Sport Science

 Football shoulder pads

SSI has designed equipment and software to evaluate the protective performance of football shoulder pads. Original testing equipment and protocols were designed and assembled, and relevant data analysis methods and performance metrics were introduced.  This testing involved measurements of large (500 lb) impact forces exerted on top of, and under, various pads attached to a manikin.  It involved comparisons of the impact-force-reduction capabilities of the various pads, and comparisons with the forces exerted (by identical impacts) when no protective pad is present.  The lab also compared the relative protective contributions of pad interior foams and exterior shells, and directly compared the effectiveness of different foam and shell combinations.

In our testing protocol, we impact a pad-protected dummy with solid loads of various weights at various speeds.  The loads are dropped onto the dummy from various heights (which determine the impact speeds).  The loads slide down a vertical rod in order to accurately aim the load and control it’s rebound.  The falling load strikes a stiff spring attached to a steel cup resting on the pad.  This load-spring-cup impact is designed to model typical game impacts with regard to impact force and impact time.  The setup is shown in Figs. 3.15 and 3.16.  Note the guide rod attached to the ceiling, the rope and pulley arrangement used to raise the load to the desired height, and the wiring that connects the force sensors to a computer.

There are three reasons for our use of a spring/cup to transmit the impact force onto a pad.  (1) The spring models the elasticity of an impacting human body.  (2) The steel cup that transmits the impacting force onto a pad can rebound in any direction, as determined by the mechanics of the interaction between the cup and the pad.  If the falling load were instead constrained to rebound directly back up the vertical rod, the created impact could be very different from the unconstrained impacts that arise in actual football games.  (3) Because our manikin impacts proceed through a stiff spring, we are able to measure the force exerted from an impact on a surface when no protective pad is in place.  The difference between this force and the force exerted from an identical impact when a protective pad is present constitutes a direct measure of the effectiveness of the pad in reducing the force created by a given impact.

We record two types of force measurements for each impact.  The force exerted onto the outside of each pad during the impact is measured using an accelerometer attached to the falling weight. In addition, we measure the actual forces transmitted through the pads onto the manikin.  These forces are recorded at five separate locations on the manikin, so that we can measure the degree to which the pads are effective in spreading out the applied impact force.  These locations are shown as black circles in Fig. 3.17.  We choose the ultimate load weight and impact speed so that the force applied to the manikin (about 450 lbs) and the impact duration (about 0.1 s) are of the order of those encountered in actual game impacts.

For each impact, we record the accelerations and forces exerted during the impacts on each of our sensors arranged as described above.  From these data, we extracted the following most relevant information:  (1) the maximum force applied on the pad, (2) the average applied force, (3) the maximum force measured on a sensor under the pad, (4) the sum of the maximum forces measured on each sensor under the pad, (5) the ratio e = v’/v of the rebound load speed and the incident load speed, and (6) the severity index (SI) corresponding to the applied acceleration profile.

The significance of this information is as follows.  (1) The maximum applied force is a measure of the effectiveness of a pad in reducing the impact force.  (2) The average applied force is a measure of the effectiveness of a pad in reducing the impact force and in spreading this force out over the impact time.  It is a measure of the severity of the impact, as described previously.  (3) The maximum force measured on a sensor under the pad is a measure of how much of the applied force is transmitted through the pad onto the sensor.  It is a measure of the effectiveness of a pad in spreading the applied force over the body of the player.  (4) The sum of the maximum forces measured on each sensor under the pad is another measure of the effectiveness of a pad in spreading the applied force over the body of the player.  (5) The speed ratio e = v’/v is called the “coefficient of restitution” (COR) between the load and the pad because it gives the fraction of the incident load speed that is restored to the load after the impact.  It is a direct measure of the kinetic energy of the load that is lost during the impact: wv^2/2g - wv’^2/2g  = (wv^2)*(1 - e^2) /2g.   (6) The SI is the integral of the acceleration a(t) of the impacting body, raised to an appropriate power p, over the duration of the impact. The smaller the SI, the less will be the bodily damage caused by the impact.

  These six quantities together characterize the effectiveness of the measured pad in reducing the severity of the impact.  For a given impact, the pad that reduces the magnitudes of these quantities the most is the pad that provides the greatest measure of safety for the football player.

A typical force verses time plot of an impact on a pad is shown in Fig. 3.18. The vertical axis specifies the measured force in pounds.  The horizontal axis specifies the elapsed time during the impact in units of 0.01 ms. (The irrelevant high-frequency vibrations created by the impact were removed using a CFC60 filter.)  The data plotted in this graph are used to evaluate the applied force metrics for the given pad. The forces transmitted through the pads were measured by force sensors attached to the manikin under the pads near the impact area.  A typical force verses time plot of these forces for an impact on a pad is shown in Fig.3.19.

To compare the effectiveness of the various pads tested, we use the metrics that we listed above.  In addition, we have introduced a more transparent way to display these metrics in terms of the IRP values for these quantities.  If the maximum force exerted on the manikin when no protective equipment is present is F0, and F is the maximum force exerted by this load when protective equipment is present, then the maximum force impact reduction percentage (IRP) of that equipment is

IRP = 100%·(F0 – F)/F0.

The IRP values can be used as the basis of a performance standard on shoulder pads in football.  For example, the SI IRP of a compliant pad could be required to be greater than 45%.

An example of some measured data for a tested pad is given in the following table.

The maximum applied force is 620 lbs when no pad is present.  This force is reduced to 459 lbs when the sample pad is present, so that the maximum force IRP is 26%.