1. Introduction
Foot drop, prevalent in various neurological disorders, significantly heightens the risk of falls and diminishes individuals’ physical independence [
1,
2,
3,
4]. This condition, marked by a decreased capacity to dorsiflex the foot during walking, results in a dragging or slapping motion of the foot against the ground. Foot drop is typically attributed to weakness in ankle dorsiflexors or over-activation in ankle plantar flexors [
3]. Consequently, individuals affected by foot drop often exhibit slower walking speeds and encounter challenges navigating uneven terrain like stairs [
2,
3,
4]. Compensatory mechanisms, such as hyper-flexion of the knee and hip joints (e.g., “steppage gait” and “hip hiking”), can lead to improper skeletal loading and injury over time [
5,
6]. Furthermore, a foot drop may restrict activities of daily living, contributing to a decline in functional independence and overall quality of life [
1].
Foot drops can stem from various neurological conditions affecting the central or peripheral nervous systems, including stroke, multiple sclerosis, cerebral palsy, spinal stenosis, peripheral neuropathy, Charcot–Marie–Tooth disease, or localized compression of the peroneal nerve [
7]. Traumatic events such as spinal cord injuries, fibular fractures, or knee dislocations can also lead to foot drop [
8]. Additionally, it may develop due to surgical positioning or prolonged habits like sitting with legs crossed [
2]. Although the incidence of foot drop is not precisely documented, it is evident that it affects diverse populations. Given the varied causes and affected populations, it is important to explore a range of treatment options to accommodate individuals with different levels of mobility.
Common treatment options include nerve decompression surgery, ankle–foot orthoses, and functional electrical stimulation [
2,
3,
4]. Surgical interventions are invasive and primarily target peripheral causes like peroneal nerve compression or direct trauma (e.g., fracture, dislocation, etc.) [
2]. While ankle–foot orthoses effectively enhance ambulatory function, they may limit ankle mobility, potentially causing discomfort and muscle contracture over time [
9,
10]. In contrast, functional electrical stimulation enables individuals with foot drops to move the affected ankle through its entire range of motion by activating the tibialis anterior (TA), the primary muscle responsible for dorsiflexion, using external electrical stimuli applied to the peroneal nerve. Research indicates that functional electrical stimulation has an immediate “orthotic effect” on walking performance, increasing toe clearance and walking speed while the device is in use [
3,
9,
10]. Moreover, it may have lasting effects on TA excitability by strengthening corticospinal connections [
11]. However, functional electrical stimulation poses challenges like difficulty in device application and skin irritation from electrodes [
12,
13]. Further research is needed to determine if voluntary control of the TA can be improved without resorting to surgery or uncomfortable assistive devices.
Gait training stands out as a promising non-invasive approach to enhance dorsiflexor function. In a study by Willerslev-Olsen et al. [
14], children with cerebral palsy exhibited a significant increase in maximal voluntary dorsiflexion torque and toe lift at the end of the swing phase after daily incline treadmill walking for 30 days. The authors proposed that uphill walking required greater voluntary activation of the TA during the swing phase, facilitating toe lift in preparation for foot strike. The gait training also induced significant shifts in corticospinal drive, evident from increased intramuscular coherence in the beta and gamma frequency bands recorded from the TA, which may have contributed to the observed improvements in gait mechanics. A similar study in spinal cord injury patients using transcranial magnetic stimulation found increased TA excitability after a 16-week, body weight-supported treadmill walking intervention [
15]. Moreover, improvements in corticospinal tract function exhibited a significant positive correlation with improvements in walking function, as assessed by the Walking Index for Spinal Cord Injury II (WISCI II). While the precise adaptation mechanism remains unclear, the authors propose that rigorous daily treadmill training involving voluntary activation of leg muscles may enhance neural control of the TA and improve toe clearance during the swing phase of walking.
