Knee OA is expected to be the fourth highest cause of disability in women and is responsible for the deterioration of quality of life and functional capacity
. A plethora of studies have investigated several aspects related to muscle function, such as strength
 and aerobic capacity
 as well as other clinical aspects such as pain
 and WOMAC index
 in patients with OA. Despite these important advances, to our knowledge, few studies have investigated the effects of different types of non-pharmacological treatments on the functional exercise capacity of patients with OA. In this context, the 6-MWT is an excellent tool to evaluate the effect of therapy on the functional exercise capacity. In this study, we found that the KIN and US procedures improved the functional exercise capacity of patients with bilateral knee OA after the intervention period; however, we found no inter-group differences. Moreover, we also evaluated the effect of the treatment period on pain using the VAS and WOMAC index, and we found that the three interventions improved the pain. The difference in this study is that our sample is homogeneous because we recruited only women with bilateral knee OA.
Pain is one of the most common complaints and disabling symptoms in OA populations. In the present study, we evaluated the efficacy of different treatment modes on knee pain, measured using the VAS and the pain dimension of the WOMAC index. We found that pain in both knees decreased in all the experimental groups. This is not the first study to demonstrate the positive effects of non-pharmacologic management on knee pain in OA patients. The Cochrane group
 systematically reviewed and combined the study results of 17 OA exercise studies (a total of 2562 participants). This group found that land-based exercise had a small-to-moderate beneficial effect on pain for people with symptomatic knee OA. Roddy et al.
 reviewed 19 randomized clinical trials investigating the effects of land-based exercise for knee or hip OA. They concluded that both strengthening and aerobic exercises performed on land could reduce pain and improve the function and health status in patients with knee and hip OA. However, these authors stated that there was not enough evidence to support or recommend specific types of exercise.
Concerning the TENS, Rutjes et al.
 conducted a systematic Cochrane review of transcutaneous electrostimulation vs. sham or no specific intervention on pain in individuals with knee OA. This systematic review found little evidence of a significant effect for electrostimulation compared to sham or no intervention on pain in knee OA. The authors attributed these results to the poor quality of the trials and the high degree of heterogeneity across the studies. Our results contradict this systematic review because we found an improvement in the pain index (VAS and pain dimension of the WOMAC index) in all the experimental groups. To evaluate the therapeutic effect of the TENS modalities, NG et al.
 studied 24 patients and compared electroacupuncture treatment and TENS, using the same parameters for both (low frequency - 2 Hz, continuous mode, pulsation of 200 μs for 20 min of application, and a control group with only educational orientations on OA of the knee) and showed that either electroacupuncture treatment or TENS are effective in pain reduction because a prolonged analgesic effect was maintained in the two groups. Another study was performed with 62 patients between 50 and 75 years of age and presenting knee OA during a four-week period. These patients were divided into four treatment groups: TENS placebo group, TENS group, exercise group and TENS plus exercise group. The results showed no significance between the different types of treatment due to the protocol duration
. This treatment was similar to ours in the number of patients and the modalities used, such as the conventional TENS and the isometric exercises. However, the TENS parameters and the application time were different and, unlike our study, did not present significance in the protocols due to the short treatment duration. In our study, the three groups (KIN, TENS and US) showed significant differences after the treatment duration.
In regards to the US, Loyola-Sanchez et al.
 conducted a meta-analysis of the efficacy of US for decreasing pain and improving physical function in people with knee OA. New evidence was found that shows that US can reduce pain by 21% compared to a control group.
Range of motion was another variable evaluated in the present study. We did not observe any difference due to the three modes of treatment used in this study. Our results agree with Tascioglu et al.
 who compared the effectiveness of ultrasound (continuous versus pulsed) therapy versus placebo ultrasound in patients with knee OA and also found no differences in ROM. These authors also found improvements in the WOMAC index and functional capacity as evaluated by a 20-m walking test.
