Apparatus for precision calibration of skinfold calipers

Skinfold calipers: which instrument to use?

The considerable amount of original and generic types of skinfold calipers available is a source of systematic measurement error. This study is a brief report that critically examines the original and illustrated structural configuration of the three main types of skinfold calipers. For more than half a century, the Harpenden®, Lange® and Slim Guide® skinfolds calipers have been widely used in clinical and research settings. It is well established that the physical, mechanical and functional specificity of each type of skinfold caliper makes its interchangeable use impossible. Our report suggests that commercially available technical specifications are insufficient to judiciously choose a skinfold caliper. The area of the jaws, the coefficient of spring and the static and dynamic downward pressure of each type of skinfold caliper must be determined in the metrological laboratory and added to the technical user manual. Choosing a type of skinfold caliper for regular use, without conflict of commercial interest, requires a critical understanding of the physical, mechanical and functional characteristics that configure it. Therefore, a new downward static calibration test and the first eligibility flowchart for a skinfold caliper have been proposed. Finally, the information gathered in this report may be useful for manufacturers of anthropometric instruments and health professionals who use the skinfold technique as a tool for diagnosis and nutritional control.

Interpreting skinfold sums. Use of absolute or relative typical error?

The main aims of this study were to: 1) quantify how divergent the sum of seven skinfolds (Sigma7(skinfolds)) of athletes have to be before a general index of measurement imprecision (typical error, TE) is no longer appropriate to very lean or somewhat overweight athletes, and 2) discuss the application of absolute or relative (TE%) typical errors. The Sigma7(skinfolds) was measured in duplicate on 101 athletes by one level 3 anthropometrist with one calibrated skinfold caliper. The TE and TE% for Sigma7(skinfolds) were calculated for all data (TE(ALL), TE%(ALL)), as well as for results less than 50 mm (TE(<50), TE%(<50)), inclusive of 50-74.9 mm (TE(50-74.9), TE%(50-74.9)), inclusive of 75-99.9 mm (TE(75-99.9), TE%(75-99.9)) and 100 mm and greater (TE(>or=100), TE%(>or=100)). At least 20 samples were taken for each measurement range. Limits of agreement (LoA) at the 68% and 95% confidence level were also calculated for all data and the four skinfold ranges. The TE, TE 68, and 95% LoA increased as a direct function of the total Sigma7(skinfolds) and the general TE(ALL) was inappropriate for TE(<50) and TE(>or=100). In contrast, the %TE was very similar within each confidence level (ranging from 1.6-2.1% and 3.1-4.2% for the 68% and 95% LoA, respectively) regardless of the skinfold total. To minimise inaccurate feedback to individuals, anthropometrists dealing with skinfolds of elite athletes should establish an absolute TE on a homogeneous sample of athletes with a Sigma7(skinfolds) in a narrow band, for instance <50 mm. We also urge prudence in interpreting change in skinfold totals and suggest that the 95% level of confidence is appropriate in most instances.

Use of anthropometric variables to predict relative body fat determined by a four-compartment body composition model

OBJECTIVE

To generate equations for the prediction of percent body fat (% BF) via a four-compartment criterion body composition model from anthropometric variables and age.

DESIGN

Multiple regression analyses were used to predict % BF from the best-weighted combinations of independent variables.

SUBJECTS

In all 79 healthy males (X+/-s.d.: 35.0+/-12.2 y; 84.24+/-12.53 kg; 179.8+/-6.8 cm) aged 19-59 y were recruited from advertisements placed in a university newsletter and on community centres' noticeboards.

INTERVENTIONS

The following measurements were conducted: % BF using a four-compartment (water, bone mineral mass, fat and residual) model and a restricted anthropometric profile (nine skinfolds, five girths and two bone breadths).

RESULTS

Stepwise multiple regression selected six (subscapular, biceps, abdominal, thigh, calf and mid-axilla) of the nine skinfold measurements to predict % BF and using the sum of these six produced a quadratic equation with a standard error of estimate (SEE) and R(2) of 2.5% BF and 0.89, respectively. The inclusion of age as a predictor further improved the equation (% BF=-0.00057 x ( summation operator 6SF)(2)+0.298 x summation operator 6SF+0.078 x age - 1.13; SEE=2.2% BF, R(2)=0.91). However, the best equation used only the sum of three skinfold thicknesses (mid-axilla, calf and thigh) and age but also included waist girth and biepicondylar femur breadth as predictors (% BF=-0.00258 x ( summation operator 3SF)(2)+0.558 x summation operator 3SF+0.118 x age+0.282 x waist girth - 2.100 x femur breadth - 2.34; SEE=1.8% BF, R(2)=0.94). Analyses of two age groups, <30 and >/=30 y, demonstrated that for the same % BF, the former exhibited a higher sum of skinfold thicknesses.

