Methods / study design

4.2.1 Subjects

The 'gender-free' study was designed to be a cross-sectional study of serving military personnel across the range of specialities in the British Army. The study design and methods have been reported by Rayson and Holliman (1995), from where much of the following information is extracted. The purpose of Rayson and Holliman (1995) was to report on the prediction of task performance of trained soldiers. Prior work (Rayson,

1998) had led to the identification of four generic criterion tasks or 'Representative Military Tasks' (RMTs) which covered the broad categories of physical tasks expected of trained soldiers. The RMTs consisted of a single lift (SL); a repetitive lift and carry (RL); a carry (C) and a loaded march (LM). Three performance standards were defined for each RMT with Level 1 being the most demanding, and Level 3 the least physically demanding. Each military specialisms was categorised by the combination of the different levels on the RMTs that reflected the physical demands of the tasks that such units are required to carry out.

Four groups of approximately 100 subjects were recruited to take part in the study. The units that the subjects in the different groups were drawn from are listed in Table 4.1 : Table 4.1: Units members of the different groups of subjects were drawn from

Group A Royal Artillery

Royal Armoured Corps

Royal Engineers___________________________________________________ Group B Army Air Corps

Royal Electrical and Mechanical Engineers Royal Army Medical Corps

Royal Army Dental Corps Group C Royal Signals

Royal Logistic Corps Adjutant General's Corps Royal Army Veterinary Corps Intelligence Corps

________Queen Alexandra's Royal Auxiliary Nursing Corps________________________ Group D Infantry

Each group of subjects was tested at appropriate levels of the RMTs. Two groups (Group A, Group D) were drawn from units whose job requirements had been largely linked to Level 1 performance on the RMTs. One group (Group B) had jobs requiring performance at Level 2 on the RMTs, and one group (Group C) had jobs requiring performance at Level 3. The precise combinations of test carried out are listed in Table 4.2. Group C were measured at their own Level for the Repetitive lift (RLIO) and also at the next higher Level (RL22).

Table 4.2: Levels of the different RMTs carried out by the groups of subjects

RMT Group A Group B Group C Group D

Single lift (SL) — SL SL SL

Repetitive lift (RL) RL44 RL22 RL10/RL22 ---

Carry (C) C C C ---

Loaded march (LM) LM20 LM20 LM15 LM25

The whole study had been approved by the Ethics Committee of the Centre for Human Sciences. Prior to testing all subjects received a detailed briefing as to the purpose and nature of the study and completed consent forms. All subjects were medically screened prior to participation in the study. Subjects wore civilian FT clothing during all fitness testing and appropriate military clothing while performing the RMTs. Soldiers were asked to perform all tasks to their individual safe maximum.

4 .2 ,2 T h e te s t b a ttery

The single lift involved progressive maximal lifting of a weighted ammunition box from the ground to two heights (1.7 m and 1.45 m). Subjects were advised on safe lifting techniques, but essentially the lift was freestyle. Subjects first attempted a load of

10 kg, and after each successful attempt 5 kg (or 4 kg after 40 kg) were successively added to the box until the subject could not safely achieve the lift within a ten second time frame, or until a maximum load of 72 kg had been achieved. An upper weight limit of 72 kg was set due to the limiting size of the ammunition box. The maximum successful load was recorded as the score.

The carry required subjects to walk up and down a 30 m course at a prescribed pace of 5.4 km/hr, carrying two fuel cans (20 kg each) for as long as possible. There were few rules, other than maintaining the prescribed pace and carrying the cans in a conventional manner. The test was continuous and no rest was allowed. The maximum duration that the subjects managed to achieve constituted the score.

The repetitive lift required subjects to lift a weighted ammunition box (10 kg, 22 kg, or 44 kg depending on role) at a prescribed rate (6, 3 or 1 shuttles per minute according to role), and carry it 10 m to and from a platform of 1.45 m for a maximum of one hour. Subjects were advised on safe lifting techniques, but essentially the manoeuvre was freestyle. The maximum duration that subjects managed to achieve, up to a maximum of 60 minutes, constituted the score.

For the loaded march subjects were required to complete a 12.8 km course as quickly as possible, with (according to role) a 15 kg, 20 kg or 25 kg backpack (Bergen). Subjects were advised to pace themselves sensibly.

