Myers et al. (1984) used the ILM to test 1003 females and 980 males about to commence basic training in the US Army. 951 of these were tested on job-related criterion tasks 8-16 weeks later. ILM lifting capacity to 1.52 m accounted for 67% of the variance in the criterion measures of physical competence. Lean body mass and an isometric upright pull accounted for an additional 3% and 1% respectively. The test had high validity (R = 0.84). Fairness analysis showed a minimal over-prediction for
women, with non-significant slope differences and only slight intercept differences. Aghazadeh and Ayoub (1985) used isokinetic lifting strength and anthropometric and static strength measurements from nine males to create models for predicting lifting
capacity. They found that dynamic strength resulted in a better model with a greater prediction capability than static strength could provide.
Mital and Karwowski (1985) found poor correlations (r < 0.4) between static strengths and psychophysical maximum lift capacity measured on 19 males and 6 females. They found better correlations (0.5 < r < 0.7) between 'Simulated Job Dynamic Strength' (SJDS), measured at 0.75 m s“‘ with the Mini-Gym, and maximum lift capacity. They therefore concluded that the dynamics of a manual handling activity cannot be ignored. Jiang et al. (1986a) developed models to predict capacity for four types of manual handling activity using data from 12 males. They found that limiting-activity based models produced the most accurate predictions and that isoinertial strength-based models had the advantage of ease of testing and good face validity.
Jiang et a l (1986b) developed models predicting psychophysically determined lifting, lowering and carrying capacities at three frequencies from isoinertial and isometric strengths. Data were collected from the 12 males used by Jiang et al. (1986a). Adding body weight to the capacities improved the correlations between them and the strength variables. The score on a 1.83 m ILM lift was the best single predictor (r = 0.85 to 0.95). Second order polynomial regression models using this score as a predictor gave R2 values between 0.791 and 0.950.
Jiang and Ayoub (1987) re-analysed data of Ayoub et a l (1978) from 73 males and 73 females. They used Principal Components Analysis to derive factor-score based models in order to predict Maximum Acceptable Load for lifting tasks. A strength and an anthropometric factor accounted for 85% of the variance among seven measures of strength and anthropometry. The model included the two factors, the frequency of lift and a constant representing the range of lift, and had an overall value of 0.924.
Three previous models developed by Ayoub et al. (1978) had R^ values ranging between 0.754 and 0.903. They commented that "since there is no sex variable in the predictive model as shown in the previous model (Ayoub et al, 1978), the sensitive problem of gender discrimination is thus avoided". They concluded that factor-score-based models have the advantage of providing a more explainable and meaningful structure for determining maximum acceptable loads than other models.
Ayoub et al. (1987) provided a brief overview of the USAF strength selection programme, of which McDaniel et al. (1983) reported part. 13 simulated tasks were devised which accounted for approximately 90% of the physically demanding tasks identified within Air Force Speciality Codes (AFSCs). Stepwise regression was used to determine which of eight selection tests were the best predictors of performance by 527 USAF personnel on the simulated tests. XI, the ILM lift to 1.83 m, was the best
predictor for most of the tasks, with correlations ranging from 0.53 to 0.87, and further variables added very little. As a result, single variable models were used but, to
normalise the data, a weighted regression of (XI y was used. The results were used to develop assignment criteria for AFSCs.
Nottrodt and Celentano (1987) reported similar work undertaken for the Canadian Forces. Analysis of military trade requirements showed that the predominant
requirement was lifting, with strength the limiting factor. 83% of lifting tasks started at floor level and finished between waist and shoulder height. Two tasks were chosen to represent 100 trades: lifting from the floor to 1.33 m, and a lift, 5 m carry, and place at a height of 0.75 m. The maximum weight that could be lifted smoothly and comfortably in a box 610 x 380 x 250 mm with handles was determined for each task for 31 males and 25 females, who were not trained lifters, nor lifted extensively as part of their job. The ILM was the best overall predictor of task performance. The use of performance standards to predict successful or unsuccessful lifting task performance was investigated using cut-off criteria of 32 kg and 41 kg for the two tasks, and two separate methods for establishing cut-off scores. They found similar results for the two techniques, resulting in 39% and 9% more correct screening decisions than using no screening test for tasks 1 and 2 respectively.
Wu and Hsu (1993), following Jiang et al (1986b), used ILM performance as a predictor of maximum acceptable weight of lift (MAWL) of a group of 12 Chinese males. They also found that it was a better predictor than isometric strengths.
Prediction models using isoinertial strength to both 1.83 m and elbow height performed best, whereas Jiang et a l (1986b) had recommended only the 1.83 m ILM test. Adding ILM performance to elbow height to the model increased from 74% to 82%.
Duggan and Legg (1993) measured performance of 384 male army recruits on a series of strength tests. Performance on a Quasi-isokinetic lift test was a poor predictor of maximum isometric lift capacity, giving rise to a much lower correlation (r = 0.46) than the modified Cybex apparatus, as used by Aghazadeh and Ayoub (1985), at a lift speed of 0.47 m-s“^ (r = 0.72). Unlike previous studies, there was not a clear superiority of dynamic tests over static tests as predictors of isoinertial lifting capacity. Using the same hydrodynamometer as Grieve (1993) they found a mean power output of 431.1 (SD 119.0) W. There was a linear correlation of 0.67 between hydrodynamometer power output and maximum incremental lifting capacity to 1.52 m on an ILM. Using multiple regression they showed that, when combined with measures of height and weight, both the hydrodynamometer and an isometric upright pull at a height of 380 mm, were equally good predictors of ILM performance (r = 0.77, = 0.59-0.60). They concluded that the hydrodynamometer lift was the most suitable of the dynamic tests, with a high level of criterion-related validity and reasonable face validity, while being significantly cheaper than the Cybex dynamometer.
CHAPTER 3
HYDRO-RESISTIVE MEASUREMENT OF DYNAMIC LIFTING