Finally, we were particular interested in evaluating the effect of KIN, US and TENS treatment on the 6-MWT performance of woman with bilateral knee OA. Several modalities are available for the objective evaluation of cardiorespiratory fitness. Some provide a very complete assessment of all the systems involved in exercise performance, whereas others provide basic information but are low-tech and easy to perform. The 6-MWT is a simple test that requires a hallway but no equipment or advanced training for the technicians. This test evaluates the global and integrated responses of all the systems involved during exercise, including the pulmonary, cardiovascular and muscular systems
. To help predict the total distance walked during the 6-MWT, Enright and Sherrill
 established a reference equation that incorporates subject characteristics such as age, body mass and height. These subject characteristics were shown to be associated with the distance walked during the 6-MWT. When applying this reference equation to the current data, the results revealed that the KIN, US and TENS groups walked 74%, 79% and 85%, respectively, of the predicted values found by the Enright and Sherrill
 equation in the pre-evaluation. These modest values demonstrate the low functional exercise capacity, and consequently low health status, of the patients evaluated in the present study. On average, our patients walked 328.8 m before the treatment. These values agree with Wang et al.
 who compared the efficacy of aquatic exercises and land-based exercises for patients with knee OA. However, these values are lower than those reported by French et al.
 (405.1 m). The difference most likely results from the poorer physical condition of our volunteers, as represented in the lower highest total score obtained for the WOMAC index compared with that in the study by French et al.
 Additionally, the sample studied by French et al.
 contained male participants with unilateral and bilateral knee OA. The difference in this study is that our sample is homogeneous because we recruited only women with bilateral knee OA.
The impact of health status on 6-MWT performance was investigated in 165 elderly people. The covered distance decreased significantly with increasing age and with worsening health status (corrected for age)
. Patients with dilated cardiomyopathy were also investigated, and the results demonstrated that the covered distance and peak oxygen uptake (cardiorespiratory fitness index) were closely correlated
. In addition, the authors found a correlation between the 6-MWT covered distance and the New York Heart Association functional class. Santana et al.
 showed that in the healthy elderly, the 6-MWT can be used to evaluate improvements in functional exercise capacity after exercise training. However, the 6-MWT is not appropriate to evaluate improvements in the cardiorespiratory fitness of elderly healthy men who have undergone exercise training because this test lacks sufficient sensitivity. Particularly in OA, French et al.
 studied the responsiveness of three physical performance measures of function following physiotherapy for knee OA and found that the 6-MWT was more responsive for the assessment of physical performance than the timed-up-and-go test and the timed-stand test.
Following 12 weeks of treatment procedures performed by the KIN and US groups, the distance covered in the 6-MWT increased by 19.8% and 14.1%, respectively. These improvements in functional exercise capacity indicate improvements in muscle strength and aerobic metabolism assuming that patients with knee OA are often physically deconditioned, interventions, as performed by current study, potentiate those muscle adaptations. Wang et al.
 investigated the effects of aquatic exercises and land-based exercises for patients with knee OA and found that the 6-MWT performance increased by 19 ± 7% and 12 ± 5%, respectively. These changes were similar to the results found in previous studies
[45, 48]. Although these articles have studied different treatment modes, the results presented here suggest an improvement in functional exercise capacity and, consequently, of the quality of life and ability to perform activities of daily living. In fact, this assertion is supported by the positive results on the WOMAC and VAS scores. This improvement in ability to perform physical effort is very important because physical exercise is considered a valuable tool to reduce the risk of cardiovascular and endocrine diseases and to improve bone and muscle conditioning. These medical conditions may affect patients with OA due to the high level of inactivity and body disuse found in these patients. Indeed, this high level of inactivity can be demonstrated by the reduced aerobic capacity in patients with severe hip and knee OA compared to controls
We assessed study outcomes only on pre- and post-tests, so we were not able to determine the outcomes of these interventions across time. The evaluation of parameters related to exercise physiology, such as the maximal oxygen uptake, economy of motion and anaerobic threshold, could provide additional information on the level of aerobic fitness of the subjects before and after the treatment period.