All subjects were also tested using a battery of Physical Selection Tests over a three day period. Each group of subjects was subdivided into groups of approximately 15. Each subject was issued with a numbered bib for ease of identification. Each subgroup arrived at the gynmasium where testing was occurring at hourly intervals but always at the same time on each day. On arrival, they were all put through a warm-up routine, were further subdivided into groups of three or four, and over a period of approximately one hour were rotated through a series of test / measurement stations. The aim of the study was to collect all measurements from all of the subjects, and all subjects in each Group were tested in the course of one day. Anthropometric measurements taken included height in cm, body mass in kg and biceps, triceps, subscapular and supra-iliac skinfolds thicknesses in mm. These skinfolds were used to estimate body fat mass using the equations of Durnin and Womersley (1973) and hence lean body mass (fat- free mass).

The Incremental Lift Machine (ILM) (McDaniel et ah, 1983) formed one test station. The maximal weights that subjects could lift to 1.7 m and 1.45 m were determined using an incremental protocol. Subjects stood on the ILM platform with feet shoulder width apart, grasped the handles of the load carriage with palms down, arms straight, knees bent and back as straight as possible and lifted the handles until they passed a mark at

the carriage was 90 lb. After each successful lift the weight was increased by 10 lb, up to a maximum of 200 lb, and the lift was repeated until the subject chose to stop or failed to reach 1.7 m. After the last unsuccessful lift the weight was reduced by 5 lb and the lift repeated. The score recorded was the greatest weight successfully lifted. After a one-minute rest the subject attempted the last unsuccessful weight to a height of 1.45 m and continued until failure at that height.

4 .2 .3 R e p e a ta b ility stu d y

On a later occasion, the Group C subjects returned for a further day of testing which was used to take repeated measures on all the tests to enable their reliability to be assessed. The group of subjects was split into four subgroups, each of which performed a quarter of the original tests.

4 .2 .4 H y d r o d y n a m o m e te r te s t p r o to c o l

Data were collected using the hydrodynamometer described in Chapter 3.3.2, the instrumentation described in Chapter 3.3.3, and the computer hardware described in Chapter 3.3.5, with the methodology of Chapter 3.3.6.

The principle of the hydrodynamometer was explained to each small group of subjects, and the device demonstrated. The need for a change of grip was mentioned. Subjects were instructed that they should start with an overhand grip and pull as hard and as fast as possible on the handle from the start height to at least 2 m high. A marker was placed on the rope which would pass another marker when the handle reached 2 m. They were told to stand on the base-board of the device with their toes on the marked line. They were told to remain on the base-board during the lift, but this was not rigorously enforced, and some stepped back part way through the lift. They were told that when they had finished the lift they should keep hold of the handle and allow the weight to lower the piston through the water.

Each subject was allowed to practice the lift at a relatively slow speed to 'get a feel for' the device and to enable them to realise the need to change grip. Each subject then took it in turns to perform lifts on the device until each one had performed two maximal lifts. This procedure ensured that subjects had a chance to rest briefly between maximal exertions. Two pulls were performed because it was known that performance tended to improve, and the subjects were not familiar with the device. Because of this effect, which was assumed to be a learning effect, it was expected that only data from the second pull would be used. Only two pulls were permitted because of the need to complete testing of each sub group of four subjects within 15 minutes.

4 .2 .5 D a ta c o lle cte d

Of the 379 subjects (304 males, 75 females) who were entered into the cross-sectional phase of the Gender-free trials, hydrodynamometer data were collected from 320 (249

males, 71 females). Due to equipment problems the data from all Group A subjects were either lost or unusable . Other data were lost due to subjects withdrawing for a variety of reasons or operator error. Usable hydrodynamometer data were obtained from 287 subjects. The data that were utilised were from the second pulls that subjects performed only, which were from 270 subjects (201 males and 69 females).

Repeatability data for the hydrodynamometer were collected from 21 subjects (11 males and 10 females) from Group C. The subject numbers are summarised in Table 4.3. Table 4.3: Numbers of males and females in the different Groups with usable hydrodynamometer data and usable data from second pulls

Males Females

All Usable 2nd pulls All Usable 2nd pulls

Group A 76 0 0 4 0 0 Group B 69 64 51 22 21 21 Group C 55 54 54 49 48 48 Group D 104 100 96 0 0 0 Totals 304 218 201 75 69 69 4 .2 .6 D a ta p r o c e s s in g

The correction for the deflection of the cantilever as the force in the rope increased was made after all data had been collected. Anthropometric data for the subjects and performance data on the RMTs and the Physical Selection Tests were made available. Each lift on the hydrodynamometer was characterised as a series of 'Events' and a series of mean values over various 'Ranges' of lift. Data analysis was carried out using

Statgraphics Plus v5.22 (Statgraphics Inc.), a statistical software package which runs under DOS on an IBM compatible PC. Graphical output from Statgraphics was exported as CGM files which were then converted to the Acorn Draw format using a utility called CGM->Draw. Tabulated output was exported as text files.

