All participants read and signed an informed consent form approved by the university institutional review board. All diagnoses were made by a local rheumatologist and were confirmed for definite OA based on a diagnostic algorithm.¹⁷ Additionally, participants had to be over 35 years of age, able to walk a city block (without the use of an ambulatory assistive device), and walk up stairs in a reciprocal manner. Participants were excluded if they currently exercised on an aquatic treadmill, had intra-articular corticosteroid injections in the past month, or reported any neuromuscular disease, cardiovascular disorders, surgeries to the lower limb (except for exploratory arthroscopy), lavage of knee joint, partial meniscetomy, or synovial fluid replacement therapy at least 1 year prior to entry into the study.

This study used a quasi-experimental crossover design to address the study purpose. Each participant completed 3 exercise sessions on an aquatic treadmill (fig 1; HydroWorx 2000^a) and on a land treadmill (Nordic Track 9600^b). The 3 exercise sessions in each condition were separated by at least 24 hours and completed within 1 week. The order of exercise mode (aquatic treadmill, land treadmill) was randomly assigned and separated by 1 rest week (table 1).

Fig 1

Experimental setup for the aquatic treadmill mode.

 

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The amount of walking for each exercise bout was 20 minutes and consisted of four 5-minutes stages at 0° incline (see fig 1). The first stage required participants to walk at a self-selected pace they considered comfortable. The second stage was .13m/s (.30 miles/h) faster than the self-selected pace, and the third stage was .26m/s (.60 miles/h) faster than the self-selected pace. During the fourth stage, speed was matched to the first stage (fig 2). These increments in speed were intended to achieve a moderate to somewhat hard rating of perceived exertion, as evidenced in previous work using aquatic treadmills.⁸ Participants performed the aquatic treadmill exercise at a water depth equal to the xiphoid process. The temperature of the water was 30°C (86°F)⁶ with air temperature set at 24°C (75.2°F). The same protocol was completed for the land treadmill exercise in the same room.

Fig 2

Average walking speeds at different stages for land and water conditions.

 

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Fourteen participants who responded to the request for volunteers met the inclusion and exclusion criteria. Physical characteristics and arthritis history for the participants are reported in table 2.

Pairwise comparisons of the self-selected speeds during exercise indicated they were not different between aquatic (.76±.24m/s) and land (.80±.26m/s) treadmill exercise (P=.13) (see fig 2). The descriptive results from the arthritis history questionnaire revealed that, on average, the amount of time between diagnosis and testing in our laboratory was 7.9±6.7 years and that the knee was the primary arthritic joint (see table 1).

Perceived pain was reduced by 100% after aquatic treadmill exercise compared with land treadmill exercise (P=.02) (table 5).

Self-selected speeds during aquatic and land treadmill exercise were not different (see fig 2). Barela and Duarte²² observed a difference in self-selected speeds during walking in a pool (.49m/s) versus walking on land (1.2m/s) in a group of healthy older adults. The disparity between studies is likely related to the different modes of exercise used. Walking on an aquatic treadmill, as performed in the present study, does not require the entire body to be displaced in the water, only the body segments required to move. In contrast, walking in a pool requires the entire body plus segments to be moved. Displacement or movement of the body produces an extra nonlinear increase in fluid drag²³ and a decrease in walking speed. Numerous aquatic treadmill studies have reported similar energy requirements during matched walking speeds in water and on land,^{24, 25, 26, 27} and 1 previous study has reported similar preferred walking speeds between environments,⁸ which supports the results of the current study.

It is also important to note that the current study's participants were all diagnosed with OA (a population that typically exhibits slower walking speeds on land),^{5, 14} while the Barela and Duarte study²² used a healthy, older adult population free from any musculoskeletal disease. Furthermore, Barela and Duarte²² observed that when walking in water at the level of the xiphoid process, older adults displayed a 14.3% decrease of the initial peak and a 9.8% decrease in the second peak vertical ground reaction force when compared with walking on land. Additionally, impact force values were 54% lower compared with walking on land. The reduction of these forces when walking underwater is likely attributed to buoyancy; and as a result, walking on an aquatic treadmill may prevent larger (more harmful) impact forces on the joints of individuals with OA.²² Because of abnormal joint loads and pain associated with OA, our participants may have had a more difficult time walking on a land than water treadmill. The findings of the current study may further support the use of an aquatic treadmill for improving exercise quality and quantity for patients with OA. Our gait kinematic values were consistent with previous research measuring joint kinematics of patients with OA.^{11, 13, 28} For example, Walker et al¹¹ reported a knee extension angle of 177°, which is consistent with our knee extension values (171°). Huang et al²⁹ reported a hip extension angle during stance of 170°, which is also consistent with our hip extension values (165°).

