en Internal Internal Original Article Original Article <span style="font-size:14px;"><span style="font-family:Times New Roman;"><span style="line-height:14pt"><b><i><span style="color:blue">Background</span></i></b><i><span style="color:blue">:</span></i> Knee osteoarthritis (OA) is one of the major causes of dysfunction that reduces patients&rsquo; quality of life. The aim of current study was to investigate the effectiveness of muscle strengthening exercises<b> </b>on the clinical outcomes of patients with knee OA.</span><br> <span style="line-height:14pt"><b><i><span style="color:blue">Methods:</span></i></b> We conducted a single-blind clinical trial on 96 patients with mild to moderate knee osteoarthritis. Patients were randomly assigned to 4 groups with 24 patients and followed-up for 8 weeks. Quadriceps, hamstring and both hamstring and quadriceps strengthening exercises were received by the first (G1), second (G2) and third (G3) groups, respectively. The fourth group (G4) was the control group and did not receive any intervention. The main outcome variables were pain, physical function and morning stiffness. This study was retrospectively registered at irct.ir (Iranian Registry of Clinical Trials) with the code IRCT20220206053950N2, 2022-09-07<span style="font-size:10.0pt"></span></span><br> <span style="line-height:14pt"><b><i><span style="color:blue">Results:</span></i></b> Our findings demonstrated a significant impact of interventions on VAS score, pain, and stiffness (All, p < 0.001). Nonetheless, the effect of interventions was not significant for physical function (P = 0.78). After adjusting for the potential confounders, similar results were observed. Besides, the results of Dunnett post hoc test showed that either G1 or G3 had the most decrease in VAS and WOMAC-morning stiffness scores compared to G2 and G4.<span style="font-size:10.0pt"></span></span><br> <span style="line-height:14pt"><b><i><span style="color:blue">Conclusion</span></i></b><i><span style="color:blue">:</span></i> Based on the evidence from the present study, we can recommend a combination of quadriceps and hamstring strengthening exercises as the most effective intervention to reduce pain and morning stiffness in patients with OA.</span></span></span><br> &nbsp; Key words: Knee osteoarthritis, Exercise therapy, Quadriceps muscle, Hamstring muscle 433 442 http://caspjim.com/browse.php?a_code=A-10-2502-1&slc_lang=en&sid=1 Alireza Sadeghi rsadeghi@zums.ac.ir 100319475328460042974 100319475328460042974 Yes Department of Internal Medicine, Vali-e-Asr Hospital, School of Medicine, Zanjan University of Medical Sciences, Zanjan, Iran Mina Rostami mina.rostami1373@gmail.com 100319475328460042975 100319475328460042975 No Social Determinants of Health Research Center, Zanjan University of Medical Sciences, Zanjan, Iran Zakiye Khanlari Zakiye.khanlari@gmail.com 100319475328460042976 100319475328460042976 No Department of Internal Medicine, Vali-e-Asr Hospital, School of Medicine, Zanjan University of Medical Sciences, Zanjan, Iran Alireza Zeraatchi dr.a.zeraatchi@zums.ac.ir 100319475328460042977 100319475328460042977 No Department of Emergency Medicine, School of Medicine, Valiasr-e-Asr Hospital, Ayatollah Mousavi Hospital, Zanjan University of Medical Sciences, Zanjan, Iran. Nooshin Jalili dr.nooshinjalili@zums.ac.ir 100319475328460042978 100319475328460042978 No Department of Internal Medicine, Vali-e-Asr Hospital, School of Medicine, Zanjan University of Medical Sciences, Zanjan, Iran Arezoo Karimimoghaddam arezookarimimoghaddam@zums.ac.ir 100319475328460042979 100319475328460042979 No ِDepartment of Ophthalmology, School of Medicine,Vali-e-Asr Hospital, Zanjan University of Medical Sciences Zhaleh Karimimoghaddam zhkarimi@zums.ac.ir 100319475328460042980 100319475328460042980 No Department of Radiation Oncology, Vali-e-Asr Hospital, School of Medicine, Zanjan University of Medical Sciences, Zanjan, Iran Ramazan Fallah rfv@zums.ac.ir 100319475328460042981 100319475328460042981 No Department of Biostatistics and Epidemiology, School of Medicine, Zanjan University of Medical Sciences, Zanjan, Iran Fateme Sangtarash ftm.sangtarash@gmail.com 100319475328460042982 100319475328460042982 No Rehabilation Faculty, Shahid Beheshti University of Medical Sciences, Tehran, Iran