4.1 Joint Actions
Before you can select exercises that match a muscle’s function, you need to know what those functions are. Every skeletal muscle produces movement by pulling on bones across a joint. A muscle’s primary actions are the motions it generates when it shortens. This is not academic detail—it is the direct link between anatomy and exercise selection. If you don’t know what a muscle does, you can’t choose exercises that challenge all of its regions and functions.
Below are the major joint actions for the muscles commonly targeted in hypertrophy training. A lift that involves the listed action will, all else equal, load that muscle and its synergists.
You do not need to memorize this table. It is a reference you will come back to when building your exercise menu in Sections 4.3 and 6.4. For now, just skim it to get a feel for how muscles and movements connect.
| Joint Action | Description | Primary Muscles | Exercise Cue |
|---|---|---|---|
| Shoulder flexion | Raising the arm forward and upward (e.g., front raise, low‑to‑high cable fly) | Anterior deltoid, clavicular pectoralis major, coracobrachialis | Arm moving forward/upward relative to the torso |
| Shoulder extension | Pulling the arm down from a flexed position (e.g., straight‑arm pulldown, pull‑over) | Latissimus, teres major, posterior deltoid, long head of triceps | Arm moving backward/downward from a raised position |
| Horizontal adduction | Bringing the arm toward the midline in front of the body (e.g., chest press, pec fly, Smith press with elbows tucked) | Pectoralis major (all fibres), anterior deltoid, coracobrachialis | Upper arm moving toward the centre of the chest from a side‑raised position |
| Horizontal abduction | Moving the arm away from the midline in the transverse plane (e.g., reverse fly, face‑pull) | Posterior deltoid, middle trapezius, rhomboids | Upper arm moving behind the body in a horizontal plane |
| Elbow flexion | Bending the elbow (e.g., curl) | Biceps brachii, brachialis, brachioradialis | Elbow angle decreasing |
| Elbow extension | Straightening the elbow (e.g., pushdown, overhead extension) | Triceps brachii, anconeus | Elbow angle increasing |
| Knee flexion | Bending the knee (e.g., leg curl) | Hamstrings (semitendinosus, semimembranosus, biceps femoris), gastrocnemius | Knee angle decreasing |
| Knee extension | Straightening the knee (e.g., leg extension, squat) | Quadriceps | Knee angle increasing |
| Hip extension | Driving the thigh backward (e.g., deadlift, hip thrust) | Gluteus maximus, hamstrings, adductor magnus (posterior fibres) | Thigh moving behind the body |
| Hip flexion | Lifting the thigh up (e.g., hanging knee raise) | Iliopsoas, rectus femoris, sartorius, tensor fasciae latae | Thigh moving forward/upward |
| Hip adduction | Bringing the thigh toward the midline (e.g., adductor machine) | Adductor group (adductor magnus, longus, brevis, gracilis) | Thigh moving toward the opposite leg |
| Hip abduction | Moving the thigh away from the midline (e.g., hip abduction machine, lateral band walk) | Glute medius, glute minimus, tensor fasciae latae | Thigh moving away from the body |
| Scapular retraction | Pulling the shoulder blades together (e.g., row, face‑pull) | Middle trapezius, rhomboids | Pinch shoulder blades back |
| Scapular protraction | Spreading the shoulder blades forward (e.g., push‑up plus) | Serratus anterior | Separate shoulder blades |
Why this matters for exercise selection
When you build an exercise menu, you match movements to the joint actions of the target muscle. For the chest, as an example:
- Horizontal adduction is the main action—trained by any pressing movement and by flyes where the arms move from wide to narrow in front of the body. For a lengthened‑emphasis exercise, choose a press or fly that puts the chest under load when the arms are behind the body (e.g., most pec flyes). For a shortened emphasis, pick a movement where the load peaks near full horizontal adduction (e.g., a Hammer Strength chest press with a cam that provides more resistance at the top).
- Shoulder flexion involves the clavicular (upper) fibres. This is trained by any press that brings the humerus forward and up, such as a low‑to‑high cable fly or an incline Smith press where the arms move upward and in front of the body. A low incline dumbbell press with the elbows tucked closer to the body also introduces shoulder flexion.
A complete chest program following the Two‑Exercise rule from 4.3.2 would therefore include:
- A lengthened‑position exercise for horizontal adduction (e.g., pec fly)
- A lengthened‑position exercise for shoulder flexion (e.g., low‑to‑high cable fly)
- If volume allows, a shortened‑position exercise for horizontal adduction (e.g., Hammer Strength chest press with a cam that peaks at full adduction)
The same logic applies to every muscle group: identify its joint actions, then select exercises that load those actions at long and short/mid muscle lengths. Combined with the active‑insufficiency and antagonist‑inhibition principles below, you now have a systematic method for choosing exercises that match a muscle’s actual function.