For individuals with limited mobility, engaging in intensive treadmill training on land may be impractical [
16,
17]. Conversely, aquatic treadmill walking is considered a safe alternative that enhances movement confidence and accommodates various levels of functional capacity [
18,
19]. Water immersion introduces an upward buoyant force, offloading body weight and reducing the impact of potential falls [
20]. Additionally, a review focused on individuals with Parkinson’s disease noted a decrease in fear of falling during exercise performed in an aquatic environment [
21]. Aside from Parkinson’s disease, prior research supports aquatic exercise as an effective intervention for addressing motor symptoms in individuals with various other neurological diseases, including stroke, multiple sclerosis, dementia, and cerebral palsy [
22]. In essence, water provides a secure environment for individuals with limited mobility to exercise with increased confidence.
Moreover, the heightened drag forces associated with walking in water may demand increased voluntary activation of the TA compared to walking on land. In both aquatic and dry land environments, fluid drag forces act on the body to impede movement. However, the density of the fluid, in this case, water, significantly influences the magnitude of the fluid drag force. Given the higher density of water compared to air, an individual experiences greater resistance to motion in water than on land [
23]. Additionally, fluid drag force intensifies with the relative velocity of the object in the fluid. During the swing phase of the walking stride cycle, the foot attains a higher linear velocity relative to more proximal lower-extremity body segments, akin to the tip of a windmill. This elevated velocity increases fluid drag, particularly at the anterior foot and ankle. Consequently, walking on an aquatic treadmill should necessitate greater TA activation to overcome heightened drag forces at the distal lower limb and lift the toes before foot strikes. Further, as indicated by Willerslev-Olsen et al. [
14], the repetitive activation of lower-limb muscles during exercises that demand heightened TA activation may enhance neural control of the ankle by strengthening corticospinal connections.
Results from previous studies examining differences in TA muscle activation between aquatic and land treadmill gaits are mixed. For instance, in studies where walking in water was performed at slower speeds compared to land or at self-selected speeds, no significant differences in peak or average TA activation have been reported [
24,
25,
26,
27]. In a study by So et al. [
28] investigating running gait while controlling for stride rate across participants, significantly greater TA activation was observed for running in water than on land, with deeper water immersion depths also leading to greater activation. However, it is noteworthy that a significant difference was observed only in the left leg, with no differences noted in the right leg. Moreover, Silvers et al. [
29] observed an increase in the absolute duration of activity and total TA activation between water and land running at matched speeds. However, the study did not find a significant difference in TA activation when expressed as a percentage of maximal voluntary contraction between the two environments. It is important to note that in the study, muscle activation measures were derived by averaging data from 10 full stride cycles without separately analyzing the swing and stance phases. Additionally, it is important to consider that findings from investigations focusing on muscle activation during running may not directly apply to walking gait. For instance, during the swing phase of running, the knee and hip exhibit greater flexion to minimize the moment arm of the leg, facilitating a faster swing phase. This altered posture may lead to reduced drag forces opposing dorsiflexion, which contrasts with the more fully extended leg position typically observed during the swing phase of walking.
The observation of greater TA activation during running at matched stride rates and speeds does, however, reflect a current need for investigation into TA activation during walking performed in water and on land at matched speeds. Moreover, there is a scarcity of research on plantar flexor (i.e., gastrocnemius) activation, and, to our knowledge, no prior study has provided insights into muscle co-activation (Co-A) between the TA and medial gastrocnemius (GM) during walking in water. Improving the ability to dorsiflex the foot involves either enhancing TA activation or minimizing GM Co-A. Consequently, it is crucial to assess the activation of both muscles. To our knowledge, no study has compared distal leg muscle activation patterns during both the stance and swing phases of gait between aquatic and land treadmill walking. This represents a notable gap in the current literature.
The purpose of this study was to compare stride kinematics and surface electromyography (sEMG) of the TA and GM in young adults performing aquatic and land treadmill walking at matched speeds. We hypothesized that (1) TA activation would be greater when walking in water than on land at matched speeds, attributed to increased fluid drag opposing the forward movement of the distal lower limb. We anticipated (2) no significant differences in the magnitude of TA activation between walking environments during the stance phase at matched speeds. Additionally, we hypothesized that (3) GM activation would be greater during aquatic walking than on land during the stance phase. This expectation stemmed from the assumption that the GM would play a more substantial role in propelling the body forward in the presence of increased fluid drag. However, we also anticipated (4) no significant difference in GM activation between environments during the swing phase. Given the relationship between relative velocity and fluid drag, we further hypothesized that (5) observed differences in muscle activation magnitude between aquatic and land walking would be more pronounced at the 3.5 mph speed condition than the 2.5 mph speed condition. The findings of this study may offer valuable insights into how the aquatic environment influences the activation of muscles spanning the ankle joint during walking and provide supporting evidence for including aquatic exercise as a component of treatment for foot drop.
