Foot arch height is generally classified into three types based on the morphology of the medial longitudinal arch: low arch (LA), normal arch (NA), and high arch (HA) (1). LA and HA are common foot morphological deviations in adults, with prevalence rates of 2%–23% for LA (2) and 10% for HA (3). Abnormal arch height alters knee kinematics (4), with low arches associated with heel valgus and tibial internal rotation (2, 5), and high arches linked to heel varus and tibial external rotation (6, 7).

Abnormal foot arch height is a significant mechanical factor influencing knee joint function during movement (4). Its effects manifest across multiple domains: static balance (8, 9), knee joint position sense (10), dynamic loading and control (where it disrupts knee joint motor control mechanisms (11), alters frontal plane kinetics (12), and increases medial tibiofemoral joint loading (13).The ultimate consequence of these pathological mechanical alterations is a significantly elevated risk of knee joint injury and degeneration, as demonstrated in studies on anterior cruciate ligament (ACL) injury (14) and the symptoms and characteristics of knee osteoarthritis (15, 16). These findings collectively establish aberrant foot arch morphology as a key risk factor (12, 17).

Knee muscle strength, as the mechanical foundation of joint stability, is directly associated with an increased risk of injury (18, 19), as well as pain and reduced quality of life (20) in patients with knee osteoarthritis (KOA). Foot arch height, as the initiating point of lower limb kinetics, is hypothesized to influence knee muscle strength (21–23); however, direct evidence remains inconsistent. For instance, Bakırhan et al. (24) found no significant association using manual muscle testing, suggesting potential obscuration of a true effect by methodological limitations. Conversely, Kim et al. (25) reported that the impact of foot arch height on knee joint moments was detectable only during walking, implying that movement velocity may serve as a critical moderating variable. Thus, moving beyond the limitations of manual testing (ambiguity) and speed-specific observations (constraints), a systematic investigation into the quantitative effects of foot arch height on knee muscle strength across varying movement velocities is essential to elucidate the underlying biomechanical mechanisms.

Isokinetic muscle strength testing represents a sophisticated method for quantitatively assessing muscle strength across the full range of motion of a joint at a constant velocity (26, 27). This approach is characterized by high efficiency (28) and reliability (29), and precisely delineates force variations throughout the range of motion—as evidenced by isokinetic moment curves (IMCs) that depict the relationship between moment and angle, curve morphology, and smoothness (30). Isokinetic muscle strength testing serves both diagnostic and screening purposes for musculoskeletal injuries of lower extremity (31), as moments measured at different angular velocities carry distinct clinical significance: ≤60 °/s measurements assess muscular strength magnitude, while ≥180 °/s measurements evaluate power or endurance (32). A meticulous analysis of IMC facilitates a nuanced understanding of muscle function at various angles and velocities, providing valuable insights into muscle performance and potential deficits. Therefore, the aim of this study was to elucidate the effect of foot arch height on knee muscle function by comparing knee IMCs among subjects with low, normal, and high arch profiles under varying angular velocities. We hypothesize that individuals with LA and HA would demonstrate distinct extension and flexion moments compared to NA controls at varying angular velocities. Elucidating these biomechanical variations holds dual clinical relevance: (1) establishing a theoretical framework for knee injury prevention and rehabilitation strategies, and (2) advancing the mechanistic understanding of foot arch height's role in lower limb kinetics.

This study examined the IMC of the dominant lower limb among three arch height groups (LA, NA, HA) during maximal effort contractions at slow (60 °/s) and fast (180 °/s) angular velocities, focusing on knee extension and flexion moments at specific joint angles. At 60 °/s (slow velocity), LA individuals exhibited greater knee extension moments than NA controls, whereas HA individuals showed the opposite trend. Both LA and HA groups demonstrated reduced knee flexion moments relative to the NA group. Notably, at the terminal phase of joint movement, LA individuals displayed higher knee flexion moments than NA individuals. At 180 °/s (fast velocity), both LA and HA groups consistently exhibited weaker knee flexion and extension moments.

Moment values at the initiation and termination of joint motion are often compromised by isokinetic apparatus lever arm effects during acceleration and deceleration phases (45–47). To address this, we applied two-point interpolation to correct data. Empirical evidence indicates that moment data derived from this method are reliable within specific angular ranges: 6°–84° at 60 °/s and 20°–70° at 180 °/s (43). Deviations in IMC have been correlated strongly with knee joint function (31, 48). Therefore, changes in joint function can be promptly assessed by examining the morphology of IMC within these reliable angular intervals. Based on this premise, the present study analyzed the functional status of knee muscles within the established reliable angular ranges (48).

