Contributors List
5. Apparel and Sport Surfaces
IMPROVING THE UNDERSTANDING OF GRIP
S.E. TOMLINSON, R. LEWIS & M.J. CARRÉ
Department of Mechanical Engineering, University of Sheffield, Sheffield, UK
This paper introduces the initial findings from frictional tests of the human finger contact on steel, glass and rubber, as used in the manufacture of rugby balls. The materials were tested using a bespoke finger friction rig, with which the normal and frictional forces are measured when a finger is dynamically moving along a material. The results showed the coefficient of friction of the rugby ball material is much greater than that of glass and steel.
The results also show and highlight the differences in the relationship between normal force and coefficient of friction for the viscoelastic and non viscoelastic surface materials, neither of which show linear relationships due to the viscoelastic nature of the finger in contact with them.
1 Introduction
The ability of a rugby player to handle a ball well can be the difference between a good or bad pass. The handling performance of a rugby ball can be quantified, in one aspect, by the coefficient of friction of the fingers with the ball material. The coefficient of friction is determined by the surface texture and also the surrounding conditions, such as temperature and the presence of moisture.
Rugby balls are made from rubber and therefore have viscoelastic properties. They also have a surface of pimples; these can be round, square, large, small, densely or sparsely populated. The fact that the materials are viscoelastic adds complexity to the situation since the majority of previous work on human skin friction has been carried out on non vis- coelastic materials.
There are 3 mechanisms of friction; adhesion, deformation and hysteresis. Adhesion is where the asperities of the two surfaces form local welds when brought together and the frictional force is the force required to shear these junctions. Deformation is the energy dis- sipation due to the deformation of the material, in viscoelastic materials, this results in hys- teresis; energy absorption resulting in a delayed response to an applied force.
Previous research has shown the coefficient of friction, between the finger and non vis- coelastic material, to vary with normal force, hydration and age (to an extent). The effect of age is only seen above the age of 50 years for glass and 70 years for sandpaper (Asserin et al., 2000), which is a relatively insignificant factor when considering professional sportsmen.
Gender and race showed not to have an effect (Sivimani et al., 2003; Spurr, 1976). The effect of normal force for the skin on non viscoelastic materials has been shown to be linear at forces between that of 0–30 g (Asserin et al., 2000). However, other work found there to be a nonlinear relationship, due to testing at higher normal forces. This non-linear relationship can be approximated by equation (1) (Comaish & Bottoms, 1971; Zatsiorsky, 2002). The nonlin- earity is due to the finger pad becoming stiffer when a larger normal force is applied.
FµNn (1)
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where Ffrictional force, Nnormal force and na constant less than 1. In most tests where nis quantified it is suggested to have a value of approximately 0.3.
The contact area changes depending on the applied force, due to the finger not being rigid. This variation in area and also the variation from person to person affect the coeffi- cient of friction, the extent of which is not yet known.
2 Method
A friction rig has been designed to measure the normal force and the frictional force when a finger is run along a surface. The surface can also be changed. An illustration of the setup is shown in Figure 1. The rig consists of two load cells attached to a flat plate; one of these measures the frictional force and the other the normal force.
A series of evaluations have been done to ensure that the tests carried out on the rig represent as closely as possible the contact experienced in a game of rugby and produce consistent results. This included validating which finger to use on the rig, the correct speed to use and the area of the finger to use.
The area of the finger to be used in tests was determined by painting the hands of a rugby player blue and delivering him a slow pass. The area of the finger used in the catch was then imprinted on the ball. The player was then asked to print varying areas of the finger in a pressing action onto some paper. This enabled the area of the fingers printed on the ball to be correlated to a pressing action, making this the standard area to be used in tests. In this particular set of tests the 32 volunteers, (6 female and 26 males aged 20–49 years), ran there fingers at a slow constant speed, along 3 different materials; glass, steel and smooth rugby ball rubber. Smooth rubber was used in this instance for a direct comparison between rubber and the smooth surfaces of the glass and steel. The volunteers were asked to vary the force applied to the material; they applied the greatest force possible, the lightest force possible (with the specified area of contact) and then three intermediate levels of force.
3 Results
3.1. Comparison to Previously Measured Coefficients of Friction
Initial analysis carried out compared the measured results with those of previous work; the plot of this comparison is shown in Figure 2. This comparison shows that the coefficient of friction on glass measured by other researchers is lower than that measured in these tests.
The main reason for this is the other glass tests (Koudine et al., 2000; Prall, 1973; Johnson
Material attached to this plate and the finger moves along it.
