower for average and square hand shapes. Although there may be several factors contributing to this difference between boys and girls in strength test reliability for different hand-shape groups, it is possible that Jamar-like handgrip dynamometers have a handle shape that impedes those girls with relatively square hands from giving reliable grip strength readings over time.