In isokinetic muscle strength analysis, moments recorded at slower velocities (60 °/s) are typically indicative of muscle strength (32). Our results revealed that at 60 °/s, individuals with LA exhibited higher knee extension moments than those with NA, whereas their knee flexion moments were comparatively lower. Notably, LA subjects demonstrated greater flexion moments than NA subjects only at the terminal phase of movement. This observation aligns with a study on ankle muscle strength, which similarly documented reduced strength in HA individuals and enhanced strength in LA subjects (22). However, previous studies have reported conflicting findings. Bakırhan et al. (24) conducted a cross-sectional study on 78 LA subjects and found no significant correlation between knee flexor and extensor strengths in LA and NA individuals. Similarly, Kim et al. (25) reported that although LA subjects exhibited greater knee abduction moments compared to NA subjects, there was no significant correlation in knee flexion and extension moments between the two groups. These discrepancies may stem from variations in the methods employed for moment measurement. Bakırhan et al. (24) used unarmed plyometrics, which is inherently subjective and constrained by its limitations. Kim et al. (25) measured ground reaction forces using a pressure plate and calculated moment magnitude indirectly by multiplying these forces by the force arm, a method limited by individual variations in force arm length. In contrast, our study employed an isokinetic muscle strength testing system, which allows for direct acquisition of moment data, thereby circumventing the limitations associated with the methodologies used by Bakırhan et al. and Kim et al. This direct measurement approach provides a more reliable and objective assessment of muscle strength and function, offering clearer insights into the biomechanical differences among the three groups.

The elevated knee extension moments observed in individuals with LA compared to those with NA may be attributed to two primary factors. First, it may represent a compensatory neuromuscular mechanism. LA individuals may require greater muscle strength to counteract knee abduction moments and maintain knee stability during daily activities. A meta-analysis (49) demonstrated that LA can increase knee abduction moments, which may necessitate higher knee adduction moments for balance and joint stability. Additionally, studies (50–52) have indicated that individuals with LA may experience impaired balance and joint position sense, further supporting the need for compensatory mechanisms. Second, this phenomenon may be related to the quadriceps angle (Q angle). Powers et al. (53) demonstrated that lower extremity alignment significantly affects patellar forces. The Q angle is formed by the intersection of the quadriceps force line and the line from the patellar center to the tibial tuberosity. External tibial rotation increases the Q angle, while internal rotation decreases it. A larger Q angle increases resistance to knee extension, reducing the knee extension moment. Conversely, individuals with internal knee rotation typically have smaller Q angles, which enhances the knee extension moment. Consequently, arch height variations lead to characteristic differences: LA (low arch) typically presents with a smaller Q-angle, while HA (high arch) demonstrates the opposite pattern. However, LA individuals exhibited lower knee flexion moments compared to NA individuals, likely due to altered gait characteristics. Marouvo et al. (4) observed greater peak knee flexion during gait in LA subjects, which may alter knee flexor contraction patterns and load distribution, potentially weakening these muscles over time.

Furthermore, in individuals with low arches, the subtalar and talonavicular joints exhibit excessive motion during the early to mid-stance phases of the gait cycle, and demonstrate a dysfunctional state in the late stance phase (54). This aberrant pattern further induces excessive foot pronation, which alters ankle kinematics and mechanical loading, thereby compromising knee joint function (55). Conversely, individuals with high arches, due to heightened foot rigidity and poor shock absorption, are prone to stress concentration along the lateral foot border (6, 7). During the gait cycle, the foot in these individuals often maintains a “locked” posture characterized by hindfoot varus and forefoot varus throughout the stance phase (3), which restricts effective stress dissipation and may predispose to fifth metatarsal stress fractures (56). Studies (57) have also indicated that high-arched athletes exhibit a higher incidence of bony structural injuries in the lateral lower limb, such as tibial and fifth metatarsal stress fractures, whereas low-arched athletes show a higher prevalence of medial soft tissue injuries, including patellar tendinopathy, Achilles tendinitis, and patellofemoral pain syndrome. These findings suggest that structural foot abnormalities can increase the risk of injuries at the ankle and knee by affecting the coordinated motion coupling mechanisms of the lower limb joints (58). Research demonstrates that personalized orthotic insoles can significantly alter plantar pressure distribution and bone stress (59, 60), indicating that future interventional studies could focus on customized footwear to explore its potential in alleviating proximal joint loads by improving foot biomechanics.

