Perform Ep 28: Identify & Fix Muscle Imbalances & Injury Risks

In this episode, I tackle muscle asymmetries and imbalances — when the differences between your two sides help you, when they hurt you, and what to actually do about them. I move past the popular "10% rule" and walk through my three I's framework: how to investigate asymmetries across morphology (muscle size and shape), quality (fat infiltration inside the muscle), and functionality (strength, power, and movement); how to interpret the data using context rather than blanket numbers; and how to intervene with a practical five-step correction program built around unilateral training and plyometrics. I cover testing tools from tape measures to MRI, the research on grip-strength asymmetry and aging, and why lower-body asymmetries warrant more attention than upper-body ones. This is for athletes, coaches, and anyone who moves and wants to reduce injury risk or perform at their best.

Scientific Articles

• A Comprehensive Examination of Age-Related Lower Limb Muscle Function Asymmetries across a Variety of Muscle Action Types (Geriatrics)
• Unilateral Plyometric Training Effectively Reduces Lower Limb Asymmetry in Athletes: A Meta-Analysis (Frontiers in Physiology)
• The Symmetry of Children’s Knees Is Linked to Their Adult Sprinting Speed and Their Willingness to Sprint in a Long-Term Jamaican Study (PLOS ONE)
• Lower Body Symmetry and Running Performance in Elite Jamaican Track and Field Athletes (PLOS ONE)
• Development of Soft Tissue Asymmetry Indicators to Characterize Aging and Functional Mobility (Frontiers in Bioengineering and Biotechnology)
• An Observational Study of Lower Limb Muscle Imbalance Assessment and Gait Analysis of Badminton Players (Frontiers in Bioengineering and Biotechnology)
• Force, Power, and Morphology Asymmetries as Injury Risk Factors in Physically Active Men and Women (Symmetry)
• The Combined Additive Effect of Inter-Limb Muscle Mass Asymmetries and Body Composition Indices on Lower Limb Injuries in Physically Active Young Adults (Symmetry)
• The Calculation, Thresholds and Reporting of Inter-Limb Strength Asymmetry: A Systematic Review (Journal of Sports Science and Medicine)
• Morphological Asymmetries Profile and the Difference between Low- and High-Performing Road Cyclists Using 3D Scanning (Biology)
• Relationship between Body Composition Asymmetry and Specific Performance in Taekwondo Athletes: A Cross-Sectional Study (Symmetry)
• Effects of Inter-Limb Asymmetries on Physical and Sports Performance: A Systematic Review (Journal of Sports Sciences)
• Morphological Inter-Limb Asymmetry in Youth Judokas Is Independent of Competitive Level and Sex (BMC Sports Science, Medicine and Rehabilitation)
• Inter-Limb Asymmetries Are Associated with Decrements in Physical Performance in Youth Elite Team Sports Athletes (PLOS ONE)
• Vertical and Horizontal Asymmetries Are Related to Slower Sprinting and Jump Performance in Elite Youth Female Soccer Players (Journal of Strength and Conditioning Research)
• Quantitative Assessment of Functional Limitations in Normal and Anterior Cruciate Ligament-Deficient Knees (Clinical Orthopaedics and Related Research)
• Gait Variability and Symmetry in World-Class Senior and Junior Race Walkers (Journal of Sports Sciences)

Books

• The Sports Gene by David Epstein
• Assess and Correct by Eric Cressey and Mike Robertson

Tools & Technologies

• DEXA scan
• Tape measure and goniometer
• Ultrasound / handheld ultrasound
• Springbok Analytics
• Functional Movement Screen (FMS)
• VALD force plates / dynamometers
• Isokinetic dynamometer
• Electromyography (EMG)
• Star Excursion Balance Test (SEBT) / Y Balance Test
• Hand-grip dynamometer
• Sit-and-reach test

People Mentioned

• David Epstein: science journalist and author who writes on athletic performance and talent development
• Jill Miller: fascia, mobility, and self-care fitness expert; founder of Tune Up Fitness
• Mike Boyle: strength and conditioning coach known for functional training and the bilateral force deficit
• Doug Larson: strength coach and Barbell Shrugged co-host
• Dr. Greg Rose: co-founder of the Titleist Performance Institute and movement-screening expert
• Eric Cressey: strength coach specializing in athletic performance and arm care; co-creator of “Assess and Correct”
• Mike Robertson: strength coach and co-creator of “Assess and Correct”
• Dr. Charlie Weingroff: physical therapist and strength coach focused on movement and rehab
• Dr. Kelly Starrett: physical therapist and mobility specialist; co-founder of The Ready State
• Aaron Alexander: movement coach and creator of the Align Method
• Dr. Ryan McGrath: researcher at North Dakota State University who runs The Hand Lab studying grip-strength asymmetry
• Bill Bowerman: legendary track coach and Nike co-founder
• Leonardo da Vinci: Renaissance artist whose Vitruvian Man represents idealized body symmetry