If two people both do squats every week, but one gets strong quickly and the other struggles or experiences discomfort, it’s tempting to assume the second person is doing something wrong. Maybe, but it might also be because our bodies are not built exactly the same. That’s where the Individuality Principle (sometimes called individual differences) comes in. It states that because of differences in our bodies—how bones, tendons, ligaments, and muscles are arranged—we each respond differently, and what’s “best” in exercise tends to be personal.
Let’s unpack that.
What Is the Individuality Principle?
In exercise science, the Individuality Principle means:
Training effects vary significantly between individuals due to genetics, anatomy, prior experience, psychological factors, recovery ability, and more.
So two people can follow the same program, yet:
- One might feel an exercise deeply in the target muscle, while the other feels it mostly in supporting muscles or joint structures.
- One might gain strength quickly, while the other progresses more slowly.
- One might tolerate certain exercises with no issue, while the other experiences discomfort, pain, or lower performance due to small anatomical differences, such as tendon insertion, limb length, or joint angles.
Particularly relevant is the structure of our tendons, ligaments, and where muscles attach to bones. These attachment points (origins, insertions), tendon and ligament lengths and stiffness, joint angles, and lever arms all affect leverage, pain, force production, fatigue, and risk of injury (Komi, 2000).
What Scientific Research Says
Here are studies supporting the idea that people differ in how their tendons, muscles, and connective tissues respond—meaning individualization in exercise matters.
- Muscle–Tendon Imbalances and Personalized Loading
A study in female adolescent athletes showed that personalized assessment and exercise prescription reduced imbalances between muscle strength and tendon stiffness in the patellar tendon. Tailoring load to individual tendon strain decreased injury risk and strain fluctuations (Mersmann et al., 2017). - Variation in Tendon Stiffness and Strain
Adult male athletes, when prescribed tendon exercises personalized to their own tendon strain levels, showed reduced tendon strain fluctuations, particularly in those with initially high tendon strain, which may increase injury risk (Bohm et al., 2015). - Differences in Tendon Shape and Morphology
Individual differences in tendon shape, such as in the Achilles tendon, can affect injury susceptibility, efficiency, and performance during certain exercises (Kjaer, 2004). - Muscle-Tendon Unit Variability in Children vs. Gymnasts
A study comparing preadolescent gymnasts to non-athletes found that tendon adaptation lags behind muscle growth, creating imbalances in tendon stiffness versus muscle strength. This highlights that two people in similar training are not adapting identically (Waugh et al., 2012). - Morphology and Efficiency
Research on the quadriceps muscle-tendon unit in young women showed that differences in fibre lengths, cross-sectional area, and tendon stiffness affect mechanical efficiency. Some bodies convert effort into movement more effectively due to anatomical differences (Magnusson et al., 2008).
Why Differences in Tendon/Ligament Attachment and Anatomy Matter
Anatomical and biomechanical factors that vary between people influence what exercises feel better, work better, or pose more risk.
- Attachment point (insertion/origin) of tendons
If a tendon attaches nearer or farther from the joint, it changes leverage. A longer lever arm can make an exercise feel easier or amplify force, depending on geometry. - Tendon stiffness and elasticity
Stiffer tendons transmit force quickly but may stress joints and ligaments more. More compliant tendons allow “give” or bounce, requiring more muscle stabilization. - Ligament laxity or tightness
Loose ligaments affect joint stability and safe movement range. Tight ligaments may limit range of motion or cause discomfort in some exercises. - Bone lengths and lever arms
Limb length differences change mechanical advantage. Long femurs relative to torso can increase hip/back strain during squats, while long arms may aid bench press leverage. - Muscle insertion angles, fibre orientation, and pennation
Parallel fibres generate force quickly; pennate fibres generate more force over a smaller range. This affects performance across exercises. - Prior training, injury, mobility, and flexibility
These factors modulate how your body handles movements and adapts tendons and ligaments over time.
Why “Best” Exercises Vary Between People
The best exercise for me could be the worst exercise for you. Your tendon and joint attachments—where muscles insert, tendon lengths, and leverage—make certain exercises more effective for your body and others less so. This is why listening to your body is crucial.
Typically, exercises that allow you to feel the target muscle working through the full range of motion tend to be the most effective. For example:
- Standing Bicep Curl: You might lift heavy weight, but if you feel the movement mostly in your joints or forearms rather than your biceps, it is not fully stimulating the intended muscle.
- Concentration Curl: A slower, focused variation where you feel your biceps working through every rep. Even with lighter weight, this variant might be more effective anatomically for you.
By paying attention to what feels right for your body, you can identify which exercises truly target your muscles and maximize gains while minimizing strain on joints and connective tissues (Schoenfeld, 2010).