4 .2 .7 I d e n tific a tio n o f 'E ven ts ' d u r in g a d y n a m ic lift

Following the example set by Canadian studies of the ILM (Stevenson et al., 1990a, Bryant et al., 1990) distinct 'Events' were identified which occurred during the course of lifts on the hydrodynamometer. The Events consisted of fixed hand heights and maxima and minima in the force, velocity and power curves. A computer algorithm was used to scan through the force data to identify either the maximum or minimum between limits which were expected to lie either side of the Event of interest. The corresponding Events in the velocity and power data were identified as the maximum or minimum values, as appropriate, within 40 samples (approximately 11.1 mm rope travel) either side of the Event in the force curve. Each set of graphs was then displayed on the computer screen to allow visual checking of the Event locations. Where an Event had been incorrectly located, the computer mouse was used to identify the region of the true location and the largest, or smallest as appropriate, value within 40 samples either side of the mouse location was returned as the new Event location.

The chosen Events are listed in Tables 4.4 and 4.5 and are marked on an example set of output graphs reproduced in Figure 4.1. It must be realised that, because of differences between individuals in terms of lifting style, not all Events occurred in all lifts.

Table 4.4: Event numbers for landmark heights

Event type Event number

Handle height of 0.7 m 1

Handle height of 1.0 m 2

Handle height of 1.45 m 3

Handle height of 1.7 m 4

Table 4.5: Event numbers for maxima and minima of the different performance measures

Event type Force Velocity Power

First peak below 0.9 m 5 12 19

Next dip below 0.9 m 6 13 20

Next peak below 0.9 m 7 14 21

First grip change below 1.7 m 8 15 22

Largest peak after (8), before any subsequent grip change 9 16 23

Second grip change below 1.7 m 10 17 24

Largest peak below 1.7 m following second grip change 11 18 25

Figure 4.1 illustrates the fact, discussed in Chapter 3, that force, velocity and power at any instant are all mathematically related to each other, and shows that Events 5, 12 and

19; 6, 13 and 20; 7, 14 and 21; 8, 15 and 22; 9, 16 and 23, 10, 17 and 24; and 11, 18 and 25 are all closely related to each other in time, if not actually co-instantaneous. The Events defined in Table 4.5 could therefore, in principle, be reduced to only seven. By contrast the inertial characteristics of the ILM result in peak force and peak velocity being separate in time because peak velocity occurs at zero acceleration.

4 .2 .8 D e fin itio n o f 'R a n g e s ’ o f a d y n a m ic lift

In addition to the Events defined as per the Canadian ILM studies, mean values over various ranges were also calculated. These are defined in Table 4.6.

Table 4.6: Numbers allocated to means of various Ranges

Range Mean force Mean velocity Mean power Mean work Mean impulse

0.4 m - Event (8) 26 40 54 68 82 0.4 m - Event (10) 27 41 55 69 83 0.4 m - 1.45 m 28 42 56 70 84 0.4 m - 1.7 m 29 43 57 71 85 0.7- 1.0 m 30 44 58 72 86 0.7 m - Event (8) 31 45 59 73 87 0.7 m - Event (10) 32 46 60 74 88 0.7 m - 1.45 m 33 47 61 75 89 0.7 m - 1.7 m 34 48 62 76 90 Event (8) - Event (10) 35 49 63 77 91 Event (8) - 1.45 m 36 50 64 78 92 Event (8) - 1.7 m 37 51 65 79 93 Event (10) -1.45 m 38 52 66 80 94 Event (10) - 1.7 m 39 53 67 81 95

LV

M \n m tfi M CO CO U) W H H U1 _W in H H in in 8 in ®3 I 8 in 8 8 I I I I I 8 in 88 8 in 8 in in in in

Figure 4.1: Screen grab of display showing displacement, force, velocity and power, with times and magnitudes of 'Events' identified

The different heights used to define the Ranges were chosen for a variety of reasons. The 0.4 m height is the starting height of the exertion; the 1.45 and 1.7 m heights were chosen because they had already been chosen as target heights for the Single Lift RMT and for lifts on the ILM. This enabled comparisons to be made between performance in other modalities of lifting test, such as the ILM and maximal box lifting, and

performance on the hydrodynamometer. The 0.7 to 1.0 m Range was chosen because early work using the hydrodynamometer (Grieve, 1993, Duggan and Legg, 1993) had measured mean power output between these heights. Events (8) and (10 ) were chosen because they represented distinct points in the output curves where velocity, and hence force and power, effectively dropped to zero, thus allowing the lifting range to be separated into discreet performance zones.

