Fig 1.
Experimental set-up used for each kick of a soccer ball.
The vertically mounted force plate collected impact force data through the PASCO Xplorer GLX™ sensor receiver unit connected to a laptop running PASCO Capstone Software™ [29]. A camera mounted on a crossbar above the force plate captured impact motion for calculating ball velocity.
Table 1.
Parameter ranges for sensitivity analysis.
Table 2.
Dry soccer ball metrics by size.
Fig 2.
Two sample impact force waves.
The first was taken from a size 4 soccer ball inflated to a pressure of 27.6 kPa and delivered at a speed of 4.57 m/s. The second was taken from a size 5 ball inflated to a pressure of 34.5 kPa and delivered at a speed of 14.6 m/s.
Table 3.
Mean peak impact force (N) (+/- Standard Deviation) of all experimental trials within a 14-17 m/s velocity range.
Table 4.
Change in soccer ball mass (kg) during water submersion.
Fig 3.
Natural log of Πo (Force) versus Πi of all 600 data points.
A distinct linear relationship suggests all necessary parameters have been considered and all soccer balls follow the same theoretical peak force relationship.
Fig 4.
Cotter’s Method sensitivity analysis considering a mass range of 0.3969—0.4797 kg, diameter range of 0.2041–0.2208 m, inflation pressures between 58.6 and 111.7 kPa, and a velocity range of 15-30 m/s.
The most important parameter was the ball velocity, but it should be noted that ball pressure exhibited a sensitivity value near the critical threshold of 0.25.
Fig 5.
Cotter’s Method sensitivity analysis taking into account the increased water absorption mass value range for a professional-level size 5 soccer ball.
Water absorption makes mass the second most important parameter.
Fig 6.
Svaldi et al.’s “High Load” and “Low Load” groups from a soccer season (High Load percentiles: 25th = 4515 g, 50th = 5615 g, 75th = 12313 g; Low Load percentiles: 25th = 2425 g, 50th = 2930 g, 75th = 3800 g) [36].
Post-season adjustments to the High Load group: 19.7% reduction from decreasing inflation pressure from 1.10 bar (16 psi) to 0.55 bar (8 psi), 7.1% reduction from downsizing a size 5, 0.55 bar (8 psi) ball to size 4.5, 0.55 bar (8 psi) ball (Table 3), and a 20% reduction in number of hits based on average PLA per hit (Adjusted High Load percentiles: 25th = 2463 g, 50th = 3063 g, 75th = 6718 g).