The results of this study support our hypothesis that some kinematic differences would occur after aquatic exercise. Hip, knee, and ankle kinematics were affected over the course of the acute training periods with most changes observed to the left limb (see table 4). It may be that we observed more changes to the left limb compared with the right because there were more subjects with left-sided OA than right-sided OA (see table 1). For example, angular velocity gain scores for left knee extension during stance, left knee internal rotation during swing, and left knee extension during swing were significantly greater after aquatic exercise versus land exercise. Similarly, angular velocity gain scores for left ankle abduction during stance and right hip flexion during swing were also greater after aquatic exercise compared with land exercise. The increases in angular velocity are important, because the values may be closer to those of a healthy population (maximum knee angular velocity during swing = 403°/s)³⁰ and may show evidence of improvements in the functional patterns of use of the knee during walking. We observed maximal knee angular velocity during swing after aquatic exercise to be 337°/s. These findings suggest aquatic therapy may be more beneficial than traditional land-based exercise for improving the angular velocity of a joint, particularly the knee, hip, or ankle, which may be critical for restoring gait quality and preventing falls. Although improving dynamic balance was not a main goal of the current study, tripping over obstacles is the chief cause of falls among older adults, and increasing angular velocity of the knee is important when avoiding loss of balance and/or falling.³¹

The mechanism for increasing angular velocity could be because of the unique environment of the aquatic exercise; there is an increased resistance to movement as a result of the drag force exerted by water against the segments of the body.²² The aquatic environment, in comparison with the terrestrial environment, could perhaps strengthen the neuromuscular aspects that affect lower-limb kinematics to a greater degree, in an effort to overcome this increase in fluid resistance that influences joint angular velocity. Clinically, these benefits of underwater treadmill exercise (ie, the warmth and pressure of the water and unloading of the joints) could be used to improve lower-limb range of motion and angular velocity of patients with OA.

Although hip flexion gain scores were higher for land exercise, we believe this result may indicate that compensatory deviations at the hip may take place to overcome limited range of motion at the knee and ankle after exercising on land.³² Participants may adopt an exaggerated hip flexion pattern when walking to overcome problems caused by the disease. This contention requires further testing using a more complete biomechanical analysis that includes joint torques and powers.

Although we hypothesized that step length and step rate would be altered after aquatic exercise, our results did not support these changes. Despite significant differences found in joint velocity and angle gain scores between land and aquatic exercises, no differences were detected between step rate and step length. In a similarly designed study, Denning et al⁸ observed that mobility, based on Timed Up & Go scores, improved after an acute bout of aquatic training as opposed to land training, but they also found no differences between stride rate and stride length (P=.16). These similar findings could infer that neuromuscular aspects of the body and balance improved in the short time period, rather than walking speed and step length. It should be noted that the present study was not designed to encourage walking speed. The participants were asked to walk at their own comfortable speed; had the study also included another condition in which participants were asked to walk as fast as possible, there may have been changes observed in step rate and step length.

The results of this study support our hypothesis that patients with OA of the knee might have less arthritis-related joint pain by training on an aquatic treadmill as opposed to a land-based treadmill. Knee pain relief may allow completion of exercise regimens that would enable patients with OA to increase the length and quality of workouts. Moreover, this improvement in higher aerobic training could result in higher cardiovascular fitness and improved physical conditioning. Our results are consistent with Denning,⁸ who observed that perceived joint pain was less immediately after aquatic treadmill versus land treadmill exercise. Others, such as Huang,²⁹ Patrick,³³ and colleagues did not observe changes in pain after aquatic exercise. This disagreement among studies may be because of differences in several factors, such as how and when an assessment was administered, type of assessment, and type of aquatic exercise (aquatic treadmill versus aquatic exercises targeted at upper- and lower-body movements). The current study and the study conducted by Denning⁸ targeted specific pain only related to knee OA, while others examined general pain. While it is beyond the scope of this study to ascertain the exact mechanism for the reduction in pain, previous authors have concluded that the benefits may be related to buoyancy, warmth, and pressure of the water.