CONCLUSIONS

Equations were generated for the prediction of % BF via the four-compartment criterion body composition model from anthropometric variables and age. Agewise differences for the sum of skinfold thicknesses may be related to an increase in internal fat for the older subjects.

Improved rig for dynamically calibrating skinfold calipers: comparison between Harpenden and Slim Guide instruments

This article describes an improved rig for the dynamic calibration of skinfold calipers. The new unit is 5% lighter and almost 60% smaller than its predecessor (Carlyon et al., 1996, 1998) with a 9.5 mm solid aluminium base and a quick release caliper mount providing stability to both the rig and caliper. Automation of the gap controller with an electric motor standardizes the jaw opening and closing velocity, thereby enabling hands-free operation. Frictional losses in the moving components of the rig have been reduced by replacing the main bush of the swing arm with a bearing, reducing the mass of the swing arm, adding a support wheel to the end of the swing arm, and replacing fishing swivels with a universal joint to allow for changes in the opening screw angle as the caliper's arm moves through its arc. This rig can also be adapted to different types of calipers by changing the position of the load cell, microswitches, and the caliper mount. A universal mounting bracket that can be secured to almost any table supports the rig in a vertical plane when calibrating the load cell. To demonstrate the versatility of the calibration rig, preliminary data are presented for the upscale and downscale jaw pressures of seven Harpenden and seven Slim Guide calipers.

Predicting the resting metabolic rate of 30-60-year-old Australian males

OBJECTIVES

This study: (a) generated regression equations for predicting the resting metabolic rate (RMR) of 30-60-y-old Australian males from age, height, mass and fat-free mass (FFM); and (b) cross-validated RMR prediction equations which are currently used in Australia against our measured and predicted values.

DESIGN

A power analysis demonstrated that 41 subjects would enable the detection of (alpha=0.05, power=0.80) statistically and physiologically significant differences of 8% between predicted/measured RMRs in this study and those predicted from the equations of other investigators.

SUBJECTS

Forty-one males ([X]+/-s.d.:, 44.8+/-8.6 y; 83.50+/-11.32 kg; 179.1+/-5.0 cm) were recruited for this study.

INTERVENTIONS

The following variables were measured: skinfold thicknesses; RMR using open circuit indirect calorimetry; and FFM via a four-compartment (fat mass, total body water, bone mineral mass and residual) body composition model.

RESULTS

A multiple regression equation using mass, height and age as predictors correlated 0.745 with RMR and the s.e.e. was 509 kJ/day. Inclusion of FFM as a predictor increased both the correlation and the precision of prediction, but there was no difference between FFM via the four-compartment model (r=0.816, s.e.e.=429 kJ/day) and that predicted from skinfold thicknesses (r=0.805, s.e.e.=441 kJ/day).

CONCLUSIONS

Cross-validation analyses emphasised that equations need to be generated from a large database for the prediction of the RMR of 30-60-y-old Australian males.

Skinfold thickness varies directly with spring coefficient and inversely with jaw pressure

PURPOSE

The main aims of this study were to: 1) determine whether heavy use of Harpenden calipers caused deterioration of the spring coefficient (force per unit length), 2) to quantify the change in skinfold thickness per unit change in jaw closing (downscale) pressure, and 3) to develop a calibration range for these calipers.

METHODS

Part a) The change in spring force per unit length after at least 100,000 cycles of opening and closing five different springs was measured on a load cell. Part b) The dynamic downscale jaw pressure exerted by six pairs of Harpenden springs was measured on one caliper. Two were new pairs of springs (N1 and N2), two were 25-yr-old springs (O1 and O2), and two pairs (S1 and S2) had been used for less 1 yr. The six spring pairs were used to measure skinfold thicknesses at nine sites, in triplicate, on 20 subjects with the order of springs randomized and counterbalanced. Part c) The downscale jaw pressure of 78 Harpenden calipers was measured at eight jaw gaps.

RESULTS

Part a) The springs did not change their characteristics after >100,000 cycles. Part b) At each skinfold site, the lowest thickness was recorded for S2 which exerted the highest jaw pressure (9.04 g x mm(-2)) and conversely the highest thickness was for N1 which exerted the lowest jaw pressure (8.02 g x mm(-2)). Increasing the downscale jaw closing pressure from 8.0 to 9.0 g x mm(-2) reduced a skinfold thickness by approximately 10%. Part c) The mean downscale jaw pressure was 7.82 +/- 0.25 g x mm(-2).

CONCLUSIONS

In summary, it is suggested that if accurate skinfold measures between different Harpenden calipers are required, the downscale jaw pressure should be in the range of 7.40-7.82 and 7.85-8.21 g x mm(-2), at jaw gaps of 5 and 40 mm, respectively. These jaw pressures can be achieved by servicing the caliper pivot and indicator gauge to minimize frictional losses, adjusting the caliper jaw alignment, and by selecting springs that have a spring coefficient in the range 1.10-1.15 N x mm(-1).