Incline walking on a treadmill results in less frontal plane knee loading and more gluteus, hamstrings, quadriceps, and triceps surae muscle activation than level walking, and therefore may benefit patients with knee osteoarthritis or those who have undergone knee replacement.
By Henry Wang, PhD, Mason Haggerty, MS, Clark Dickin, PhD, and Jennifer Popps, PhD
Walking is a daily activity and a common exercise for all age groups, and can provide many health benefits, including a reduced risk of type 2 diabetes, cardiovascular disease, and obesity.1 Other benefits include strengthening the muscles of the quadriceps, hamstrings, and triceps surae.2 However, despite its benefits, fitness walking also involves risk factors, such as increased frontal plane knee loading. Over time, the repetitive nature of fitness walking can expose the knee joint to an increased cyclic mechanical loading.
Large frontal plane knee loads, represented in many biomechanical studies by high abduction moments, could lead to cartilage degeneration.3-5 Research has determined that 41% of the adult population has a varus knee alignment, which can further contribute to elevated internal knee abduction moments during daily activities and increase the risk of developing knee osteoarthritis (OA).6 This suggests that individuals with a varus knee alignment may experience high medial knee loading when participating in level fitness walking, and that practitioners should consider alternate exercise programs for adults with varus knee alignment. Activities such as swimming and cycling have been promoted for adult fitness, and incline walking is also considered an alternative exercise prescription.
Incline treadmill walking is a popular form of exercise used frequently for fitness and rehabilitation purposes.7 Incline walking is associated with a higher metabolic rate, which can result in faster calorie consumption and burning of fat tissue, than level walking over a shorter time.8,9 Research has also shown that incline walking demands more muscle activation from the quadriceps, hamstrings, and triceps surae than level walking.8-13 These increases in muscle activation could be beneficial in strengthening the lower limb muscles. For example, Lange et al suggested that incline walking with a gradient above 12%, which increases quadriceps and hamstrings activation, could be beneficial for knee rehabilitation.10
Given these potential benefits associated with an alternative means of promoting cardio- vascular health, incline walking has been integrated into exercise prescriptions for obese individuals and rehabilitation protocols after knee surgery, including total knee replacement (TKR).7,8 In some cases, patients with TKR are prescribed incline treadmill walking five to eight weeks after surgery.7 However, these patient populations also are often the focus of efforts to reduce knee loading because of the risk or existence of knee OA.
Despite the popularity of incline walking, there is limited research available that has described its effect on knee joint mechanics. Haight et al reported that slow incline walking resulted in smaller compressive tibiofemoral forces than fast level walking.14 Ehlen et al also found that slow incline walking reduced external knee adduction moment compared with faster level walking.8 It is still unclear whether incline walking is associated with large frontal plane knee joint loading when walking speed is at a fitness walking pace, defined by the American College of Sports Medicine as 1.34 m/s.1
The internal knee abduction moment has been a good indicator of mechanical loading placed on the medial knee compartment,4 and researchers have reported that a large abduction moment is associated with an increase in the rate of development of medial tibiofemoral OA.15 Thus, a large knee abduction moment is considered a risk factor for medial knee OA.5,6,15 It is essential to examine frontal plane knee loading during incline walking and determine if it is suitable for knee surgery patients and obese and older adults for achieving fitness while reducing joint loading, OA risk, or damage to an existing TKR.
To address this information gap, we conducted a study to quantify frontal plane knee moment during incline walking. We hypothesized that the frontal plane knee moment would decrease as the treadmill gradient increased. We also hypothesized the existence of a dose-response relationship between the frontal plane knee moment and treadmill gradient.
We recruited 15 healthy male volunteers (mean age, 20.80 ± 1.78 years; weight, 77.96 ± 10.32 kg; height, 1.79 ± .07 m; body mass index, 24.23 ± 2.07 kg/m2) from Ball State University in Muncie, IN. The participants had no neuromuscular or lower extremity conditions that could prevent them from level or gradient walking; no previous knee surgery; and no lower extremity injuries within the past two years. In addition, we examined participants’ knee alignment using motion-capture data and found that all had well-aligned knee joints. The Ball State University Institutional Review Board approved the study, and all participants provided written informed consent.
Participants came to the Ball State Biomechanics Laboratory for data collection. All wore a compression shirt, compression shorts, and the same type of athletic shoe. We took anthropometric measurements, including height, weight, leg length, knee and ankle width, and distance between anterior superior iliac spines. For motion analysis, 14-mm reflective markers were placed bilaterally on the following locations: the manubrium, sternum, acromio-clavicular joint, anterior superior iliac spine, posterior superior iliac spine, mid-thigh marker, four-marker thigh cluster, lateral and medial epicondyles of the knee, midshank marker, four-marker shank cluster, lateral and medial malleoli of the ankle, second metatarsal head, fifth metatarsal head, and the calcaneus.
Once the participants were prepped, they completed a three-minute warm-up on the treadmill at 1.34 m/s and then began the walking trials. The men walked at 1.34 m/s on a tandem dual-belt force-instrumented treadmill at gradients of 0%, 5%, 10%, 15%, and 20% in random order. We chose these gradients because they fall within the range of those used in previous studies and because having consistent increments between gradients (ie, 5%) is helpful for identifying positive and inverse relationships between knee mechanics and gradient.8,12,13,16-20
The participants walked in the center of the treadmill for three minutes at each gradient, and data collection began after the one-minute mark. This allowed the men to become comfortable walking on the treadmill and establish their normal gait pattern. During the final two minutes, we collected five 10-second trials. The marker trajectories were recorded using 14 cameras sampling at 120 Hz. The two force plates on the force-instrumented treadmill sampled at 1200 Hz. Data collection for the specific gradient concluded once the participants had walked for three minutes at that gradient. A five-minute break was given before the next walking trial began, and this protocol was repeated for each of the remaining trials.8
We used motion-analysis software to reconstruct and label the motion-capture trials and to perform link-model based calculations. Hip and knee joint centers were defined using a functional joint procedure.21 Net joint moments were calculated using standard inverse dynamics equations,22 and all data were normalized to body mass. The 3D motion data were filtered at 8 Hz, and the ground reaction force data was low-pass filtered using a cutoff frequency of 40 Hz. Frontal plane knee moment was the dependent variable.