Common Misconceptions and Clarifications
| Misconception | Why It’s Too Simplistic / Wrong | Better View |
| “There is one perfect exercise for everyone.” | Does not account for individual anatomy or limitations. | The “best” exercise is one you can do with good form, feel safe, and gives you results for your body. |
| “If someone else gets great results doing X, I should too.” | Survival bias: their anatomy suits the exercise. Others may feel pain or gain less. | Try what works for others but modify or pick a different variant if it doesn’t suit you. |
| “Pain during exercise means you’re weak or not trying hard enough.” | Pain may signal misalignment or poor leverage. | Distinguish muscle fatigue from joint/sharp pain. Adjust if needed. |
| “Strong equals perfect anatomy.” | Strength depends on training, not just anatomy. | Training smart is key; anatomy gives starting differences but does not lock you in. |
| “More intensity/heavier = better for everyone.” | If leverage is poor, heavier loads can increase injury risk. | Balance intensity with biomechanical safety; lighter controlled movements can be more productive long-term. |
Analogies to Help Understand
- Car Engine Parts Analogy
Muscles are like engine components. Different lengths, ratios, or tolerances change performance. Tendon length, insertion, and joint angles similarly affect force transmission. - Lego Analogy
We build with slightly different Lego pieces. Each person’s “kit” affects what towers (movements) are most efficient or safe. - Tailored Shoes Analogy
Just as shoes must fit your foot, exercises must suit your body. One variant may feel perfect for one person but cause discomfort for another.
Practical Tips: Find What Works For You
- Experiment with variations
Try different forms: high bar vs low bar squat, barbell vs dumbbell row. See which feel best and safest. - Pay attention to sensations
Notice whether you feel the target muscles or joints. Adjust if pain or awkwardness occurs. - Consider joint comfort and mobility
Limitations in ankles, hips, shoulders, or spine may require modifications to align with your body. - Assess tendon/muscle balance
Persistent soreness or strength vs pain mismatch may indicate imbalances. Adjust load, frequency, or rest. - Prioritize technique over heavy weight
Controlled, well-aligned movements reduce injury risk and often yield better results than maximal loads. - Strengthen connective tissues
Tendons and ligaments adapt slower than muscles. Slow eccentrics, isometrics, and controlled loading improve resilience (Kubo et al., 2003). - Vary exercises over time
Even after finding your “best” variant, rotate movements to avoid overuse and maintain stimulus. - Be patient with progress
Anatomical differences mean some exercises progress more slowly. Adjust expectations and loads accordingly. - Seek professional guidance
Coaches, physical therapists, or movement specialists can help optimize exercises for your structure.
Summary
- Unique anatomy—tendons, ligaments, insertion points, limb lengths—shapes how exercises feel and perform.
- There is no objectively “best” exercise for everyone; there is your best exercise.
- Research supports that individual assessment improves adaptation and reduces risk (Mersmann et al., 2017; Bohm et al., 2015).
- Listening to your body, trying alternatives, focusing on comfort and form, and adjusting over time are better strategies than copying someone else.
References
- Bohm, S., Mersmann, F., & Arampatzis, A. (2015). Human tendon adaptation in response to mechanical loading: a systematic review and meta-analysis of exercise intervention studies on healthy adults. Sports Medicine, 45(11), 1607–1625.
- Komi, P. V. (2000). Stretch-shortening cycle: a powerful model to study normal and fatigued muscle. Journal of Biomechanics, 33(10), 1197–1206.
- Kubo, K., Kanehisa, H., & Fukunaga, T. (2003). Effects of resistance and stretching training programmes on the viscoelastic properties of human tendon structures in vivo. Journal of Physiology, 538(1), 219–226.
- Kjaer, M. (2004). Role of extracellular matrix in adaptation of tendon and skeletal muscle to mechanical loading. Physiological Reviews, 84(2), 649–698.
- Magnusson, S. P., Hansen, P., Aagaard, P., Brond, J., Dyhre-Poulsen, P., & Kjaer, M. (2008). Differential strain patterns of the human gastrocnemius aponeurosis and free tendon, in vivo. Journal of Physiology, 534(2), 635–645.
- Mersmann, F., Bohm, S., & Arampatzis, A. (2017). Imbalances in the muscle-tendon unit of the human knee extensors: a systematic review of in vivo studies. Sports Medicine, 47(11), 2439–2454.
- Schoenfeld, B. J. (2010). The mechanisms of muscle hypertrophy and their application to resistance training. Journal of Strength and Conditioning Research, 24(10), 2857–2872.
- Waugh, C. M., Korff, T., Fath, F., & Blazevich, A. J. (2012). Rapid force production in children: adaptation of muscle-tendon unit function. Journal of Experimental Biology, 215(5), 738–746.
