The hammer toe is one of the relatively common deformities at the foot. Investigation of hammer toe side effects on the other parts of the lower limb can be helpful for better identification of this deformity as well as prevention of progressive pain. For this aim, the finite element model including soft and hard tissue, ligament, and cartilage were used as well as musculoskeletal modeling. The validation of the finite element results was performed by comparing the results of the predicted plantar pressure distribution of the foot with the results of the experimental pressure pad at previous study . In order to validate musculoskeletal model, the results of the predicted intensity of muscle activities were compared with EMG signals.
As our previous study  and soft tissue von Mises stress distribution in Fig. 5 show, the presence of a hammer toe, not only causes the stress concentration in the forefoot, but also will increase plantar pressure and internal stresses and will increase the risk of injury and ulcers. In the present study, it is tried to show the other aspects of hammer toe deformity effects on the lower limb.
Wang et al.  showed that body weight force (BWF) transmits by the medial metatarsals (1st, 2nd, and 3rd toes and metatarsals), which is in line with the peak plantar pressure region in Akrami et al.  results. In addition, Kalra et al.  showed the important role of medial metatarsals, especially the second one in BWF transmission by investigating on 10 cadaveric feet. As shown in Fig. 5, the hammer toe changes the location of transfer body weight force line to the lateral side of the foot, and the 3rd, 4th, and 5th metatarsal had a significant function in weight-bearing. The function of several parts of the lower limb may be affected as a result of this condition. Previous researches on healthy feet found that fifth metatarsal has lower maximum stress compared to other metatarsals [67, 69, 70] while fifth metatarsal fractures are thought to occur more frequently than any other metatarsal fractures , while the hammertoe increased stress in the fifth metatarsal, indicating an increased risk of fifth metatarsal fracture. In Fig. 4, the results of lateral/medial displacement of hammer toe foot show that the displacement of the little toe, especially in the metatarsal region, is considerable and indicates expectancy and the probability of bunionette (tailor's bunion) as the subsequent possible deformity. At the same time, the results of the stress distribution, as well as the displacement of the first toe in the MTP joint, do not demonstrate the possibility of Bunions (Hallux Valgus), as a side effect of the hammer toe.
The contact area of the foot with the ground is reduced when the hammer toe occurs, and the toes are less involved in weight bearing during walking. As shown in Fig. 7, MTP joint reaction force for hammer toe foot is less than the healthy foot, and also the stress distribution results in Fig. 5 indicate the significant difference between metatarsals and toes stress, and this issue shows a shift in the contribution of the toes in weight bearing to the metatarsals. Furthermore, this means a reduction in foot length which acts flat spring-like. The stiffness of the flat spring rises as its length decreases, so the hammer toe exhibit more rigid foot, and this condition has an impact on the lower limb's induced forces. A rigid foot is less likely to be able to absorb shocks, resulting in intense ground forces being imposed on the foot [72, 73]. Rigid foot causes higher ankle, knee, and hip joint reaction forces, which in this study appeared in knee forces with 50% higher than a healthy foot and also disappearing the double hump in vertical knee and hip reaction forces that were monotonically increasing in most of the stance phase, as shown in figures B1 and B2 at appendix B.
The results of the joint reactions indicate the distinction of the knee abduction moment pattern, as well as the amount of force on the knee and hip between hammer toe and healthy foot as shown in Appendix A. Our results of the healthy foot (pattern and magnitude) are in line with previous studies [74, 75]. A notable point on the peak of the reaction moments is that since the hammer toe shifted the position of the COP to the lateral, the moment arm of the total GRFs has been reduced on joints, which is aligned with the previous finding . For this reason, the peak reaction moments for hammer toe foot are lower than the healthy foot.
Based on the plantar fascia tension shown in Fig. 6, the productivity of plantar capacity does not occur in the hammer toe foot, and as a result, there is less shock and energy absorption in the plantar fascia of the hammer toe foot. Caravaggi et al.  showed that the overall tensional plantar fascia load decreases from medial to lateral in healthy foot and also the previous studies showed that the medial side of the plantar has higher stress during walking , and the medial side was reported as maximum stress area [44, 66] which show that hallux plantar ray has a significant role in elastic energy storage. On the other hand, plantar fasciitis occurs as a consequence of excess mechanical load on the fascia [79, 80]. Results of this study show that for the hammer toe as the deformity and consequent poor windlass mechanism, the maximum tensional load does not occur at the medial side of the plantar fascia, and the first ray of the plantar fascia under the 1st MTP has lower tensile force during walking shown in Fig. 6. Furthermore, the result of PF stresses shows as well as 2nd plantar ray, lateral side of the PF is expected to be more prone to the onset of small tears and injury in HTF as a result of the continues forces induced during the stance phase of gait. The need for more muscular forces will be reduced by storing energy in tendons and ligaments during walking [42, 81, 82] expected up to 17% of the overall mechanical energy spent during walking . As shown in Fig. 3, due to reduction of the elastic stored energy at the PF, required power and muscle forces for HMT foot is higher than HF, and this issue raises metabolic cost. The soleus, medial and lateral gastrocnemius make a contribution to 93 percent of the plantar flexion torque during a step . The tensions in the Achilles tendon and plantar fascia are mechanically connected. Increasing the PF tension, in addition to providing integrity to the bony arch structure, increases the tension in the Achilles tendon, which is likely to reduce the metabolic cost of walking . As shown in Fig. 2, PF force significantly reduced in HMT foot, and as shown in Fig. 3, this issue demands extra soleus, medial and lateral gastrocnemius muscle force for effective propulsion and this matter makes the body use more energy during walking.
According to Zhang et al. , injuries in joints, such as the knee joint and osteoarthritis, will increase the force induced on foot. Also based on the results of soft tissue stress distribution shown in Fig. 5, deformity at the metatarsal head makes the stress concentration and increases internal and plantar stresses in hammer toe foot which is consistent with previous studies [22, 60].