IDeA Center, University at Buffalo

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Memorandum: 

Date: 4/23/2013

From: Clive D’Souza and Edward Steinfeld, IDeA Center, University at Buffalo 

To: Rex Pace, Technical Assistance Coordinator, U.S. Access Board 

RE: Revised Knee Clearance Depth and Footrest Clearance dimensions for sizing of mammography 

equipment

Introduction

This memo provides a revised description and analyses examining knee clearance dimensions in 

mammography equipment (as requested in your email dated: 4/15/2013). The analyses pertain to:

1. Abdomen depth (AD)

2. Footrest clearance height and depth

1. Abdomen depth (AD)

Abdomen depth was analyzed as the overall clearance depth beneath the mammography machine, and 

measured from the anterior-most aspect of the abdomen to the anterior-most aspect of the occupied 

wheelchair (e.g., to the footrest or toe-tip). The data were analyzed using both, univariate and 

multivariate methods. 

Univariate analysis

The univariate analysis examines AD alone and is appropriate when no other clearance dimension is 

constrained or bounded (e.g., knee clearance height, foot clearance height or depth). Table 1 and Figure 

1 show the percentiles and maximum observed values for male and female wheelchair users. Within 

device type, female users had slightly less AD compared to male users. The 95th percentile value for 

female users of manual and power wheelchairs was approximately 30.5 in. and would accommodate 

about 95% of female users assuming no other clearance dimensions is restricted or bounded. Based on 

the current study sample, accommodating the entire user population would require a clearance depth 

between 33 in. – 37 in.

Table 1: Percentile and maximum values for Abdomen Depth (in.) obtained from univariate analyses

Percentile value

User Group Median – 50th 75th 90th 95th Max

Female MWC users (n=130) 23.2 26.4 29.2 30.6 33.0

Male MWC users (n=147) 24.8 28.3 31.3 32.9 37.7

Female PWC users (n=89) 21.6 24.8 28.1 30.4 36.7

Male PWC users (n=100) 24.6 27.7 31.2 32.0 34.0
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Figure 1: Boxplot showing the distribution for abdomen depth (inches) across wheelchair type and 

occupant gender. The dotted line (30.5 in) depicts the 95th percentile value for female wheelchair 

users, both manual and power.

Note on Multivariate Analysis

The above univariate analysis is only applicable in the case of one critical dimension (i.e., abdomen to 

anterior-most point depth) and all other knee clearance dimensions are unrestricted or unbounded. If 

more than one dimension is bounded (e.g., combinations of knee clearance height or depth, foot 

clearance height or depth, occupied width, etc.) then a multivariate analysis which involves

simultaneous consideration of multiple dimensions is more appropriate for performing clearance 

assessments. As the number of dimensions in the multivariate analysis increases it is possible that the 

resulting space requirements are larger in comparison to the univariate case where each dimension is 

analyzed separately.

2. Analysis of footrest clearance height and depth

Any base support or flange for the mammography machine at floor level (inside the clear floor space 

width) would obstruct some wheelchair users from approaching the mammography machine. The 

purpose of this analysis was to determine suitable dimensions for the base flange depth and thickness at 

floor level to minimize obstructions in forward approach for such users.

Over 30% of manual wheelchair users in this sample did not use footrests. Detailed footrest 

measurements (i.e., foot rest height and corner points on the top surface) were collected from 158 
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manual wheelchair users and 108 power wheelchair users. Footrests were present and recorded on the 

right-side for 101 (64%) manual wheelchair users and 105 (97%) of power wheelchair users. Front castor 

and footrest measurements were available for 67 power wheelchair users (excluding front-wheel drives 

for this analysis, due to time constraints). Three dimensions were computed on the right-side for the 

sub-set of wheelchairs with footrests:

1. Abdomen-Castor edge depth: measured as the horizontal distance from the abdomen to the 

distal (forward-most) edge of the front castor measured at castor hub height

2. Abdomen-Footrest distal depth: measured as the horizontal distance from the abdomen to the 

distal (forward-most) edge of the footrest

3. Footrest height: measured as the height from the floor to the top surface of the footrest at the 

proximal (near) outside corner

Both depth dimensions are referenced to the abdomen assuming that the wheelchair occupant makes 

contact with the breast-platform at the abdomen. The range and median values measured on the rightside for these three dimensions are presented in table 2. Critical values are highlighted in yellow. For 

each dimension, the more conservative estimate across manual and power wheelchairs is 

recommended. Based on the data in Table 2 we recommend that:

