Optical motion analysis (MA) is a useful tool for evaluating musculoskeletal function in health and disease. MA is particularly useful in quantifying joint kinematic and kinetic abnormalities accompanying osteoarthritis. However, current practice does not allow the joints of the foot to be measured since the foot is treated as a single rigid segment. To develop a multi-segment kinematic model of the foot for use in a clinical motion analysis laboratory. Apply the model to a healthy population during normal walking and gait intentionally disrupted by a high arch orthotic.

The foot was defined as five rigid segments: hindfoot (calcaneus), midfoot (tarsus), medial forefoot (first metatarsal), lateral forefoot (fifth metatarsal) and the hallux (both phalanges). Each of these segments were tracked individually using custom-built marker triads attached to the skin. Thirty healthy subjects (eleven male, nineteen female; mean age 27.7 years, range 19–53) were examined using MA (eight Eagle camera, EvaRt system, Motion Analysis Corp., Santa Rosa, CA, USA) during normal walking and gait disrupted with a high arch orthotic taped to the plantar surface. All trials were performed barefoot. The special foot marker system was applied to the right foot with the remaining markers in the Helen Hayes configuration. Three motions are reported. The hallux-medial forefoot angulation (HA) is reported in the sagittal plane (plantar-dorsiflexion). The hindfoot-midfoot angulation (HFA) is also reported in the sagittal plane (plantar-dorsiflexion). The height-to-length ratio of the medial-longitudinal arch (MLA) is reported, normalised to zero in quiet standing. Paired t-tests compared the normal and disrupted gait conditions. All angles were compared at the instant of foot flat.

HA was not significantly changed between normal and disrupted conditions: from 8.5° ± 6.4° to 8.6° ± 7.4° (p=0.88). The HFA plantar-flexion significantly increased from 0.5 ° ± 3.3° (normal) to 2.9° ± 4.4° (disrupted; p< 0.01); mean difference = +2.5° (95% CI: 0.81 to 4.1°). The MLA was significantly increased (arch raised) from 0.004 ± 0.018 (normal) to 0.017 ± 0.021 (disrupted; p< 0.01); mean increase = +0.012 (95% CI: 0.00421 to 0.021).

A multi-segment kinematic model of the foot has been successfully implemented in an optical motion analysis laboratory. The model was sensitive to an intentional disruption of normal foot kinematics during walking in a healthy population.