Wheelchair Work Measurements. The work required to propel a wheelchair in a straight path provided an indication of the firmness of the surface. The work required to propel a wheelchair through a 90 degree turn provided an indication of the stability of a surface. Wheelchair work per meter values were not obtained for sand because it was not possible to propel the wheelchair through this surface. The work to negotiate a ramp was linearly related to the angle of the ramp. The work required to propel across the level surface was compared to the work required to propel up or through a turn on various ramp angles in order to relate the work required for the surface to ADAAG specifications for "sufficiently accessible" levels of work (e.g., 8.3% grade for 30 feet).

The dirt (DIRP), chipped brush (CPBR), and engineered wood fiber (EWFK and EWFJ) surfaces had higher work per meter values compared to the other exterior surfaces (Figure 2, Figure 3).

Chipped brush and the engineered wood fibers (J and K) all required more work to roll on than a 3% ramp (chipped brush equates approximately to a 3.7% ramp). These three surfaces also required more work to negotiate than the ADAAG course and the crushed granite with stabilizer. It is recommended that these surfaces be considered moderately firm and stable, and be suitable for use in level areas for a limited distance (e.g., around a campsite) or for shorter distances on trails.

Figure 2. Wheelchair Work Values for Straight Propulsion (bar graph of results)D

Note: The "I" shaped vertical lines indicate the standard error of the mean, a statistic that estimates the variability expected if repeated samples of the same size are taken. It is calculated by dividing the standard deviation of the observations by the square root of the number of observations.

Figure 3. Wheelchair Work Values for Turning (bar graph of results)D

Rotational Penetrometer Measurements of Firmness and Stability. The Rotational Penetrometer penetrated significantly more on the dirt (DIRP), chipped brush (CPBR), engineered wood fiber (EWFJ and EWFK), and sand (SAND) surfaces compared to the other exterior surfaces, indicating decreased firmness and stability (Figure 4, Figure 5). The results of the Rotational Penetrometer correlated with those of the Wheelchair Work Measurement Method.

All of the exterior surfaces, except sand, became less firm in wet conditions (Figure 6). All exterior surfaces, except chipped brush (CPBR), became less stable when wet (Figure 7). While engineered wood fiber K (EWFK) became only slightly less stable, engineered wood fiber J (EWFJ) and path fines (PAFN) became much less stable. Path fines without stabilizer and dirt, while stable in a dry condition, became unstable and moderately stable when wet, respectively.

Figure 4. Rotational Penetrometer Firmness MeasurementsD

Figure 5. Rotational Penetrometer Stability MeasurementsD

Figure 6. Rotational Penetrometer Firmness Measurements for Dry and Wet SurfacesD

Figure 7. Rotational Penetrometer Stability Measurements for Dry and Wet SurfacesD


Characteristics of Study Participants

Thirty-nine (39) subjects (23 female, 16 male) participated. Subjects were classified into one of four groups: 1) No disability - no known disability or mobility limitation; 2) Ambulatory with limited mobility - persons whose mobility was impaired, but who did not use any type of assistive device for ambulation; 3) Ambulatory with assistive device - persons with a mobility limitation who used an assistive device, such as crutches, a cane, a walker, a surgical implant or prosthetic limb; or 4) Wheelchair user - manual wheelchair users (Table 3). The subjects within each disability category had a wide range of fitness and ability levels.

A statistically significant correlation between total energy required for the ADAAG course and the energy required for each surface (except sand) indicates that for a given subject the level of community mobility is related to the "accessibility" of outdoor surfaces.

On the surfaces objectively measured as firm and stable, subjects with higher fitness levels had lower heart rate and RPE scores (i.e., walking was less difficult). On dirt, wood chips, and engineered wood fibers J and K, fitness level was correlated with all measures of surface "accessibility." Higher fitness levels resulted in lower energy consumption, higher velocity, lower heart rate, and lower RPE.

