By K. Missy Ball, MT, PT, ATP, Pediatric NDT  | When prescribing seating and wheeled mobility assistive technology, the seating team will take actual client measurements to determine the size and other specifics of the seating system. Length of time for funding approval needs to be considered here as well.  Longer than a 3 month period between seating evaluation and delivery will require child to be re-measured to ensure best fit and maximum growth in chair. These measurements are best taken with client on firm surface supported in best alignment for function.  The following is a list with a description of how to take the measurement.

Seat depth is measured (with pelvis neutral, if possible, in upright seated position) from back of pelvis to popliteal fossa (back of the knee joint).  In the growing child, when specifying frame depth or seat depth another 3” is added for future growth. The seat depth is set at delivery  ½” shorter than actual client seat depth. With Convaid products, this is achieved in the EZ Rider, Cruiser, or Rodeo with extended upholstery piece and seat rail extensions.  On the Trekker, the seat pan has 6”of depth adjustment.  In custom seating, the seat depth is fabricated 3” longer than depth needed with back moved forward to achieve appropriate seat depth (future seat depth extends rearward from back).

Seat width is measured at widest point, which could be hips, upper thighs or lateral knee to knee with lower extremities in slight abduction & external rotation. This measurement in conjunction with shoulder width usually determines the actual frame width.  If working in pediatrics, 3” of additional width is added to actual client width (1 ½” either side) if they are dependent for propulsion.  If pediatric client can self –propel, 2” maybe sufficient with wheels cambered for best propulsion.  With an active, independent self- propelling adult,

1” of additional width maybe sufficient for future weight gain.  Wheel placement for propulsion is crucial to reduce shoulder repetitive strain injury and promote efficient propulsion with least energy consumption.  For a client with a structural windswept deformity of the hips, seat width would be measured from the most lateral abducted knee horizontally to a line coming off the greater trochanter on the opposing side. The point – measurements need to reflect the client’s skeletal limitations and the chair needs to accommodate these issues.

Seat to the top of the head is measured if client lacks head control, cannot sustain head control or if a tilt or recline system will be considered.  This measurement is important when considering height of user in chair for van access for example.

Seat to shoulder will be used for the client with limited trunk control, such as progressive muscle disease or low tone cerebral palsy.

Seat to inferior angle of the scapula minus 1-2” can be effective for full shoulder girdle excursion during self –propulsion on a client with spina bifida for example.

Seat to posterior superior iliac spine can be useful when considering support at pelvic/sacral angle for more active paraplegic SCI or if considering a biangular back to provide segregated support of back for more involved clients.

Seat to axilla (underarm) is usually measured if client needs lateral trunk supports. The height of the lateral trunk supports is influenced by this measurement, as well as the need for specific adjustment and placement of pad to reduce scoliosis.  Chest Depth, depth from breast at the nipple line to the back of client, is also needed when specifying lateral trunk supports.  The length of the pad is determined by this chest depth, the load being absorbed by the pad, and whether the pad is linear or curved.  Linear pads are usually 1” shorter than chest depth, while curved pads are 1” longer than actual chest depth.

Leg length is measured at popliteal fossa (back of knee) to the heel of foot or the most distal fixed point.  This measurement is needed to determine front rigging length. Usually a 2” clearance is needed between front rigging and ground.  Front rigging angle also needs to be considered.  Knee joint range of motion (hamstring flexibility) affects front rigging angle which in turn can impact on front rigging length.  A client with limited knee flexion may not be able to handle a 90* fixed front end.  A variety of front rigging angles 85-60* are available on different manufacturers frames.  Convaid has elevating legrests to accommodate this client in the Rodeo, Trekker or Safari.  With knee flexion contractures , choosing front rigging that can flex somewhat under the seat and clear caster rotation will be important.  Convaid’s adjustable front rigging also can accommodate knee flexion contracture of 95*.

Client measurements are primary to determine specifics of frame and seating components, but how frame parameters impact on functionality is also important to consider.  For example, the frame length, which could be rear anti-tipper to front of footplate, affects turning radius in the home or work environment, as well as space needed in van for WC-19 tie down system or to collapse and store chair in car or van.  Frame width, outside wheel to wheel usually (or push rim), is critical to ensure access through door frames, hallways, ramps and transport. The overall height of chair with occupant seated is important to ensure clearance of head through van entrance, as well as from roof during transport.  Seat to floor height is important with regard to independent transfers from one surface to another, (example commode), independent propulsion for best ergonomic set up to wheel, or to allow access to table tops and sinks. Seat to floor height can be measured from front and rear of seat rail, and is adjustable on frame with adjustable axle placement.  Also, front rigging angle can impact on table and sink access as well.

In summary, the seating team needs to balance client dimensions with their functional capabilities, daily activities, and home, work, transportation environments when choosing equipment for best outcome for the client.


Karen “Missy” Ball, MT, PT, ATP, who is the former Acting Director and Assistant Director of the Physical Therapy Department at Children’s Hospital in New Orleans.  She was the co-director of the seating program at the hospital for a 10-year period as well as a consultant for the United Cerebral Palsy Center’s seating clinic. She served as educational specialist for Freedom Designs for 21 years and presently is an educational consultant for Convaid Products, LLC and R82, Inc. She has lectured both nationally and internationally on seating and mobility for over 20 years. She has a private practice specializing in pediatric neurology. She is an ATP and is certified in pediatric NDT with a focus on improving function through treatment and equipment.

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