Let’s dive into why moment arms may be important to understand in correlation to hypertrophy. 

First, what is a moment arm? A moment arm is considered the perpendicular distance between the axis of rotation (joint) and the line of force (muscle or external resistance). It determines how much torque (rotational force) a muscle must produce to move a joint. To better understand this, let me break down why understanding this will matter when it comes to resistance training.

• Longer moment arms require a greater torque force in order to move the given weight in question. 
• Shorter moment arms require less torque. 
• The longer a moment arm on the concentrated area or target muscle, the higher the mechanical tension.
• Allow for optimized exercise selection to help choose activities with longer moments arms. 
• Movements can be more difficult when they are longest. 
• Examples: Knees traveling forward in a sissy squats increases the moment arm in the squat, causing the quads to work harder. When the hip complex travels father back into the posterior chain (pulling the femur towards & knees with it) the moment arm for the quads decreases, while the moment arm for the glutes & hamstrings increases. 

So, why are moment arms considered important to understand for hypertrophy training? 

First things first, moment arms can influence mechanical tension. Mechanical tension is the primary driver of muscle growth. I went further into detail about this in 2 of the blog posts on my website. 

“The Connection Between Time Under Tension & Hypertrophy”

https://www.cptlij.com/blog?id=67d247cc15cd4e7a11afb4bd

Mechanical Stress/Tension

https://www.cptlij.com/blog?id=67c9164f15cd4e7a11afb4ba

Increasing muscle tension is largely crucial for stimulating hypertrophy. Not only is the mechanical tension important due to the muscle fibers growth response to sustained tension, but here are some other reason as well. 

• Progressive overload drives the process of adaptation: Without progressively increasing mechanical load, the stimulus becomes insufficient, leading to plateaus in growth.
• Ability to maximize motor unit recruitment: Forced recruitment of high-threshold motor units.
• Micro trauma @ the physiological level that allows for muscle remodeling. 
• Training under mechanical load conditions increases muscle endurance and force production, making the muscle more resistant to fatigue over time.

What are some of the other reasons why it is important to understand moment arms in resistance training?

• Better muscle concentration: Adjusting moment arms efficiently can help shift workload onto the intended area, boosting concentrated activation on the target muscles. 
• Joint stress: Optimizing moment arms helps ensure that muscles bear the load/stress, not primarily the joints. 
• Resistance profiles: Some exercises have the ability to naturally increase or decrease moment arms thought movements. 

Understand resistance profiles play a crucial role in hypertrophy due to the fact that it helps with the ability to match the resistance curve of an exercise to the strength curve of a muscle. This will inevitably maximize mechanical tension through the movement. Ensure consistent mechanical tension, align with strength curves, improve controlled contractions, improve the ability to apply time under tension efficiently for mechanics purposes, reduce strength gaps + plateaus, & enhance exercise selection. 

Peer review articles that support the information above:

Influence of Muscle Hypertrophy on the Moment Arm of the Triceps Brachii Muscle

https://journals.humankinetics.com/view/journals/jab/31/2/article-p111.xml?utm_

Influence of muscle hypertrophy on the moment arm of the triceps brachii muscle

https://pubmed.ncbi.nlm.nih.gov/25411895/

Biomechanical implications of skeletal muscle hypertrophy and atrophy: a musculoskeletal model

https://pubmed.ncbi.nlm.nih.gov/26644989/

The moment arms of the muscles spanning the glenohumeral joint: a systematic review

https://pmc.ncbi.nlm.nih.gov/articles/PMC6284439/

Muscle moment-arms: a key element in muscle-force estimation

https://pubmed.ncbi.nlm.nih.gov/23998280/

Practical applications of biomechanical principles in resistance training: Moments and moment arms

https://www.researchgate.net/publication/260791576_Practical_applications_of_biomechanical_principles_in_resistance_training_Moments_and_moment_arms

