Effects of Squat Exercise Variations on Lower Limb Biomechanics and Functional Outcomes: Implications for Physiotherapy and Rehabilitation: A Systematic Review

Objectives: To consolidate the evidence regarding the effects of squat exercise variations on lower limb muscle activation, joint kinematics, joint kinetics, force-related outcomes, and functional performance, and to examine their implications for physical therapy, physiotherapy, and rehabilitation. Methods: The study conformed to the PRISMA 2020 guidelines. Before the data extraction process started, the protocol for this review was registered in the PROSPERO database (CRD420261390860). A search strategy was developed to systematically review six electronic databases: PubMed/MEDLINE, Embase, Scopus, Web of Science Core Collection, CINAHL Ul-timate, and SPORTDiscus, from inception to 1 April 2026. All articles published in English reporting results of randomized controlled trials and cross-sectional comparative studies involving squat exercise variations in humans were considered eligible. The extracted data pertained to lower limb muscular activation, joint movement, forces, torques, and functional outcomes. Risk of bias was assessed with RoB 2 and a customized appraisal informed by AXIS and JBI, and certainty of evidence was assessed with GRADE. Given the considerable heterogeneity among squat exercises, testing protocols, measurement techniques, and outcome variables, the evidence was synthesized narratively using a structured direction-of-effect approach. Results: Twenty-four studies were included: 23 acute cross-sectional comparative investigations, predominantly laboratory-based studies, and one randomized controlled trial; no study recruited a clinical population. The study sample primarily comprised healthy male adults; however, older adults and females were underrepresented. In relation to the synthesis of qualitative literature, deeper squats were associated with greater joint excursion and knee extensor demands, whereas gluteus maximus activation findings varied. Stance width altered hip and knee demands in a protocol-dependent manner. While front squats showed muscle activity similar to back squats, they had a more upright trunk posture and, in one study, less knee compression. Unstable and unilateral squat exercises increased activation of hip stabilization muscles but were accompanied by lower maximal force output. The certainty of evidence was very low for all outcome domains. Conclusions: Very low-certainty evidence suggests that squat exercise variations are associated with differences in muscular demands, joint motion, joint loading, and functional performance. Depth, stance, bar placement, stability, and fatigue appear to alter the mechanical characteristics of the exercise differently. These findings may inform individualized squat selection in physical therapy, physiotherapy, and rehabilitation, but longitudinal trials in clinical populations are needed before specific variations can be recommended.