Our study found that knee extension and flexion moments were lower in HA individuals compared to NA individuals under slow-speed exercise conditions. This finding aligns with the work of Zhao et al. (61), who reported an inverse relationship between arch height and lower limb muscle strength in adult males, with HA individuals exhibiting diminished muscular performance. Williams et al. (7) demonstrated through electromyographic analysis that lateral femoral muscles in HA individuals activate earlier during exercise, indicating early knee extension activity. This early activation may increase limb stiffness upon foot contact with the ground (22). Additionally, Mao et al. (11) noted differences in anterior-posterior and internal-external ground reaction forces during the stance phase between HA and NA runners, which may be related to reduced lower limb muscle activity and altered muscle work. Thus, it is hypothesized that increased leg stiffness and reduced lower limb muscle activity may contribute to weaker knee musculature in HA individuals, potentially predisposing them to knee injuries.

Additionally, muscle strength performance in both LA and HA individuals is influenced by the Q angle. Powers et al. (53) demonstrated that normal lower extremity alignment is crucial for patellar outward force. A larger Q angle increases the lateral vector acting on the patella during knee extension, thereby augmenting the resistance to extension and reducing the knee extension moment. In HA individuals, external tibial rotation laterally displaces the tibial tuberosity, increasing the Q angle and the associated lateral vector, which in turn yields a smaller knee extension moment. Conversely, in LA individuals, internal tibial rotation medially shifts the tibial tuberosity, decreasing the Q angle and the lateral vector, thereby producing a larger knee extension moment. Accordingly, LA individuals typically exhibit greater knee extension strength than HA individuals.

In high-speed (180 °/s) sports, muscles primarily rely on the recruitment of fast-twitch muscle fibers (type II fibers) to generate force rapidly. These fibers are suited for short-duration, high-intensity activities such as sprinting and weightlifting (62, 63). In our study on the peak knee moments during rapid (180 °/s) motion, NA individuals exhibited the highest flexion and extension moments, followed by LA, with HA individuals showing the lowest values. This finding aligns with the work of Guenka et al. (23), who reported reduced isokinetic moment in the ankle joint among HA and LA individuals relative to NA individuals. Although their study focused on ankle musculature, the consistency in outcomes suggests a potential mechanistic link—specifically, that LA and HA individuals may possess a lower proportion of type II fibers. Reduced type II fiber content may decreases the muscle's capacity to produce maximal force in a short period, negatively impacting athletic performance (64, 65). Additionally, athletes with lower type II fiber content experience slower recovery after high-intensity exercise, increasing injury risk (66). To mitigate injury risk in LA and HA individuals during high-speed activities, several studies have emphasized the importance of targeted fast muscle resistance training to enhance the recruitment and function of fast-twitch fibers (63, 67, 68).

While the present study offers valuable insights into the relationship between arch height and knee isokinetic muscle strength at different angular velocities, several limitations should be acknowledged. First, the exclusive inclusion of young adults (20–30 years) may limit the extrapolation of findings to other age demographics, suggesting the need for broader age representation in future studies. Second, the singular focus on isokinetic muscle strength assessment precluded complementary evaluations of neuromuscular activation (via electromyography) and metabolic responses, which could offer more comprehensive mechanistic understanding. Third, the absence of participant exercise habits data represents an uncontrolled variable in our analysis. Fourth, the unilateral assessment protocol may constrain the generalizability of bilateral lower limb function inferences. Fifth, the use of a fixed testing order may introduce potential order effects. Sixth, although torque data from non-isokinetic phases were analytically excluded, the dynamometer may still struggle to maintain the preset speed perfectly within extremely short angular ranges, potentially leading to minor velocity fluctuations. Future studies could employ equipment with higher velocity-sampling precision or utilize velocity spectrum analysis to control this variable more rigorously. While our primary objective focused on examining knee flexion and extension muscle function across different foot arch types, subsequent research would benefit from incorporating bilateral measurements alongside integrated electrophysiological and metabolic assessments to more thoroughly characterize these functional relationships.

In summary, our study demonstrates that arch height is associated with knee muscle strength under varying exercise speeds. Specifically, at 60 °/s, individuals with LA exhibit greater muscle strength in the knee extension and the posterior half of the knee flexion range. Conversely, at 180 °/s, both LA and HA individuals display diminished knee muscle strength. These findings suggest that individuals with non-normal arch heights may benefit from incorporating targeted high-speed knee exercise training to enhance muscle performance and mitigate injury risk in competitive sports. Future studies will focus on the arch characteristics of patients with sports-related knee injuries and further explore the influence of arch height on knee function in this population.