Normal force load cell Frictional force
load cell
Figure 1. Illustration of the finger friction rig.
et al., 1993) were all done on different areas on the body, mostly the forearm, not the finger pad. The skin on the finger pad has much more pronounced ridges so the surface characteris- tics are very different, changing the coefficient of friction. The sample size of people tested in these experiments is greater than the majority of the tests in the literature. The perspex tested by Fuss et al.(2004) against the finger, shows a higher coefficient of friction than that meas- ured with the glass. Perspex is used as a substitute for glass, however the molecular structure and physical properties are different so differences in the coefficients of friction would be expected.
The previous steel coefficients of friction are also lower than the results gained in this test. Again the main reason of this is because the site of testing for the literature values is not the finger pad. Roberts measured the coefficient of friction of the finger against latex gloves.
The coefficient of friction of this rubber is a lot lower than the rubber tested in these experi- ments; however the properties of the two rubbers are very different so there is no surprise to see a difference.
3.2. Comparison of Rubber to Standard Materials
The coefficient of friction between the finger and rubber is greater than that between a finger and steel or glass, as illustrated in Figure 2. This can be seen to be significantly greater, almost twice as high. The coefficient of friction was found to be slightly higher for glass than for steel, however this is not a statistical difference when compared over the full range of forces.
3.3. Coefficient of Friction with Variable Normal Force
The coefficient of friction was found to decrease with increasing normal force for the finger pad contact when tested on polycarbonate (Bobjer et al., 1993) and acrylic (Han et al.,
Improving the Understanding of Grip 131
Glass n=173 F=1.6-58N
Rubber n=169 F=1.6-58
Steel n=169 F=2.4-75.8N
Roberts (1992)_Latex F=4N, s=1mm/s El Shimi (1977)_Steel
Sivimani (2003)_Steel n-4, F=0.0005-200N, s=10m/s Koudine (2000)_Glass
Prall (1973)_Glass
Johnson (1993)_Glass Fuss (2004)_Perspex
0 0.5 1 1.5 2 2.5
Coefficient of Friction
Figure 2. Measured coefficients of friction over full spectrum of forces and the coefficients of friction found in the literature.
1996). The coefficient of friction for glass decreases exponentially with normal force, as shown in Figure 3, agreeing with this previously found trend. However, the coefficient of friction for the steel-finger contact seems to be fairly constant across all forces, this could be because the extent of the exponential decrease is less due to the lower coefficient of fric- tion. The coefficient of friction varies from person to person as does the extent of the decreasing trend in coefficient of friction, so more work needs to be done on the effect of other factors such as area, to gain a fully integrated solution.
The main interesting factor that can be seen in these results is that the rubber follows a different trend to that of the standard engineering materials. Here it can be seen that the coefficient of friction increases with increasing normal force, to a point, at which it starts to decrease again, following a more parabolic trend. The main reason for this difference is that the rubber is a viscoelastic material, so therefore has very different material properties to that of either glass or steel. The rubber will also have variable stiffness, like the finger, so hysteresis is a friction mechanism associated with both rubber and fingers.
4 Discussion
Skin has varying properties depending on where it is on the body. This difference is clearly seen when comparing the skin of the palm to the forearm. Not only does the skin on the palms have more pronounced ridges, it also has a much thicker epidermis. These differing characteristics change the values of the measured coefficient of friction. This is illustrated when the results of these tests are compared to those of previous studies. This difference is also due to the test equipment; much of the work on the forearm was carried out using a probe. The hysteresis friction mechanism has a large influence on the coefficient of friction measured, so changing the profile of deformation by using a probe instead of a flat surface will change the coefficient of friction. There is also a large difference in coefficient of friction
0 0.5 1 1.5 2 2.5 3
0 2 4 6 8 10 12 14 16 18 20
Normal Force (N)
Coefficient of Friction
Glass Rubber Steel Rubber
Glass Steel
Figure 3. Measured values from one of the volunteers showing the general trend seen.
from person to person. This is due to several points. Firstly the characteristics of the skin vary from person to person; they may have dryer hands and everyone has a different pattern of undulations. The area used will also have a large effect. Skin is a viscoelastic material;
the frictional properties of contacts with viscoelastic materials are dependant on area. The effect of area may be both the size of the finger and also the difference in the way it deforms with an applied load. This area effect is related to the force applied, so further analysis will be carried out on these results to find the effects of area from person to person and also the change in area with applied load for a single person.
The results showed that for glass, the coefficient of friction decreases with increasing normal force. This is the trend shown in the literature (Bobjer et al., 1993; Han et al., 1996).
