According to the World Health Organisation, falls are the second leading cause of accidental or unintentional injury deaths worldwide, and 37.3 million falls severe enough to require medical attention occur each year. Studying falls requires an understanding of how pedestrians typically move as they walk, the mechanics of the interaction between a person’s foot and the surface they are walking on, as well as the sorts of obstacles that generally make walking surfaces unsafe. Therefore, investigations of slip and fall or trip and fall incidents are multidisciplinary, involving knowledge from kinesiology, biomechanics, physics, mechanical and civil engineering.
The Typical Gait Cycle
In modern homo sapiens, the typical gait cycle, which has been pretty much the same for at least 100,000 years, has been broken down by biomechanists from medical, kinesiology and engineering disciplines. The cycle begins and ends with the heel of one foot striking the ground, and can be divided into two phases: the stance phase, during which the foot is in contact with the ground; and the swing phase, during which the foot is lifted off the ground and swung through the air.
A more detailed model of the gait cycle consists of seven stages, which are illustrated in the figure below.
The stages of the Gait Cycle are:
- Heelstrike – This is where the foot makes first contact with the ground via the heel.
- Flatfoot – The entire sole of the foot is now in contact with the ground, and the body’s centre of mass moves forward towards the planted foot.
- Midstance – The body’s centre of mass passes over the foot on the ground as the opposite foot rises in the air.
- Pushoff – This is when the toes push off and the heel of the planted foot begins to raise.
- Acceleration – The foot begins to rise into the air and gain speed.
- Midswing – This is where the foot is entirely off the ground and moves forward towards the body’s centre of gravity.
- Deceleration – The foot slows down and lowers to the ground again, heel first.
As slips and trips are the results of different mechanisms, they tend to occur in different points of the gait cycle.
Slip and falls
A slip occurs when there is insufficient friction between the foot and the ground to complete the gait cycle normally, and the foot slides unexpectedly on a slippery surface. Slips occur most frequently during the heelstrike stage of the gait cycle. During a slip, the heel slides forward relative to the rest of the body, causing the centre of mass to be displaced rearward, starting a backwards fall.
Trip and falls
A trip occurs when the foot makes contact with a discontinuity or change in elevation in the walking surface. Trip and falls occur most frequently during the midswing stage of the gaitcycle. As the foot swings forward, it strikes a discontinuity in the walking surface, and the body undergoes a forward rotation, resulting in the centre of mass being displaced forward of the normal balanced position, starting a forwards fall.
Mechanical Measurement and Analysis of Slip and Trip Hazards
In a slip scenario, it is the lack of adequate friction that causes the disruption of the gait cycle. We can measure friction, the unit less measurement of the ratio of horizontal to vertical forces between two surfaces sliding against each other. It is a measure of slipperiness in terms of slip index.
When measuring the slip index on a particular surface, it is important to take into account the relative motion between the foot and the surface. During a heel strike, only a small portion of the heel contacts the ground at first, and the surface must provide sufficient friction to allow the heel to remain stationary as the rest of the foot comes down (foot flat) and the body’s centre of mass moves forward towards the midstance stage of gate.
In order to measure the slip index of a surface, we use an Excel Variable Incidence Tribometer with sequencer (pictured on the left). The Excel mimics the motion of a heel strike, therefore providing a close approximation of the interaction between the foot and the ground during gait. After measurement, the slip index of the surface in question can be compared to relevant material and civil engineering standards. This slip index can be measured on a dry or wet surface to best represent the conditions of the surface at the time of the slip event in question (see our article on measuring slip index for more details).
The generally accepted minimum slip index for a safe walking surface is 0.50 for an unloaded, normal walking condition (i.e. individual not carrying an object, walking forwards on a dry floor). This slip index value allows for a factor of safety, as research has shown that most individuals only require traction of approximately 0.30 for normal walking and up to 0.36 for more abrupt maneuvers such as turning. It is typically the slip index or coefficient of friction that is less than 0.25 in one area on an otherwise slip resistant surface which causes most slip and falls.
In a trip scenario, it is a usually a surface discontinuity or transition in the walking surface that poses the hazard. It is typically an unexpected single step or change in elevation of 12 mm or more that cause most trip and falls, because that can exceed the minimum toe clearance during the swing through phase of the gait, especially for older pedestrians (see our article on the effect of age on slip / trip and falls). In order to incorporate a factor of safety, standards require a sloped transition in cases where the change in elevation is greater than 6 mm.
Depending on the situation, discontinuities are documented using a variety of appropriate mechanical measurement tools, and compared against relevant local regulations, standards (such as the Provincial Building Code, or Minimum Maintenance Standards) or Industry accepted standards such as ASTM (American Society for Testing and Materials). Rulers and combination squares graded down to 0.5 mm increments, or vernier calipers with precision and accuracy of +/- 0.01 mm may be utilized depending on the height of the discontinuity. The image to the left shows a measured surface difference in elevation of 8 millimeters using a combination square.
The following video illustrates how even a small unexpected irregularity can pose a trip hazard:
Investigations of slip and fall or trip and fall scenarios are multidisciplinary, as they combine knowledge from biomechanics (the gait cycle), mechanical engineering (taking the appropriate measurements of surfaces, slip index or discontinuities), and civil engineering (understanding the relevant codes and standards that apply to the location of the incident).
References J. M. Miller, “Slippery work surface: toward a performance definition and quantitative coefficient of friction criteria”, J. Saf. Res., 14:145/158, 1983.
 H. Fischer, S. Kirchberg, and T. Moessner, “Biomechanical Gait Analysis for the Extraction of Slip Resistance Test Parameters” Industrial Health 2009, Dresden, Germany (Apr 21, 2009), article 617–625. J. M. Burnfield, Y. Tsai, and C. M. Powers, “Comparison of utilized coefficient of friction during different walking tasks in persons with and without disability,” Gait & Posture, Vol. 22 (2005), pp 82 – 88.  American Society for Testing and Materials, “Standard Practice for Safe Walking Surfaces”, ASTM F1637-09.
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