A common surgical treatment for posterior tibial tendon dysfunction (and the resulting flat foot) is the correction through a calcaneal lengthening osteotomy of the lateral column (LLC). Clinical studies showed pain relief and functional improvements through different scores. However, according to clinical experience, some patients complain about a limited ankle dorsiflexion after LLC surgery. Several joints of the foot (talocrural, subtalar, talonavicular, calcaneocuboid) contribute to the overall range of motion in foot plantarflexion/dorsiflexion and pronation/supination. Changes in the range of motion in one joint can affect all the other joints. For instance, it was shown that a fusion of the talonavicular joint removes most of the residual hindfoot motion in plantarflexion/dorsiflexion and pronation/supination. Because the lengthening of the lateral column presumably decreases the mobility of the medial column and thus of the talonavicular joint, this surgery can influence the range of motion of the other joints, and hence contribute to the reported decreased ankle dorsiflexion motion.

Patients after LLC have less plantarflexion of the first metatarsal throughout stance of level walking and less inversion of the hindfoot during push-off compared to healthy subjects. Uphill walking requires more ankle plantarflexion and dorsiflexion than level walking. A limitation of the ankle joint mobility especially in dorsiflexion could therefore lead to additional or greater changes in gait patterns (hindfoot and forefoot kinematics) during uphill walking.

The primary objective is:

• To compare differences in hindfoot and forefoot kinematics between level and uphill treadmill walking in relation to passive range of motion

The secondary objectives are:

• To compare lower leg muscle activation during level and uphill treadmill walking between patients after LLC and healthy subjects
• To test the association between muscle strength, muscle activation patterns and hindfoot and forefoot kinematics during level and uphill walking and heel rise
• To relate clinical outcome of LLC surgery by functional scores to passive range of motion

At the initial assessment, written informed consent will be obtained before participants will undergo a clinical exam (inspection and palpation of the foot, measurement of bilateral passive ankle range of motion). All participants will complete the Short Form (SF)36 and the Foot Function Index20 to obtain pain and functional scores (approximate duration: 30 minutes). Participants will be able to familiarize with treadmill walking at their preferred walking speed. Surface electrodes will be placed bilaterally over the tibialis anterior, gastrocnemius medialis and lateralis, soleus, and peroneus brevis. Isokinetic muscle strength in ankle plantarflexion/ dorsiflexion and inversion/eversion will be tested using the Biodex system 4 Pro (approximate duration: 45 minutes). Reflective surface markers will be placed bilaterally on anatomic landmarks according to the Plug In Gait model and a specific foot model. These markers are seen by 6 Vicon MX cameras. Participants will be asked to stand on the treadmill (h/p cosmos, Zebris), and data for a standing reference trial will be collected. Single-limb heel rise performance with each leg will be tested on the treadmill while kinematic, electromyography (EMG), and pressure data will be measured. Participants will then walk barefoot for 2 minutes at 0% slope while kinematic, EMG, and pressure data will be recorded. Subsequently, the treadmill incline will be increased to 15%, and data for 2 minutes walking at this slope will be recorded followed by three heel rises (approximate duration: 45 minutes). The estimated total time for each participant is 120 minutes.

Inclusion Criteria:

• age > 18 years
• age < 70 years
• Patients:
• Unilateral surgery for posterior tibial tendon dysfunction by flexor digitorum longus transfer and lateral lengthening osteotomy of the calcaneus
• Minimum of 2 years postoperatively

Exclusion Criteria:

• Neuromuscular disorders affecting gait
• Cardiovascular disease
• Diabetes
• Pregnancy (if unknown, a pregnancy test (urine test) will be performed)
• Body mass index > 35 kg/m2
• Patients:
• Additional pathologies that influence the mobility of the ankle joint
• Bilateral surgery
• Use of walking aids
• Healthy control group:
• Lower extremity surgery
• Pain in the lower extremities within the last 6 months

20 patients who had undergone lengthening osteotomy of the lateral column of the foot 2 to 10 years prior and 20 age and sex matched healthy subjects