In our study, no statistically significant difference was observed in terms of the ROM of the ankle between the two groups; however, statistically significant differences were noted in terms of isokinetic muscle strength parameters. This difference was more prominent in the PF muscle groups than in the other groups.
Venous diseases are progressive diseases that are frequently observed in the lower extremities, and such diseases can occur in various forms, ranging from telangiectasias that cause cosmetic problems to painful varicose veins and even skin ulcers that develop due to severe venous insufficiency . According to the Edinburgh Vein Study data, CVI is observed in 9% of males and 7% of females, and it increases with age and is not correlated with social status . On the basis of the result of the 13-year follow-up of the same study, the incidence rates of CVI were 10.6% in males and 8.1% in females .
Progression of the disease is not associated with age, sex, or whether the disease is in the right or left leg. However, CVI will progress in the extremity initially diagnosed with venous reflux, and it is more commonly observed in ageing females . Overweight, obesity, and history of 4 or more pregnancies are risk factors for the development of C2 varicose vein and C3–C6 CVI. However, smoking, use of oral contraceptive medications, hormone replacement treatment, and active work life are not correlated with the incidence of the disease . In our study, female dominance was observed in the sex distribution of patients with C3 CVI. Furthermore, similar to those in other studies, our study groups comprised overweight and middle-aged individuals.
Normal venous return depends on the venous valves and the strength of the muscles surrounding the veins. Muscle pump dysfunction and a decrease in lower extremity muscle strength play important roles in the pathophysiology of CVI .
CVI classification is extremely important in terms of clinical follow-up and patients’ treatment. PPG is an effective method for determining the presence and severity of venous hypertension and CVI classification. It is advantageous in terms of diagnosis and staging compared to other methods as it provides quantitative data about muscle pump measurement and is affordable, non-invasive, easy to apply, and portable [19,20].
Isokinetic muscle strength measurement systems have high reliability and accuracy in determining muscle strength and torque value. Isokinetic dynamometers using the load at a maximal level at all points along the ROM of a joint are preferred by users in rehabilitation and in dynamic muscle test applications . During these measurements, the ROM of the joint for which the test will be performed is important for the standardization of the isokinetic muscle strength .
In the literature, different researchers have used various diagnostic and measurement methods to demonstrate venous pump function and calf muscle insufficiency [18,21–23]. In this study, we preferred the use of PPG to measure calf muscle pump function and isokinetic systems to measure muscle strength.
The severity of the problems in muscle pump function and calf muscle dysfunction is correlated to the stage of the disease, and as the disease progresses particularly in patients with ulcer, a decrease in the muscle pump function, ROM of the joint, and muscle strength is observed [18,24].
Araki et al.  have classified patients who were clinically diagnosed with CVI as non-ulcerated and with healed ulcer or active ulcer and have examined the calf muscle pump functions using air plethysmography. Results of their study showed that the venous refilling index was not normal and those with active ulcer had a lower ejection fraction and a high residual volume fraction. In conclusion, venous insufficiency played a role, but not sufficient, in the development of ulcer, and the venous ulceration level was significantly associated with calf muscle pump insufficiency.
According to the CEAP classification, Dix et al.  have reported that upon selecting all groups in CEAP0–6 as cases, the ROM of the joint in CEAP4,5 and CEAP6 in both the PF and DF directions was correlated to the stage of the disease. The PF and DF ROM for CEAP0, CEAP2, CEAP4,5, and CEAP6 were 47° and 14.4°; 39.5° and 10.2°; 32.9° and 9.2°; and 37.5° and 3.4°, respectively.
The calf muscles are affected in patients diagnosed with CVI. Qiao et al.  have performed muscle biopsy to show the effects of having affected calf muscles on these patients. Thus, myofibril structure was found to be affected by venous hypertension in the gastrocnemius muscle.
Van Uden et al.  have found that gait and endurance of the calf muscle were lower even if it had been healed or had an active ulcer, on the basis of evaluation of the functional status of the calf muscles of patients with CVI using the heel rise test and gait parameters. In the study of Heinen et al.  conducted on patients with a venous ulcer, 35% of the patients were found incapable of walking for more than 10 minutes due to calf muscle insufficiency.
Yang et al. ’s study that included a group of patients with CVI (n=49) and a control group has shown statistically significant differences between the groups in terms of ankle isokinetic PF muscle strength, PT/BW, and TWD. Cetin et al. ’s study has compared the group of patients with CVI and the control group and has found that the calf and thigh muscle strengths of patients diagnosed with non-ulcerated CVI were weak. Upon performing PPG and CVI staging, they reported that the ankle PF muscle strength decreased and the visual analog scale pain score increased as the disease stage progressed. A muscle ratio of more than 33% for the ankle DF/PF obtained from the isokinetic muscle strength measurements and strength imbalance between muscles were found to be correlated to the stage of CVI.
De Moura et al.  have selected the CVI case group diagnosed with CEAP4–6 in their studies and have compared the gait speed, functional capacity, PF and DF ROM of ankle joint, and isokinetic muscle strength measurements of such group to those of the control group. As a result, the ROM of the joint of patients diagnosed with CVI was more limited, the isokinetic PF PT/BW and PF strength results were lower, and the social restriction amount was higher. Upon subgrouping the patients according to the CEAP classification, no difference was observed in terms of the results of the DF ROM of joint and isokinetic PF PT/BW and PF strength; however, it was observed that the limitation in the PF ROM of the joint results increased as the stage of the disease increased.