KIN 322 Kinesiology · Unit 1: Foundations and the axial skeleton · on Exam 1
1.3 Connective tissue and muscle mechanics
Study notes for Connective tissue and muscle mechanics in KIN 322 Kinesiology at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- Tissues are loaded in tension, compression, shear, bending and torsion. Stress = force ÷ area and strain = change in length ÷ original length. A stress–strain curve has a toe region (crimp straightening), an elastic region, a plastic region (permanent deformation) and failure; the slope is stiffness.
- Connective tissues are viscoelastic: creep (lengthening under a constant load), stress relaxation (falling stress at a constant length) and rate dependence. Ligaments join bone to bone and tendons muscle to bone (dense, parallel collagen); cartilage resists compression; bone remodels to the loads it carries (Wolff’s law), and disuse weakens every tissue.
- Muscle architecture: parallel (fusiform) fibers, as in sartorius and biceps, favor range and speed; pennate fibers (unipennate, bipennate such as rectus femoris, multipennate such as deltoid) pack in more fibers, raising the physiological cross-sectional area and therefore force.
- Length–tension: active force peaks near resting length, where actin and myosin overlap best; passive tension rises with stretch. Force–velocity: concentric force drops as speed rises, and eccentric force can exceed maximal isometric force. Active insufficiency: a two-joint muscle shortened across both joints produces little force (a weak fist with the wrist flexed). Passive insufficiency: a two-joint muscle cannot stretch across both joints at once (hamstrings limit hip flexion with the knee straight).
- Contraction types: concentric (shortening), eccentric (lengthening under load, controlling a lowering) and isometric (no length change). Roles: agonist (prime mover), antagonist, synergist, stabilizer and neutralizer. When gravity drives a motion, the muscle on the opposite side controls it eccentrically.
Key formulas
Stress
$$\sigma = \dfrac{F}{A}$$
Strain
$$\varepsilon = \dfrac{\Delta L}{L_0}$$
Worked example
When you slowly sit down into a chair, which muscle group controls the knees, and what type of contraction is it?
Show the solution
The knees flex under gravity, so the quadriceps (knee extensors) contract eccentrically to control the lowering. Gravity does the flexing; the muscle on the other side of the joint brakes it.
Common mistakes
- Naming the hamstrings for lowering into a squat because the knee is flexing. With gravity driving the motion, the knee extensors work eccentrically.
- Thinking pennate muscles are weaker because their fibers are angled. More fibers fit, so force is higher.
- Mixing creep (constant load) with stress relaxation (constant length).
- Swapping active and passive insufficiency.
Exam tipFor every movement question ask three things: which joint motion happens, whether it is with or against gravity (or a load), and therefore which muscle group works and how.
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