Rationale:
The literature currently provides few if any discussions regarding the dynamics of locomotion on surfaces that are either rocky or uneven. Not all occupational settings or the activities of daily living involve level, smooth firm surfaces, and a large percentage of work-related injuries are associated with work that is done on altered surfaces. In 2004 the Railroad reported that walking was the cause of 14.9% and 19.3% of all railroad worker injuries and days absent, respectively.

Hypothesis:
The purpose of this study was to determine the feasibility of using force plates to measure accurately the ground reaction forces (GRF) during gait on ballast surfaces.

Methods:
1. Used a known magnitude of vertical and shear loads at the surface of the ballast to verify that the magnitude of forces being applied at the force plate was equivalent
2. Comparisons of the magnitude and timing of the GRF data during locomotion on a level surface with GRF data of locomotion on ballast of varying depths wer made
3. n = 5 ( four male, one female – age: 22-25 mean 23.6 ± 1.4)
Exclusion Criteria:
- A history of or current neurological or orthopedic disease
- Any difficulty that would impede normal walking
- Any cardiopulmonary condition
- Osteoporosis
- Balance or dizziness problems
- Pregnancy

Setup:

• Each subject was given the same brand and model of polyvinyl chloride hard-soled shoes with new socks.
• 5 m wide/ 8.5m long vinyl tile walkway GRF measured at 1,080 Hz.
• Ballast surface conditions consist of 1 1/4in. marble ballast covering the walkway surface at two depths of approximately 2.5 in. or 4 in.
• Indoor outdoor carpet placed between ballast and vinyl flooring to act as a binder of the ballast to adhere to the floor surface with minimal movement between teh ballast and the floor surface.
• Statics loads of 2.114. N and 511.5 N placed at five locations on each force plate.
• Then the static load was pulled horizontally to provide shear force to the surface of the ballast until movement was initiated and continued following the initiation of movement until exiting the force plate.
• Human Gait:
  - Five subjects walked at self selected pace on the three surface types, vinly tile, 63.5 mm ballast, 101.6 mm ballast
  - Total of 25 trials performed with in each condition of each subject.
  - 20 N threshold set to determine heal strike and toe off.
  - GRF data were filtered using a 4th order zero lag butterworth digital low-pass filter with a cutoff frequency set at 25 hz.
  - GRF peak values normalized to body mass
  - Anterior-posterior shear were recorded as the maximum from 0 to 50% stance and 5 to 100 % stance
  -Medial-lateral shear was recorded as the maximum and minimum from 0 to 100% stance
  -Significance was set at alpha level of p ≤ .05

Results:
Shear forces (AP and ML) measured by the force plate generated by pulling horizontally on the two static loads showed slight systematic difference, preceding and following the initiation of movement, from the load cell measurements between 63.5 mm and 101.6 mm ballast conditions; however, these differences were not statistically significant.

The three-component part GRF time history curves during walking on the no-ballast condition, the 63.5 mm ballast condition, and the 101.6 mm ballast condition showed very similar GRF time history curves and nonsignificant difference in shear and normal forces.

Discussion:
Tee study indicates that a traditional 6-degree-of-freedom force plate system embedded in a walkway can provide accurate GRF measures of individuals walking over rough, uneven surfaces, specifically ballast.

Strengths:

Weaknesses:
This method is only truly validated for loading in the approximate center of the force plates.

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References:
Getting Started with LaTeX – David R. Wilkins