Useful biomechanics: levers, torque and resistance curves
Moment arms, the length–tension relationship and resistance profiles. The part of the physics that actually changes how you pick and perform exercises.
Two exercises can train the same muscle and feel completely different. The explanation is almost never mystical: it is geometry. This chapter covers the minimum biomechanics that changes real programming decisions.
Torque: what the muscle actually overcomes
Your muscles do not lift kilos: they generate torque around a joint. The external torque you have to overcome is force × moment arm, where the moment arm is the perpendicular distance between the line of action of the resistance and the joint axis.
- A dumbbell curl is hardest at 90° of elbow flexion because that is where the external moment arm peaks, not because the biceps is “weaker” there.
- A high-bar and a low-bar squat move the same weight while splitting torque differently between knee and hip.
- Tilting the bench changes the moment arm at the shoulder and redistributes load between the clavicular and sternal heads of the pec.
Internal moment arm
The muscle has its own moment arm too, and it changes with joint angle. When several muscles cross a joint, their relative internal moment arms determine which contributes most force at each angle. That is why some exercises share the work differently even when the movement looks alike.
The length–tension relationship
A muscle does not produce the same force at every length. There is an optimal intermediate length (maximum filament overlap) and force falls off when it is very shortened or very stretched. In practice, working in stretched positions also appears to hold an advantage for growth.
Loading a muscle in the stretched position favours hypertrophy
Reviews comparing partial reps at long versus short muscle lengths find greater hypertrophy at long lengths. Against full range, lengthened partials produce similar adaptations. In other words: do not switch to partials, but pick exercises that load the stretched portion well.
| Muscle | Exercise loading the stretch | Exercise loading the contraction |
|---|---|---|
| Pecs | Dumbbell flyes, decline press through a long range | High-to-low cable crossover at the end of the range |
| Biceps | Incline curl (shoulder extended) | Preacher curl in the final portion |
| Hamstrings | Romanian deadlift, seated leg curl | Lying leg curl |
| Quads | Deep squat, hack squat | Leg extension at lockout |
| Glutes | Long-stride lunge, deep squat | Hip thrust at lockout |
Resistance curves
Every exercise has a resistance profile: how the difficulty changes across the range. Knowing it explains why some exercises bite at the start and others at the end.
| Type of resistance | Where it is hardest | Practical consequence |
|---|---|---|
| Free weight | Where the segment is horizontal (moment arm at its peak) | There is usually a clear sticking point and some easy zones |
| Cable | Depends on the cable direction, which you can choose | Lets you hold tension where a free weight loses it |
| Cammed machine | Designed by the manufacturer; often flatter | Spreads the effort across the whole range |
| Resistance band | At the end of the range (more stretch = more force) | Barely loads the stretched position: pair it with other options |
How to use this when building a plan
Per muscle, combine one exercise that loads the stretch with one that loads the contraction or has a flat profile. Two well-chosen exercises cover the whole curve without piling up redundant work.
Machines and free weights
For hypertrophy, machines and free weights are comparable
A within-subject randomised trial on the knee extensors found comparable regional hypertrophy training with machines or with free weights. The 2026 ACSM position stand points the same way: the type of equipment is not decisive, and non-traditional methods such as bands or bodyweight are highly effective.
- Free weights: more transfer to sport and daily life, more stability demand, a steeper learning curve.
- Machines: easier to get close to failure safely, less systemic fatigue, ideal for accessory volume and for beginners.
- The right question is not “which is better?” but “which lets me progress measurably and without pain for months?”.
Joint actions: the map
Choosing exercises per muscle is easier if you think in terms of the joint action that muscle produces. It is the scheme used by classic kinesiology directories such as ExRx.
| Joint action | Main muscles | Typical exercises |
|---|---|---|
| Knee extension | Quadriceps | Squat, leg press, leg extension |
| Knee flexion | Hamstrings | Leg curl |
| Hip extension | Gluteus maximus, hamstrings | Deadlift, hip thrust, lunge |
| Shoulder horizontal adduction | Pectoralis major | Bench press, flyes |
| Shoulder extension / adduction | Latissimus dorsi | Pull-ups, pulldown, pullover |
| Shoulder abduction | Middle deltoid | Lateral raises |
| Scapular retraction | Middle traps, rhomboids | Rows, face pull |
| Elbow extension | Triceps | Dips, cable extensions |
| Elbow flexion | Biceps, brachialis | Curls |
| Plantar flexion | Gastrocnemius, soleus | Standing and seated calf raises |
Takeaways
- How hard an exercise is comes from the moment arm, not the weight on the bar.
- Prioritise exercises that load the stretched position of the muscle well.
- Pick pairs of exercises with complementary resistance profiles.
- Machine or free weight: whichever lets you progress pain-free for months.
Sources for this chapter
- [1]Ulster University (revisión sistemática y meta-análisis) (2025). Muscle hypertrophy from partial repetition at long vs. short muscle length: a systematic review and meta-analysis. Ulster University Research Portal. meta-analysis
- [2]Larsen S, Swinton P, Sandberg NØ, et al. (2025). Lengthened partial repetitions elicit similar muscular adaptations as full range of motion repetitions during resistance training in trained individuals. PubMed. trial
- [3]Investigación aleatorizada intra-sujeto sobre extensores de rodilla (2025). Comparable regional hypertrophy of the knee extensor muscles in response to resistance training with machines versus free weights. Journal of Science and Medicine in Sport / ScienceDirect. trial
- [4]American College of Sports Medicine (2026). Position Stand: Resistance Training for Healthy Adults. ACSM. position stand
- [5]ExRx.net (2026). Kinesiology: estructura, función y directorio muscular. ExRx.net. resource