4.3 Results

4 .3 .1 A n th r o p o m e tr ic ch a ra cte ristics o f su b je c ts

Table 4.7 summarises the anthropometric data collected from the 270 subjects from whom usable data from their second pulls on the hydrodynamometer were obtained. Table 4.7 shows that the male sample was normally distributed with regard to height and fat-free mass, but not body mass. The female sample was normal with respect to stature and body mass, but not fat-free mass. Both distributions were positively skewed with regard to isometric lifting strength at 850 mm and age, indicating long tails of more than expected stronger individuals and older individuals in the distributions. The male distribution of isometric strength and both age distributions were also positively kurtic, (leptokurtic) indicating that the distributions were more peaked than a normal distribution, i.e. more individuals than expected were near the mean.

Comparison of height and weight shows that the male subjects matched the British male population exactly on mean height but had a slightly smaller coefficient of variation. Females were 15 mm taller than the population, though this was not statistically significant. On body mass, both genders matched the population to within 1 kg.

Table 4.7: Characteristics of 201 males and 69 females whose hydrodynamometer data were used, with stature of British adults aged 19-25, and body mass of British adults aged 19-65 (Pheasant, 1986)

Variable: Stature (mm) Body mass (kg)

Gender All Males Females All Males Females

Sample size 270 201 69 270 201 69 Mean 1728.2 1760.0 1635.5 71.0 74.1 62.1 Std. deviation 83.3 62.3 65.7 11.1 10.4 7.6 Minimum 1482 1579 1482 49 50 49 Maximum 1916 1916 1845 112 112 81 Std. skewness -2.602 -1.252 1.169 3.883 3.720 1.883 Std. kurtosis -0.974 0.161 0.885 1.521 1.921 -0.394 GB mean 1760 1620 75 63 GB std. dev. 73 61 12 11

Variable: Fat-free mass (kg) Isometric strength @ 850 mm (N)

Gender All Males Females All Males Females

Sample size 270 201 69 270 201 69 Mean 55.4 59.9 42.2 1299.4 1469.7 801.4 Std. deviation 9.6 6.2 3.8 438.3 351.2 244.3 Minimum 35.0 42.8 35.0 245.3 686.0 245.0 Maximum 78.5 78.5 54.3 2943.0 2943.0 1510.0 Std. skewness -1.666 1.649 2.905 2.537 5.892 2.397 Std. kurtosis -2.273 1.071 2.117 2.072 6.106 1.300

Variable: Age (years)

Gender All Males Females

Sample size 270 201 69 Mean 23.97 23.97 23.99 Std. deviation 4.61 4.83 3.91 Minimum 18 18 19 Maximum 41 41 40 Std. skewness 8.041 6.758 4.493 Std. kurtosis 4.880 3.339 4.912

4 3 .2 C o rre la tio n b e tw e en a n th ro p o m e tr ic va riables

Table 4.8 gives the matrix of correlation coefficients of the measures of stature, body mass, fat-free mass and isometric lifting strength at 850 mm.

Table 4.8: Correlations between the anthropometric characteristics of the 270 subjects whose hydrodynamometer data were utilised

Stature Body mass Fat-free mass

Body mass 0.6331

Fat-free mass 0.8093 0.8425

Isometric strength at 850 mm 0.7497 0.6824 0.8080

4 .4 .3 C o rre la tio n s o f m e a s u r e s o f d iffe r e n t R a n g e s

Appendix 6 contains a full correlation matrix of the values obtained from the different Ranges. Since not all Events occurred in all pulls, missing values were eliminated pairwise when calculating the correlation coefficients. Values greater than +0.7 and -0.7 were marked and, where possible grouped, to show where common variance of greater than 49% occurs. The ways in which the Range variables formed groups are shown in

Table 4.9. Relationships between variables are considered further in Chapter 7, where Principal Components Analysis is used to explore them more formally.

In Table 4.9 correlations between the different measures over a single Range are high because all the measures are functions of force; the Ranges in Group 1 overlap to a great extent, causing the measurements to correlate very highly; the measurements between the first change of grip and 1.7 m overlap to a greater or lesser extent with the other measurements in group 2; and in groups 3 and 4 measurements from either the first or second change of grip to 1.45 and 1.7 m overlap and therefore correlate.

Table 4.9: Groups of highly related Range variables on a hydrodynamometer pull

In document Analysis of dynamic lifting exertions performed by males and females on a hydrodynamometer (Page 103-113)