1.1 Musculoskeletal Anatomy & Kinesiology
Key Takeaways
- Synovial joints are classified by shape: hinge (elbow/knee), ball-and-socket (hip/shoulder), pivot (atlantoaxial), saddle (thumb CMC), condyloid (MCP/wrist), plane (intercarpal)
- Muscle contractions are concentric (shortening), eccentric (lengthening under load), and isometric (no length change); eccentric generates the most force
- Osteokinematics is bone movement (flexion, abduction, rotation); arthrokinematics is joint-surface movement (roll, glide, spin)
- The concave-convex rule: a convex surface glides OPPOSITE the bone motion; a concave surface glides in the SAME direction
- The gait cycle is ~60% stance and ~40% swing, with two periods of double-limb support per cycle
- Type I fibers are fatigue-resistant and aerobic (endurance/posture); Type II fibers are powerful but fatigable
- Closed kinetic chain fixes the distal segment (squats); open kinetic chain frees it (knee extensions)
- Wolff law: bone remodels in response to the mechanical stress placed on it
The musculoskeletal system is the structural framework for movement and the foundation of physical therapy practice. The NPTE expects fluency in joint structure, muscle physiology, and the biomechanical rules that govern how PTs assess and mobilize joints.
Joint Classifications
Synovial Joint Types
Synovial joints are the freely movable joints of the body, classified by the shape of their articular surfaces and the motions they permit:
| Joint Type | Shape | Example | Degrees of Freedom |
|---|---|---|---|
| Hinge | Convex into concave | Elbow (humeroulnar), knee, ankle (talocrural) | 1 (flexion/extension) |
| Ball-and-Socket | Spherical head into cup | Hip, shoulder (glenohumeral) | 3 |
| Pivot | Ring rotates on axis | Atlantoaxial (C1-C2), proximal radioulnar | 1 (rotation) |
| Saddle | Reciprocally concave-convex | 1st carpometacarpal (thumb) | 2 + opposition |
| Condyloid | Oval convex into oval concave | MCP joints, radiocarpal (wrist) | 2 |
| Plane (gliding) | Flat surfaces | Intercarpal, intertarsal, acromioclavicular | Gliding/translation |
The shape dictates available motion: a hinge permits one plane, a ball-and-socket permits three, and the saddle thumb adds opposition that makes the human grip possible.
Osteokinematics vs. Arthrokinematics
PTs distinguish two layers of joint motion:
- Osteokinematics — the visible movement of bones in space: flexion, extension, abduction, adduction, rotation.
- Arthrokinematics — the movement of the joint surfaces themselves: roll, glide (slide), and spin.
The Concave-Convex Rule governs the direction of the arthrokinematic glide and is essential for joint mobilization:
- When a convex surface moves on a fixed concave surface, the glide is opposite the bone's direction.
- When a concave surface moves on a fixed convex surface, the glide is in the same direction as the bone.
Worked example. During shoulder flexion the convex humeral head moves on the concave glenoid, so the head glides inferiorly even though the arm rises superiorly. By contrast, during open-chain knee extension the concave tibial plateau moves on the convex femoral condyles, so the tibia glides anteriorly — the same direction as the lower leg. Misapplying this rule reverses your mobilization and is a frequent NPTE trap.
Muscle Physiology
Contraction Types
| Type | Description | Example |
|---|---|---|
| Concentric | Muscle shortens while producing force | Lifting a biceps curl |
| Eccentric | Muscle lengthens while producing force | Lowering the curl slowly |
| Isometric | Force without length change | Holding the weight at 90 degrees |
The force hierarchy is eccentric > isometric > concentric. Eccentric loading produces the greatest tension and is the basis of evidence-based tendinopathy programs (for example, eccentric heel drops for Achilles tendinopathy and eccentric wrist extension for lateral epicondylitis).
The length-tension relationship explains why a muscle generates peak force near its mid-range resting length (optimal actin-myosin overlap) and less at fully shortened or fully lengthened positions. The force-velocity relationship adds that, in concentric action, force falls as contraction velocity rises, whereas eccentric force actually increases with velocity.
Muscle Fiber Types
| Fiber Type | Other Names | Traits | Role |
|---|---|---|---|
| Type I | Slow-twitch, slow oxidative | High mitochondria, dense capillaries, aerobic, fatigue-resistant | Posture and endurance |
| Type IIa | Fast oxidative-glycolytic | Moderate fatigue resistance, aerobic + anaerobic | Power-endurance |
| Type IIx | Fast glycolytic | Low fatigue resistance, anaerobic | Maximal force and speed |
Kinetic Chains and Bone Adaptation
Closed kinetic chain (CKC) exercises fix the distal segment (foot or hand) — squats, lunges, leg press, push-ups. They promote joint stability through co-contraction and compressive (rather than shear) forces, which is why CKC quadriceps work is favored early after ACL reconstruction. Open kinetic chain (OKC) exercises free the distal segment — seated knee extensions, biceps curls — isolating a muscle but increasing joint shear.
Wolff law states that bone remodels in response to the mechanical stress imposed on it: loaded bone becomes denser and stronger, while unloaded bone (bed rest, immobilization, disuse) demineralizes. This principle justifies weight-bearing and resistance exercise for osteoporosis prevention. The soft-tissue analogue is Davis law, describing how soft tissue remodels along lines of stress.
Gait Analysis
The gait cycle is one stride of a single limb, divided into stance (~60%) and swing (~40%), with two brief periods of double-limb support (at the start and end of stance).
| Phase | % of Cycle | Description |
|---|---|---|
| Initial Contact | 0% | Heel strikes the ground |
| Loading Response | 0-10% | Weight acceptance, shock absorption |
| Midstance | 10-30% | Single-limb support; body advances over the foot |
| Terminal Stance | 30-50% | Heel rises; body advances past the foot |
| Pre-Swing | 50-60% | Toe-off; second double-support period |
| Initial Swing | 60-73% | Foot clearance via hip/knee flexion |
| Mid Swing | 73-87% | Limb advances |
| Terminal Swing | 87-100% | Deceleration before next contact |
Key gait parameters: cadence (~110-120 steps/min in adults), step length (heel-strike to opposite heel-strike), stride length (= 2 step lengths, same foot), and walking velocity (~1.2-1.4 m/s). During midstance, the hip abductors (gluteus medius) must hold the pelvis level; weakness produces contralateral pelvic drop, the Trendelenburg sign.
Clinical Tie-In: Why These Fundamentals Are Tested
The NPTE rarely asks a fundamental in isolation; it embeds it in a decision. Knowing that the gluteus medius stabilizes the pelvis at midstance lets you predict a Trendelenburg gait from a weak abductor and choose abductor strengthening. Knowing the force hierarchy (eccentric > isometric > concentric) lets you justify eccentric loading for a tendinopathy. Knowing the concave-convex rule lets you mobilize in the correct direction. Treat each fact here as the first step of a clinical chain the exam will ask you to complete.
During shoulder flexion, which direction does the convex humeral head glide on the concave glenoid?
Which muscle contraction type generates the most force?
Which are characteristics of Type I (slow-twitch) muscle fibers? (Select all that apply)
Select all that apply
What percentage of the gait cycle does the stance phase normally occupy?
By the concave-convex rule, when a convex surface moves on a concave surface, the glide is in the _____ direction of the bone movement.
Type your answer below
The first carpometacarpal (CMC) joint of the thumb is which type of synovial joint?
During which gait phase is single-limb stability demand highest, making gluteus medius critical to prevent a Trendelenburg sign?
Wolff law states that bone:
A closed kinetic chain (CKC) exercise is characterized by: