# Incorporation of Torsion Springs in a Knee Exoskeleton for Stance Phase Correction of Crouch Gait
## 1 Introduction  
Crouch gait is defined as flexion in excess of 20\({}^{\circ}\) during the stance phase of the gait cycle [1, 2]. Crouch gait is common in those with severe diplegia or spastic quadriplegia caused by conditions such as cerebral palsy (CP), stroke, multiple sclerosis (MS), and other neurological disorders. The causes of crouch gait include contracted hamstrings, contracted hip flexors, impaired balance, foot deformities, impaired proprioception, and muscle weakness [3, 4]. In addition, excessive flexion occurs during the stance phase of the gait cycle, resulting in the degradation of joints, pain, and loss of ambulation over time [4, 5, 6]. There are two major phases of the normal gait cycle: the stance phase and the swing phase. The stance phase is the point of the heel strike to toe-off, which occurs from 0% to 60% of the gait cycle [7]. The swing phase is defined as the point from toe-off from the previous phase at 60% of the gait cycle to the follow up heel strike at 100% of the gait cycle. Figure 1 describes the normal knee angle throughout one complete gait cycle for the right knee, where normal knee angles provided by OpenSim 4.3 were plotted with respect to the proportion of the gait cycle. The knee angle for the normal gait cycle (red) is compared to four different crouch gaits. The normal rotation of the knee ranges between \(-2.27^{\circ}\) and \(-58.25^{\circ}\). For each crouch gait, the joint rotation has a severe limitation in the range of motion. As a result, the knee cannot overcome extreme flexion during the stance phase, which ends at approximately 60% of the gait cycle. The knee extension and flexion are generated by force actuation in the quadriceps and hamstrings. [PERSON] et al. notes that an imbalance in these results in a deviation of gait,resulting in flexed knee gait (FKG) or crouch gait, wherein excessive flexion occurs during the stance phase of the gait cycle [5]. The acceleration of the gluteus maximus and vasti are responsible for knee extension; thus, lower velocities inhibit knee extension [8]. Causes of FKG include hamstring shortening, fixed flexion deformity of the knee, quadriceps weakness, patella tendon lengthening, and soleus weakness. Due to the quadriceps having to work constantly to stabilize the knee, as opposed to a short duration during a normal gait, FKG causes high energy consumption, resulting in fatigue. Inefficiency in gait is well documented [9; 10; 11; 12]. Complications of uncorrected crouch gait result in increasing load on the knee joint [2; 13] which likely causes knee pain [5; 14; 15] and joint deterioration [2; 16; 17]. The stance phase of the gait cycle is the weightbearing portion of the gait cycle. When the foot maintains contact with the ground, the weight bearing axis should align centrally from the ankle through the knee [18] to the hip. In crouch gait, the weightbearing axis shifts to the posterior of the knee [19]. Tibiofemoral forces corresponding with ground reaction force peaks in the stance phase of the gait cycle are more than double [13] in crouch gait compared to unimpaired gait, depending on the severity of the crouch gait [2]. In crouch gait, [PERSON] et al. attributed the increasing tibiofemoral force to increased force in the quadriceps throughout the stance phase. Additionally, crouch gait severity results in increased patellofemoral contact pressures, as well as the location [13] of the patellofemoral contact pressure. [PERSON] et al. discussed altered tibiofemoral kinematics and the resulting increased posterior translation of the tibia with increasing flexion and the correlation to knee pathology with increasing patellofemoral contact pressures [20] and hypothesized that balancing hamstrings and quadriceps forces could reduce patellofemoral contact pressure. In many cases, crouch gait is a result of torsional deformities [17; 21], which tends to cause either an inward or outward position of the feet and ankles.