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NASM Certified Personal Trainer · Basic and Applied Sciences and Nutritional Concepts

NASM Certified Personal Trainer: Anatomy and Biomechanics Concepts

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Written by AI. Only units that passed a separate AI check (re-reading facts and numbers) are published. Tell us if you find a mistake.

Overview

This chapter covers how the body moves: planes of motion, muscle roles, types of muscle action, and the sensors in muscles and tendons. These ideas show up in many other chapters, so they are worth learning well.

Movement happens in three planes. The sagittal plane holds forward and backward motion, such as flexion and extension in a squat. The frontal plane holds side-to-side motion, such as abduction and adduction in a side lunge. The transverse plane holds rotation, such as a trunk twist.

Muscles play different roles. The agonist is the prime mover, the antagonist works against it, the synergist helps the prime mover, and the stabilizer holds a joint steady. A muscle can shorten (concentric), lengthen under tension (eccentric), or stay the same length (isometric).

Two sensors help control movement. Muscle spindles sense how long a muscle is and how fast it stretches. Golgi tendon organs sense tension where the muscle meets the tendon.

Key concepts

  • Planes of motion

    Sagittal plane: flexion and extension (squat, bicep curl). Frontal plane: abduction and adduction (side lunge, jumping jack). Transverse plane: rotation (trunk twist).

    Memory tipSagittal = Straight ahead. Frontal = side to side. Transverse = Turning.

  • Agonist and antagonist

    The agonist is the main muscle doing the movement. The antagonist is the muscle on the opposite side that works against it. In a bicep curl, the biceps is the agonist and the triceps is the antagonist.

    Memory tipAgonist = Action. Antagonist = Against.

  • Synergist and stabilizer

    A synergist helps the prime mover. A stabilizer holds a joint or body part steady so the movement can happen.

    Memory tipSynergist = helper. Stabilizer = holder.

  • Concentric, eccentric, isometric

    Concentric: the muscle shortens (lifting). Eccentric: the muscle lengthens under tension (lowering). Isometric: the muscle works but does not change length (holding).

    Memory tipEccentric = Exit, the muscle gets longer on the way down.

  • Muscle spindle

    A sensor inside the muscle that detects changes in muscle length and how fast it stretches. A quick stretch makes it tell the muscle to contract.

    Memory tipSpindle = length sensor.

  • Golgi tendon organ

    A sensor where the muscle joins the tendon. It detects tension, and when tension is high or held long enough, it helps the muscle relax.

    Memory tipGolgi = tension sensor in the tendon.

Often tested

  • Flexion and extension are sagittal; abduction and adduction are frontal; rotation is transverse.
  • Eccentric means lengthening under tension, such as lowering a weight.
  • Golgi tendon organs sense tension; muscle spindles sense length. They are often swapped.
  • The agonist is the prime mover; the antagonist opposes it.

Easy to confuse: Planes of motion

PlaneMotionsExample exercise
SagittalFlexion, extensionSquat, bicep curl
FrontalAbduction, adductionSide lunge, lateral raise
TransverseRotationTrunk twist, cable wood chop

3 sample questions

Verified practice questions from this unit.

Practice questionReviewedBasic and Applied Sciences and Nutritional Concepts › Anatomy and Biomechanics★★★★☆

Question 1. A muscle produces force while it lengthens. Which type of contraction is this?

  1. ① Eccentric
  2. ② Concentric
  3. ③ Isometric
  4. ④ Isokinetic
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Answer: ① Eccentric

Key point: Eccentric = muscle lengthens under tension

Lowering a weight slowly is an eccentric contraction. The muscle makes force as it gets longer.

Wrong choices

  • Concentric: In a concentric contraction the muscle shortens.
  • Isometric: In an isometric contraction the muscle stays the same length.
  • Isokinetic: This is constant speed movement.
Practice questionReviewedBasic and Applied Sciences and Nutritional Concepts › Anatomy and Biomechanics★★★★☆

Question 2. A biceps curl happens mainly in which plane of motion?

  1. ① Sagittal plane
  2. ② Frontal plane
  3. ③ Transverse plane
  4. ④ Oblique plane
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Answer: ① Sagittal plane

Key point: Flexion and extension = sagittal plane

The sagittal plane divides the body into left and right. Bending and straightening movements, like a curl, happen here.

Wrong choices

  • Frontal plane: The frontal plane includes side-to-side movements.
  • Transverse plane: The transverse plane includes rotation.
  • Oblique plane: This is not a main plane of motion.
Practice questionReviewedBasic and Applied Sciences and Nutritional Concepts › Anatomy and Biomechanics★★★★☆

Question 3. Which movement happens mainly in the frontal plane?

  1. ① A biceps curl
  2. ② A squat
  3. ③ A trunk rotation
  4. ④ A side lateral raise
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Answer: ④ A side lateral raise

Key point: Abduction and adduction = frontal plane

Side-to-side movements like a lateral raise (abduction) happen in the frontal plane.

Wrong choices

  • A biceps curl: This is in the sagittal plane.
  • A squat: This is mainly in the sagittal plane.
  • A trunk rotation: This is in the transverse plane.
Practice 5 questions from this unit →