4. Discussion
This study aimed to investigate lower-limb muscle activation during walking gaits performed by young adults in water versus on land. Our hypothesis that TA activation would be greater during the swing phase of aquatic treadmill walking compared to land treadmill walking at matched speeds was confirmed. Additionally, we hypothesized that the difference in TA activation between aquatic treadmill walking and land treadmill walking would be more pronounced at 3.5 mph compared to 2.5 mph, which was also confirmed. We observed a significant 15% increase in TA RMS sEMG during the swing phase while walking in water compared to walking on land at 2.5 mph (Cohen’s d = 0.42). The increase in TA RMS sEMG was considerably more pronounced for walking in water compared to on land at 3.5 mph (123%; Cohen’s d = 1.13). We also observed meaningful effects on swing phase TA activation between water walking conditions. For instance, TA RMS sEMG increased 94% (Cohen’s d = 1.77) during walking in the water at 3.5 mph compared to 2.5 mph. Additionally, TA RMS sEMG significantly increased by 52% (Cohen’s d = 1.11) for walking in water at 3.5 mph performed with 75% jet resistance compared to without jet resistance.
Indeed, our findings regarding the heightened TA activation during the swing phase of water walking compared to land walking are compatible with the effects of fluid drag. Fluid drag is known to have a linear relationship with fluid density and a quadratic relationship with relative speed. In our study, the water temperature was carefully regulated and maintained at approximately 29.5 °C, resulting in a water density estimate of 995–996 kg/m
3 [
36]. This density is approximately 800 times greater than that of ambient air [
37]. In addition, by increasing the walking speed in water or by adding jet resistance, fluid drag (water’s resistance to the motion of the foot) increases quadratically, further amplifying the muscle activation necessary to maintain the position of the foot [
23].
Our findings indicate that TA activation during water walking is modulated by either increasing the walking speed or by introducing jet resistance opposing the direction of the walking gait. We observed an increase in stride rate both on land and in water when walking speed was raised to 3.5 mph compared to 2.5 mph (+15%, Cohen’s d = 1.40), which coincided with decreases in both stance and swing time, particularly in water. These findings align with the notion that increased lower limb velocity was accompanied by increased fluid drag, attributable to the quadratic relationship between the velocity of the foot relative to the surrounding water. Stride rate also increased for the 3.5 mph water walking condition when jet resistance was added; however, this increase appeared to be largely due to a reduction in stance time as there was no significant difference in swing time between the 3.5 mph water walking and 3.5 mph water walking with added jet resistance. This indicates that the application of jets led to greater motor drive as participants exerted more effort to maintain a stationary walking position on the aquatic treadmill. Anecdotally, many participants self-reported localized fatigue in the TA after completing the 3.5 mph water walking condition with jets, which supports the observed increase in TA activation.
Our hypothesis regarding greater GM activation in water compared to land during the stance phase of walking gait was not supported by the findings of this study. GM RMS sEMG was lower during the stance phase of water walking compared to land walking, with a more substantial effect observed at the matched 2.5 mph speed condition (+52%; Cohen’s d = 1.82) relative to the 3.5 mph speed condition (+24%; Cohen’s d = 0.65). While we expected the GM to play a larger role in propelling the body forward in the presence of increased fluid drag, the findings suggest that the buoyancy effect of water immersion, which offloads body weight, reduces GM activation during stance. Interestingly, as fluid drag increases with walking speed, differences in GM activation between water and land walking decrease. We also hypothesized that there would be no significant difference in GM activation during the swing phase of walking in water compared to on land. This hypothesis was partially supported, with no significant difference observed between water and land walking at the matched 3.5 mph speed condition. Curiously, however, GM RMS sEMG was significantly reduced for the 2.5 mph water walking condition relative to all other conditions. This observation may reflect a differential balance of contributions from buoyancy and fluid drag expressed at different walking speeds in water.