- The total knee clearance depth should preferably be no less than 30 in. to minimize contact with 

the footrest (i.e., 95th percentile value for Abdomen-Footrest Distal depth). The abdomenanterior-most point depth presented in the previous section is a preferred dimension as the 

feet/toes often go past the footrest. 

- Any flange or base support should preferably be no deeper than the overall clearance depth 

(e.g., 32 in. or 30 in.) minus the 95th percentile value for abdomen-castor edge depth. For 

example, for power wheelchair users, recommended flange depth = 30 in. – 19.3 in. = 10.7 in.

- Any flange or base support thickness should preferably be less than the 5th percentile value of 

footrest height for manual wheelchair users minus the thickness of the footrest (assuming 0.5 

in.), i.e., approx. 1.7-0.5 = 1.2 in.

Table 2: Observed ranges and percentile values for Abdomen-Castor edge depth, Abdomen-Footrest 

edge depth, and Footrest Height, measured on the right-side for manual and power wheelchair users 

(Note: combined data for male and female). Critical values are highlighted in yellow.

Measurement Dimension Device Type Min 5 10 50 90 95 Max

Abdomen-Castor Edge Depth Manual (n=101) 2.0 5.6 6.8 11.4 17.1 18.5 20.3

Power (n=67) 4.1 6.0 7.8 10.9 17.6 19.3 21.9

Abdomen - Footrest Distal Depth Manual (n=101) 12.3 15.0 16.0 21.1 27.8 28.4 34.6

Power (n=67) 13.7 14.2 15.0 21.0 27.2 29.9 35.1

Footrest Height Manual (n=101) 1.2 1.7 2.2 4.1 7.2 10.0 36.2

Power (n=67) 1.8 2.7 3.1 4.8 8.8 10.5 16.9
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Figures 2 and 3 provide scatter-plots showing the relationship between Abdomen-Castor edge depth 

(and castor hub height) and Abdomen-Footrest distal depth (at footrest height) referenced from the 

abdomen point measured on the right side for manual wheelchair users (fig. 2) and power wheelchair 

users (fig. 3) respectively. 

Assuming a 30 in. knee clearance depth, sample profiles of flanges based on the footrest measurements 

for manual and power wheelchair users are also shown. Although the flange depth and thickness is 

limited (e.g., 1-1.5 in.) the data suggests that the flange thickness could be increased closer to the base 

of the machine e.g., a triangular profile.

Further, it can be assumed that individuals that did not have a footrest (over 30% of manual wheelchair 

users) have some lower extremity mobility and would be able to lift and place their feet on top of the 

flange/base.

Figure 2: Scatter-plot showing observed values footrest height (on the vertical axis) and CastorFootrest Depth (on the horizontal axis) measured on the right-side for manual wheelchair users

referenced to the abdomen point. The grey curves show sample profiles of castor-wheels.
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Figure 3: Scatter-plot showing observed values footrest height (on the vertical axis) and CastorFootrest Depth (on the horizontal axis) measured on the right-side for power wheelchair users 

referenced to the abdomen point. The grey curves show sample profiles of castors-wheels.

Conclusions

Design criteria based on the univariate analysis of abdomen depth is sufficient if all other clearance 

dimensions are unbounded (i.e., unrestricted) and abdomen depth is the only relevant dimension. The 

95th percentile value for female users of manual and power wheelchairs was approximately 30.5 in. and 

would provide adequate clearance depth such that approximately 95% of female users can approach the 

machine up to the abdomen. 

Analysis of the footrest clearance height, abdomen-castor depth and abdomen-footrest depth suggests

a flange of depth 10-10.7 in. and thickness 1-1.2 in. maximum. However, a graphical plot of the data 

suggests that the flange thickness could be increased closer to the base of the machine e.g., with a 

gradient or triangular profile.