Significant differences between the five walking conditions were tested using an analysis of variance (ANOVA) with repeated measures and a p-value of less than or equal to .05 was used. A Bonferroni correction was used for the pairwise comparisons.
The peak abduction moments for the 0%, 5%, 10%, 15%, and 20% gradients were .54 ± .15, .50 ± .15, .46 ± .16, .42 ± .18, and .37 ± .18 Nm/kg, respectively. The peak abduction moments for the 10%, 15%, and 20% gradients were significantly lower than the peak abduction moment for the 0% gradient (p < .05). A dose-response relationship between the peak knee abduction moment and treadmill gradient existed in 10% gradient increments. The peak knee abduction moment significantly decreased from the 0% to 10% gradient (p = .001), from the 5% to 15% gradient (p = .037), and from the 10% to 20% gradient (p = .015). Figure 1 displays the differences in the peak knee abduction moments for various levels of inclined walking.
Incline walking is a popular form of exercise used frequently for exercise and rehabilitation purposes due its health benefits.7-11,16 However, there is minimal research available that describes the effect of incline walking on knee joint mechanics, and it is unclear whether the exercise is associated with increased joint loading.
We hypothesized that the frontal plane knee moment would decrease as the treadmill gradient increased, and this hypothesis was partially accepted. The peak knee abduction moment significantly decreased from level walking at all gradients except 5%.
Previous research showed that incline walking requires greater knee extensor moment to overcome the elevated slope.12,13 Increased knee extension moment is accompanied by increased knee extensor (quadriceps) activation. In this study, as the treadmill slope increased, the difficulty of maintaining a constant walking speed increased, which was signified by greater knee extension moments and greater quadriceps activity (these data have not yet been analyzed in detail).
The initial double support phase during stance is when the body weight shifts from the trailing leg to the leading leg. It is possible that participants moved their upper bodies over the leading legs quickly so that the center of mass of the body could be positioned right above the leading knee upon heel strike, reducing the external knee flexion moment while maintaining a constant walking speed. As the center of mass of the body is positioned above the knee of the leading leg, the ground reaction force vector is shifted toward the center of the knee joint, which reduces the moment arm of the ground reaction force with respect to the knee center. For the given ground reaction forces encountered, the frontal plane knee moment is reduced due to a shortened moment arm. In this study, the decreases in the knee abduction moment suggest that increases in treadmill gradient are associated with decreased medial knee loading.
During exercise individuals with varus knee alignment or with knee replacements should avoid high loading in the medial compartment of the knees to lengthen the lives of their cartilage or TKR components. Incline walking can strengthen leg muscles while introducing less joint load to the medial knee. Thus, it can be an ideal exercise for these individuals.
We also hypothesized that a dose-response relationship between the frontal plane knee moment and treadmill gradient would exist. This hypothesis was accepted. Specifically, the peak knee abduction moment significantly decreased when the treadmill incline was raised in 10% increments. This important finding can provide guidance for the development of rehabilitation and exercise protocols using incline walking. It offers information on how to safely adjust the treadmill grade when designing rehabilitation and exercise prescriptions.
As incline walking promotes cardiovascular function and strengthens leg muscle groups, clinicians and practitioners can develop exercise prescriptions integrating it into programs for patients who need to regain knee function and strength. Incline walking programs can be initiated with small gradients such as 5% to accommodate patients’ ability to walk. As leg muscle strength improves, clinicians can introduce steeper gradients in 5% to 10% increments. Based on the dose-response relationship between the incline gradients and frontal-plane knee joint loading, patients participating in such incline walking programs should experience a continuing reduction in frontal plane knee loading as the incline gradient increases. Thus, patients can improve their cardiovascular health and develop their leg strength through incline walking exercise while having minimal concerns on the negative effect from high frontal plane knee loading.
Treadmill gradients above 5% significantly decreased frontal plane knee loading compared with level walking values. Also, a dose-response relationship between the frontal plane knee moment and treadmill gradients existed in gradient increments of 10%. Treadmill gradients of 10% to 15% or greater are recommended for rehabilitation and exercise protocols. This is in agreement with Lange et al, who suggested using a treadmill gradient just greater than 12% for knee rehabilitation purposes.10 Using a treadmill gradient in this recommended range would result in reduced frontal plane joint loading at the knee and greater use of the gluteus, hamstrings, quadriceps, and triceps surae muscles. This recommendation allows an individual to walk safely on an inclined treadmill and increase muscle activation of the lower limb for rehabilitation or exercise goals. The dose-response relationship findings may help clinicians create appropriate rehabilitation and exercise protocols.
Henry Wang, PhD, is an assistant professor of exercise science at Ball State University in Muncie, IN, and conducts research in biomechanics. Mason Haggerty, MS, is a sales representative with Stryker Orthopedics in the Washington, DC, area, and was formerly a graduate assistant in the Ball State University Biomechanics Laboratory. Clark Dickin, PhD, is an assistant professor of exercise science at Ball State University and conducts research in biomechanics and motor control. Jennifer Popps, PhD, is an assistant professor of athletic training at Ball State University and conducts research in athletic training.
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