While our study did not specifically hypothesize about Co-A indices, we did observe that the Co-A of the GM during the swing phase was greater on land compared to in water, irrespective of the matched speed conditions at 2.5 mph (56%; Cohen’s d = 0.88) and 3.5 mph (37%; Cohen’s d = 0.70). Swing phase Co-A of the GM was influenced by a combination of increased TA RMS sEMG across both speed conditions and reduced GM RMS sEMG for the environment-matched 2.5 mph speed condition. This suggests the potential that net ankle torque is shifted toward dorsiflexion when walking is performed in water compared to on land at 2.5 mph. We also observed that the Co-A of the TA during the stance phase was greater in water compared to on land (44%; Cohen’s d = 1.05), a finding that was explained by reduced GM activation, given that there was no significant difference in TA activation between land and water conditions.
Collectively, the medium-to-large effects observed in TA and GM activation during water walking compared to land support the notion of accentuated TA activation when walking is performed in water. However, it is important to contextualize these findings within the existing body of research. Prior studies have not observed significant differences in TA activation between land and water walking performed at self-selected speeds [
24,
25]. This lack of disparity may be attributed to participants’ tendency to opt for slower walking speeds in water [
24]. Consequently, mismatched speeds resulting in a greater speed of walking on land, paired with a reduction in fluid drag in the water, likely contribute to the absence of a significant effect of the environment on TA activation [
24,
25,
26,
27].
To target TA activation through aquatic treadmill walking exercise, it is important to optimize walking intensity through variables such as speed, water resistance application, and water depth. Moreover, further research is warranted to determine whether TA activation magnitude, duration, or a combination of both has a greater impact on longitudinal biomechanical and neural adaptations. While our study focused on RMS sEMG, future studies could explore muscle activation in terms of absolute duration and total activation [
38]. Furthermore, it is essential to customize optimal walking conditions for specific populations, especially those with neurological or mobility limitations. Further research is needed to explore the optimal combination of walking speed, water resistance, and water depth to maximize TA activation and toe lift during the swing phase while also considering movement constraints. This is particularly relevant when extrapolating our study findings to populations with foot drops. For example, although our study introduced jet resistance only at 3.5 mph, future investigations should examine its implementation at slower speeds and with a sufficient immersion depth [
28], which may better suit individuals with limited mobility or gait impairments. Given the observed influence of walking speed and jet resistance on the effect sizes, clinical populations such as those with foot drops may walk at a slower pace. Increasing the jet resistance may be an important consideration when extrapolating the findings of the current study to individuals with foot drops. It is also reasonable to assume that individuals with foot drop experience more drag opposing dorsiflexion during the swing phase, as foot drop results in a greater projected area. This increase in drag could enhance TA activation during the swing phase, especially if individuals with foot drop are unable to walk at a 2.5 mph pace in water due to their limitations. It is also important for future research to consider the water immersion depth, with chest-deep immersion recommended, as increased water depth has been observed to enhance the effect of fluid resistance on muscle activation during gait performed in water [
28].
Several study limitations need to be acknowledged. Firstly, a convenient sample of healthy, recreationally active adults was chosen, limiting the generalizability of the findings to clinical populations. Future research should include individuals with conditions commonly associated with foot drop, such as cerebral palsy, multiple sclerosis, or spinal cord injuries, to assess the consistency of cross-sectional findings and the potential effectiveness of aquatic treadmill walking in reducing foot drop. Additionally, while increased TA activation during the swing phase in water was observed, this study did not establish a causal link between increased TA activation and biomechanical or neural adaptations in dorsiflexor function. Future studies should investigate the effects of aquatic treadmill interventions on TA excitability, voluntary dorsiflexor torque, and toe lift/clearance while walking, as demonstrated in previous research [
14,
15]. Lastly, it is important to consider the limitations of sEMG when interpreting the findings. Internal noise and movement artifacts stemming from factors like fat, skin, and muscle can directly influence sEMG recordings, potentially impacting the quality of the signal collection.