The coefficient of friction decreases because the skin becomes stiffer. This reduces the hys- teresis effect so the coefficient of friction decreases. Increasing area will increase the adhe- sion mechanism, however this is to a much lesser extent than the decrease in hysteresis. The rubber showed a different trend, here the coefficient of friction increased with increasing normal force and then began to decrease after a certain load. The exact relationship differs from person to person, but they all follow this same trend. Further analysis will be carried out to try and quantify this relationship. The section of increasing coefficient of friction can be accounted to the larger contact area increasing the adhesion and therefore frictional force. That is to say that in this section the adhesion mechanism is more dominant than the hysteresis mechanism. The section of decreasing coefficient of friction can be explained in the same way to that on glass, however it is to a much greater extent because both materials are viscoelastic so the effect of reducing this mechanism is greater.
5 Conclusion
These experiments clearly show that the frictional properties and rules for rubber are very different to that of standard engineering materials. The analysis so far, goes some way to explain the trend of frictional force with normal force, however further analysis will be car- ried out to quantify this relationship. Once the relationship of the normal force on the rub- ber in the test situations has been completed this can be applied to an actual game of rugby by recording the forces involved in different passes and then using the force relationship to calculate the friction involved in that instant for the pass or throw concerned.
References
Asserin J., Zahouni H., Humbert P., Couturaud D. and Maougin D. (2000) Measurement of the friction coefficient of the human skin in vivo. Quantification of the cutaneous smoothness. Colloids and Surfaces B: Biointerfaces, 19, 1–12.
Bobjer O., Johansson S.E. and Piguet S. (1993) Friction between hand and handle. Effects of oil and lard on textured and non textured surfaces: perception of discomfort.
Applied Ergonomics, 24/3, 190–202.
Comaish S. and Bottoms E. (1971) The Skin and Friction: Deviations from Amonton’s laws, and the effects of hydration and lubrication. British Journal of Dermatology, 84/1, 37–43.
El-Shimi A.F. (1977) In vivo skin friction measurements. Journal of the Society of Cosmetic Chemists, 28, 37–51.
Fuss F.K., Niegl G. and Tan A.M. (2004) Friction between hand and different surfaces under different conditions and its implication for sport climbing. In: Hubbard M., Mehta R.D.
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and Pallis J.M. (Eds.), The Engineering of Sport 5, Vol. 2, International Sports Engineering Assocation, Sheffield, UK, pp. 269–275.
Han H.Y., Shimada A. and Kawamura S. (1996) Analysis of friction on human fingers and design of artificial fingers. International conference on robotics and automation.
Minnesota, April.
Johnson S.A., Gorman D.M., Adams M.J. and Briscoe B.J. (1993) The friction and lubrica- tion of human stratum corneum. 19th Leeds-Lyon Symposium on Tribology. Elsevier, pp. 663–672.
Koudine A.A., Barquins M., Anthoine P.H., Auberst L. and Leveque J.L. (2000) Frictional properties of the skin: proposal of a new approach. International Journal of Cosmetic Science, 84, 37–43.
Prall J.K. (1973) Instrumental evaluation of the effects of cosmetic products on the skin surfaces with particular reference to smoothness. Journal of the Society of Cosmetic Chemistry, 24, 693–707.
Roberts A.D. and Brackley C.A. (1992) Friction of Surgeons’ gloves. Journal of Physics:
Applied Physics, 25, A28–A32.
Sivamani R.K., Wu G.W., Gitis N.V. and Maibach H.I. (2003) Tribological testing of skin products: gender, age and ethnicity on the volar arm. Skin research and technology, 9, 299–305.
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Tribological Studies in Man – An Overview. Skin Research and Technology, 9, 227–234.
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Zatsiorsky V.M. (2002) Kinetics of Human Motion. Leeds, Champaign.
IONISED SPORTS UNDERGARMENTS: A PHYSIOLOGICAL EVALUATION
A.R. GRAY, J. SANTRY, T.M. WALLER & M.P. CAINE
Progressive Sports Technologies Ltd., Innovation Centre, Loughborough University, Loughborough, Leicestershire, UK
Sports apparel is becoming increasingly technical from both a feature and fabric perspec- tive. A propriety technology has now made it possible to ionically treat garments. Existing literature suggests exposure to negatively charged particles may be beneficial to sports per- formance. Six recreational games players and six university 1st team players were recruited to quantify the influence of ionic garments (I) and non ionic garments (N) on discrete phys- iological parameters associated with rugby. Mean power was found to be higher (2.7%, P0.05) when wearing garment I than garment N in a second Wingate test. Additionally all rugby players tested achieved higher mean power outputs in this second test. During rest minimum heart rate was found to be lower (53 bpm, P0.05) wearing ionised garments than control (56 bpm). Other parameters of performance and recovery which included the measurement of haemodynamics, pain perception during sub-maximal arm crank ergome- try and maximal power output during incremental cycle ergometry were not found to be significantly different. This is the first study of ionically treated garments and so further research is required to substantiate current findings and more clearly elucidate the effects of ionised garments in a sport specific setting.
