## 4 Discussion  
The current study compares cycling, the most-used exercise modality among individuals with lower-extremity pain, with HFgait, an alternative novel exercise modality. Thus, this study is also the first to describe HFgait lower limb kinematics. Moreover, given that current study hypothesized differences in kinematic patterns between the two modalities, a phase offset reduction technique was applied to improve the contrast between the two cyclical movements. The main differences between the two exercise modalities were found at the hip. Compared to cycling, HFgait has reduced hip flexion during the cycle except during the peak hip flexion, which corresponds to the period from 70 to 90% of the HFgait cycle. During HFgait, the hip has a larger sagittal plane range of motion, ranging from about 10\({}^{\circ}\) of extension to 95\({}^{\circ}\) of flexion, whereas cycling maintains an orientation of hip flexion throughout and promotes a muted range of motion of about 36\({}^{\circ}\) to 82\({}^{\circ}\) of flexion. The range of hip-joint excursion during HFgait was different from that during cycling; suggesting that HFgait has effects that cannot be achieved by cycling. Particularly, reductions in peak hip extension angle have been strongly associated with increased severity of disability in patients with hip and knee osteoarthritis [19]. Therefore, kinematically, HFgait might be recommended for rehabilitation outcomes that benefits from a large hip range of motion [20,21]. Additionally, because the hip reaches extension during HFgait, which does not occur during cycling, and maintains a less flexed position for a longer duration of the HFgait cycle than during cycling, HFgait might be a valid alternative to individuals with lower back pain [22], or vascular pathologies that might be aggravated with cycling [23]. HFgait also involved a larger hip range of motion in the frontal plane, maintaining a less abducted hip angle compared to cycling, such that the frontal plane hip-joint angle was oriented closer to a neutral position during most of the gait cycle. This further supports the use of HFgait for interventions that target hip mobility with an exercise component, such as the clinical practice guidelines for the management of hip osteoarthritis [24, 25, 26]. Differences in knee kinematics between HFgait and cycling were observed essentially only in the sagittal plane. Similarly to the hip, cycling maintains the knee in a more flexed position than HFgait during the cycle, while HFgait transitions from very little flexion (less than 20\({}^{\circ}\)) during the first half of the cycle to a large peak flexion (approximately 120\({}^{\circ}\)) at about 80% of the cycle. The most common overuse injury in cycling is observed at the patellofemoral joint [23,27]. This is attributed to the force generated at this joint at the beginning of the downstroke. At this point, the quadriceps femoris muscle group develops a large force at the patellofemoral joint surface that is made even greater by the amount of knee flexion during this phase (peak knee flexion of approximately 110\({}^{\circ}\)) [28]. This pattern is not observed during HFgait. Although peak knee flexion during HFgait is similar to cycling, in HFgait, this phase takes place during the swing phase when no external forces are being applied at the knee joint. Bike configuration and saddle height have been shown to alter lower limb kinematics during cycling and limit overuse knee injuries [29]. However, this will most likely involve bike fitting protocols with bike professionals that are not accessible to most individuals that wish to use cycling as an exercise modality. Our discrete analysis reported differences between HFgait and cycling for the ankle joint at all discrete parameters except minimum sagittal place. However, while our continuous analysis also identified some differences for ankle joint motion, these were short in terms of duration during the cycles.