Table 3. Demographic Characteristics of Study Participants (Mean ± 1 SD, (min - max)

Dis G N Age (yrs) Yrs with Disability
All F 23 35.9 ± 9.0 (22 - 49) 15 ± 12 (3 - 47)
  M 16 34.3 ± 7.7 (24 - 46) 11 ± 9 (3 - 29)
NoDis F 7 32.7 ± 9.1 (22 - 45) NA
  M 7 33.2 ± 7.7 (24 - 44) NA
AwLM F 8 37.4 ± 10.6 (22 - 47) 17 ± 15 (3 - 47)
  M 1 44.5 (44.5) 3 (3)
AwAD F 4 36.5 ± 8.0 (25 - 41) 12 ± 9 (3 - 24)
  M 2 30.7 ± 8.3 (25 - 37) 14 ± 9 (7, 20)
Wc F 4 37.5 ± 8.4 (29 - 49) 13 ± 13 (5 - 32)
  M 6 35.0 ± 8.1 (26 - 46) 11 ± 9 (3 - 29)


  • Dis = Disability
  • G = Gender
  • N = Number of subjects
  • NA = Not applicable
  • All = All subjects combined
  • NoDis = No known disability
  • AwLM = Ambulatory with limited mobility
  • AwAD = Ambulatory with assistive devices
  • Wc = Manual wheelchair user

Energy Requirements for Different Surfaces

The energy consumption results are shown in the following table and figures.

Table 4. Average Energy Consumption (mlO 2/kg/m) for Each Surface by Subject Group

 Energy Consumption above Resting Values
No Dis 2.42 0.10 0.09 0.10 0.09 0.09 0.11 0.10 0.10 0.11 0.16
AwLM 2.50 0.14 0.15 0.12 0.14 0.15 0.16 0.16 0.14 0.18 0.12
AwAD 2.95 0.12 0.15 0.18 0.16 0.18 0.18 0.21 0.21 0.22 0.26
Wc 3.05 0.14 0.10 0.12 0.12 0.10 0.20 0.28 0.31 0.37 1.31

Where: Rest = Resting energy consumption

Figure 8. Average Energy Consumption for All Subjects CombinedD

Figure 9. Average Energy Consumption for Subjects with No Disability (NoDis), Ambulatory D

Figure 10. Average Energy Consumption for Subjects with No Disability (NoDis) and Manual Wheelchair Users (Wc)D


Objective measures of surface firmness and stability were compared to energy costs (energy consumption, velocity, RPE and level of difficulty) to negotiate the surfaces. All human subject measures were significantly correlated to the objective measures. All correlations were strong (>0.85; maximum 1.0 for a perfect correlation), except for the medium correlation between penetrometer firmness and velocity (r = -0.6). Velocity was inversely related to firmness and stability. All other variables were directly related to firmness and stability.

There was a strong, second order polynomial relationship between the Rotational Penetrometer stability measurements and the average energy cost for wheelchair users. As the surface became less stable (Rotational Penetrometer displacement increased), the energy cost for wheelchair users increased more dramatically than for ambulatory individuals with and without disabilities.


What would happen with other disability groups? Overall, our results are very similar to the published literature for similar populations. Therefore, we would hypothesize that our results would also be similar if we had tested other disability populations or older adults. The published literature for children indicates that their energy consumption levels are considerably higher than for adults with similar disabilities. In general, children have higher levels of energy expenditure because they are less "practiced" and therefore less efficient in their movement. However, generally children can also tolerate higher levels of energy expenditure, and therefore higher levels of energy consumption do not necessarily relate to a lack of accessibility. (See the Technical Report for detailed comparisons and complete references).

What proportion of the population could negotiate these surfaces? Energy consumption, level of difficulty rating and rating of perceived exertion were evaluated to determine the proportion of our subjects who considered each surface to be "accessible." An "accessible" surface was one that required less than 0.20 mlO2/kg/m energy consumption, a level of difficulty rating less than 6 ("difficult"), or a rating of perceived exertion less than 13 ("somewhat hard"). Over 90% of our subjects found the objectively firm and stable surfaces to be "accessible," while over 80% of the subjects considered the packed dirt surface "accessible." In comparison, less than 70% of the subjects considered the wood products or sand surfaces to be accessible. Further, the percentage of subjects who considered sand to be accessible is probably artificially high because many subjects, particularly those using wheelchairs, refused or were unable to complete the sand test. If we assume that other disability groups would have similar results (as indicated above), we can hypothesize that in general, at least 80% of the population would consider the surfaces that meet the proposed criteria for "firm and stable" to be accessible.