Physiology of Stretch-Mediated Hypertrophy and Strength Increases: A Narrative Review

https://link.springer.com/article/10.1007/s40279-023-01898-x

Mechanisms of mechanical overload-induced skeletal muscle hypertrophy: current understanding and future directions

https://journals.physiology.org/doi/full/10.1152/physrev.00039.2022

Longer moment arm results in smaller joint moment development, power and work outputs in fast motions

https://www.researchgate.net/publication/9069911_Longer_moment_arm_results_in_smaller_joint_moment_development_power_and_work_outputs_in_fast_motions?utm_

What is a Moment Arm? Calculating Muscle Effectiveness in Biomechanical Models Using Generalized Coordinates

https://www.researchgate.net/publication/267491173_What_is_a_Moment_Arm_Calculating_Muscle_Effectiveness_in_Biomechanical_Models_Using_Generalized_Coordinates?utm_

The moment arms and leverage of the human finger muscles

https://www.sciencedirect.com/science/article/abs/pii/S0021929020306047?utm_

Muscle Moment Arm–Joint Angle Relations in the Hip, Knee, and Ankle: A Visualization of Datasets

https://www.biorxiv.org/content/10.1101/2024.06.22.600198v1.full

Sexual Dimorphism Impact on the Ground Reaction Force Acting on the Mediolateral Direction During Level Walking: Hip Abductor Muscle Biomechanics and Its Correlation to GRF Moment Arm

https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2022.863194/full

Effects of range of motion on muscle development during resistance training interventions: A systematic review

https://pmc.ncbi.nlm.nih.gov/articles/PMC6977096/

The influence of variable resistance moment arm on knee extensor performance

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://hal.univ-reunion.fr/hal-01232338v1/document

Moment arm length variations of selected muscles acting on talocrural and subtalar joints during movement: An In vitro study

https://www.sciencedirect.com/science/article/abs/pii/0021929095000259

Optimizing Resistance Training Technique to Maximize Muscle Hypertrophy: A Narrative Review

https://www.mdpi.com/2411-5142/9/1/9

Anatomical and Neuromuscular Determinants of Strength Change in Previously Untrained Men Following Heavy Strength Training

https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2019.01001/full

Biomechanics of the knee-extension exercise. Effect of cutting the anterior cruciate ligament.

https://journals.lww.com/jbjsjournal/abstract/1984/66050/biomechanics_of_the_knee_extension_exercise_.11.aspx

Muscle Hypertrophy Response to Range of Motion in Strength Training: A Novel Approach to Understanding the Findings

https://www.researchgate.net/publication/362864671_Muscle_Hypertrophy_Response_to_Range_of_Motion_in_Strength_Training_A_Novel_Approach_to_Understanding_the_Findings

Effects of variable resistance training versus conventional resistance training on muscle hypertrophy: a systematic review

https://link.springer.com/article/10.1007/s11332-023-01103-6

Resistance training prescription for muscle strength and hypertrophy in healthy adults: a systematic review and Bayesian network meta-analysis

https://bjsm.bmj.com/content/57/18/1211

Resistance Training Variables for Optimization of Muscle Hypertrophy: An Umbrella Review

https://www.frontiersin.org/journals/sports-and-active-living/articles/10.3389/fspor.2022.949021/full

Does Varying Resistance Exercises Promote Superior Muscle Hypertrophy and Strength Gains? A Systematic Review

https://www.researchgate.net/publication/358212528_Does_Varying_Resistance_Exercises_Promote_Superior_Muscle_Hypertrophy_and_Strength_Gains_A_Systematic_Review

Muscle hypertrophy and muscle strength: dependent or independent variables? A provocative review

https://pmc.ncbi.nlm.nih.gov/articles/PMC7582410/

An Evidence-Based Narrative Review of Mechanisms of Resistance Exercise–Induced Human Skeletal Muscle Hypertrophy

https://pmc.ncbi.nlm.nih.gov/articles/PMC9390238/

Higher resistance training volume offsets muscle hypertrophy nonresponsiveness in older individuals

https://journals.physiology.org/doi/full/10.1152/japplphysiol.00670.2023

Comparing the effects of variable and traditional resistance training on maximal strength and muscle power in healthy adults: A systematic review and meta-analysis

https://www.sciencedirect.com/science/article/pii/S1440244022002201