1 Introduction
Sports apparel is becoming increasingly technical from both a feature and fabric perspective.
Advancements in apparel technology have focused mainly on the enhancement of ther- moregulation and muscular compression. A propriety technology has now made it possible to ionically treat garments, though benefits afforded to a wearer remain unknown. Existing literature has documented benefits of negatively charged particles via inhalation, in param- eters including increased work capacity (Minkh, 1961), favourable haemodynamic (Ryushi et al., 1998), heart rate (Yates et al., 1986) and lactate responses, as well as improvements in various psychological state indices (Fornof & Gilbert, 1988). However conflicting research exists and any mechanisms in action remain unclear.
The present study aimed to investigate the influence of negative ions, via ionically treated garments, on a wide range of physiological performance and recovery parameters.
2 Methods
Following informed consent, twelve male participants (222.8 years, 85.714.7 kg, 10.02.8% body fat; meanSD) consisting of recreational games players (n6) and uni- versity first team rugby players (n6), took part in a double blind randomised cross-over study.
Before testing participants were familiarised to equipment and requested to refrain from alcohol and caffeine for at least 24 hours. Dietary intake was recorded and matched
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for both trials 24 hours prior testing, while water was provided ad-libitum during the trials.
A minimum of 72 hours was required between each trial.
On arrival, height, mass, and percentage body fat were measured using a stadiometer, weighing scales and skin fold callipers, respectively. Garments were then worn, where trial order of use was determined independently to ensure counter balance and double blind design remained.
Participants then assumed a semi-recumbent position for 15 minutes on a physiotherapy bed. Systolic and diastolic blood pressure, along with heart rate were monitored at 3 minute intervals (Vital Signs Monitor, Hunleigh Healthcare). A 3 minute low intensity warm-up was then performed on the cycle ergometer (894E, Monark), followed by three vertical jump repetitions with hands on hips at all times. An approximate 90° knee joint angle was maintained for 1 second before jumps.
A 15 second maximal effort Wingate test utilising a resistance equal to 0.075 kgkg body mass1, was performed on a cycle ergometer (894E, Monark). After a 2 minute recov- ery a second identical 15 second Wingate test was performed. Mean power, peak power, time to peak power and fatigue were measured. Heart rate was recorded throughout (Team System, Polar). Participants performed three further repetitions of vertical jumps using the same tech- nique as described earlier. A 10 minute semi-recumbent recovery period was then followed.
Participants performed a five minute arm-crank ergometer test (modified cycle ergometer RB1, Reebok) where load was 0.07 kgkg body mass1. Localised muscular pain perception was measured using a 0–10 point pain scale self assessment method (Cook et al., 1997), along with heart rate at 1 minute intervals.
Finally, a five minute recovery was allowed before participants performed an incremental (25 Wmin1) ramped cycle ergometer (839E, Monark) test, where participants were asked to cycle starting at 100 W until exhaustion. Maximal power and heart rate were measured.
Participants then immediately assumed a semi-recumbent position during recovery, systolic and diastolic blood pressure and heart rate were measured at 3 minute intervals as before. Post testing self assessment of localised muscle soreness was made by a 0–10 scale chart (Thompson et al., 1999) for 72 hours post testing at 24 hour intervals.
3 Results 3.1. Mean Power
It was found that average power achieved between the second of the two Wingate tests was significantly higher (730114 W) when garment I was worn in comparison to garment N (711126 W). Across the entire cohort this equated to an increased performance of 2.73.3%. Of the six rugby players tested, all showed improvements in mean power in the second Wingate test when wearing garment I, equating to an increase of 3.22.5%. Only four of the six recreational players were observed to have higher mean power when wearing gar- ment I, equating to a mean improvement of 2.14.1% which was not significantly different to garment N.
3.2. Minimum Resting Heart Rate
It was also found during the 15 minute semi-recumbent resting period before testing that minimum heart rate was significantly higher while wearing garment I (568 bpm) than garment N (537 bpm).