Sources of variation in back and leg dynamometry

Three investigations were carried out to examine the concurrent validity and the reliability of a portable dynamometer (Takei Kiki Kogyo) for the measurement of back and leg strength. First, leg extension strength of 19 subjects was measured using the Takei dynamometer and compared to the isometric knee extension strength of the dominant (right) leg measured using a computer-controlled dynamometer (Lido Active, Davis, CA). The back extension strength of 18 subjects was also compared between the two dynamometers. Second, back and leg strengths of 36 subjects, aged 19-30 years, were measured twice using the Takei dynamometer. Six days separated the test and retest. Third, back and leg strengths of four subjects, aged 21-30 years, were measured at six different times of the solar day. Significant relations (p <0.001) were obtained between the Takei and Lido dynamometers for leg strength (r = 0.90) and back strength (r = 0.79). Significant test-retesi correlations (<0.001) were found for leg strength (r=0.80) and back strength (r = 0.91). Group mean (± SD) leg strength values of test (1450.4 + 428.6N) and retest (1432.8 ±449.1N) did not differ ((p<0.05). A small (4.5%) but significant difference was found between the test (1057.2 ± 309.9N) and retest (1106.2 ± 334.4N) mean values for back strength (p < 0.05). A time of day effeel was evident for back and leg strength (p < 0.05); on average the peak times occurred at 16:53h and 18:20h, respectively. Peak lo trough variations of back and leg strength throughout the day were 21.1% and 17.9% of the respective means. Results suggest that portable dynamometry can be used for determining leg and back strength in held conditions, provided that the measurement protocol is standardized and controlled for time of day.