Understanding Joint Actions

Understanding Joint Actions: A Key to Better Movement and Health

When we talk about human movement, it’s crucial to understand how bones move relative to each other at a joint. These movements are called joint actions. By understanding joint actions, fitness professionals, physical therapists, and sports medicine experts can effectively communicate and analyze movement, design exercise programs, and provide therapeutic interventions. Let’s break down what joint actions are, why they matter, and how they relate to different types of movement.

What Are Joint Actions?

Joint actions describe the movement of bones around a joint and are a way of categorizing how our body moves. These movements are important in various activities, including exercise, daily tasks, sports, and therapeutic interventions. By understanding joint actions, we can determine the muscles being used, the mechanics of the movement, and how to optimize performance or rehabilitation.

Note: Joint actions are distinct from arthrokinematic motions (like spin, glide, or roll), which refer to the motion of the joint surfaces themselves, rather than the bones. When you see the term “osteokinematic motions” or “osteokinematic joint actions,” it’s referring to the movement of the bones around the joint.

Why Are Joint Actions Important?

1. Better Communication: For professionals in sports medicine, fitness, and rehabilitation, knowing the specific joint actions makes communication clearer and more precise. For example, when discussing exercises or therapies, you can talk about exactly which movement is occurring at a specific joint (e.g., flexion at the elbow, extension at the knee).
2. Exercise & Technique Selection: Joint actions guide exercise selection. For instance, knowing that a pull-up involves shoulder adduction or that a lunge involves hip and knee extension can help in programming exercises that target specific movements and muscles.
3. Analyzing Motion: By understanding joint actions, you can analyze how your body moves during different activities, which is crucial for performance optimization and injury prevention.

Joint Actions by Plane of Motion

Joint actions are categorized based on the plane of motion in which they occur. These planes describe how movement happens in relation to the body’s three-dimensional space: sagittal, frontal, and transverse planes.

1. Sagittal Plane Joint Actions

The sagittal plane divides the body into left and right halves. Movements in this plane usually involve forward or backward motions.

• Flexion: Flexion is a decrease in the joint angle. For example, when you bend your elbow or knee, you’re performing flexion. This action reduces the angle between two body parts, like when you curl your arms in a bicep curl or squat down.
• Extension: Extension is the opposite of flexion, where the joint angle increases. Straightening your arm or leg is an example of extension. For instance, extending your elbow after a bicep curl or standing back up after a squat.

Examples in exercise: Running involves repetitive flexion and extension of the hip, knee, and ankle joints.

2. Frontal Plane Joint Actions

The frontal plane divides the body into front and back portions. Movements in this plane typically involve side-to-side motions.

• Abduction: Abduction occurs when a body part moves away from the body’s midline. Think of raising your arms to the side or spreading your legs apart during a jumping jack. In these movements, the limbs move outward from the center of the body.
• Adduction: Adduction is the opposite of abduction, where a body part moves back toward the body’s midline. For example, bringing your arms back down to your sides after abduction or bringing your legs back together after spreading them apart.
• Lateral Flexion: This is a side-to-side movement of the trunk, like when you tilt your torso to one side while standing or sitting.

Examples in exercise: Side lunges or lateral raises involve abduction, while squats involve adduction of the legs.

3. Transverse Plane Joint Actions

The transverse plane divides the body into upper and lower halves. Movements in this plane involve rotations and horizontal actions.

• External (Lateral) Rotation: External rotation refers to the outward rotation of a body part, like turning your leg outward from the hip joint. It involves the anterior side of a body segment moving away from the midline of the body.
• Internal (Medial) Rotation: Internal rotation is the opposite, where the anterior side of a body part rotates toward the midline. For example, rotating your arm inward after an outward rotation.
• Horizontal Adduction (Flexion): Horizontal adduction involves moving a limb horizontally toward the body’s midline. A good example is bringing your arms toward your chest when performing a chest fly or a cable chest press.
• Horizontal Abduction (Extension): This is the opposite of horizontal adduction. It involves moving the arms or legs away from the body’s midline, like when your arms open up during a reverse chest fly.

Examples in exercise: Rotating your torso during a cable woodchop involves internal or external rotation. Chest flies or reverse flies involve horizontal adduction and abduction.

How Do Joint Actions Affect Muscle Recruitment?

Every joint action is tied to the recruitment of specific muscles. By understanding which joint actions occur during an exercise, you can better understand which muscles are being activated.

• Flexion and extension often engage muscles that are responsible for bending and straightening limbs, like the biceps (during elbow flexion) or the quadriceps (during knee extension).
• Abduction and adduction recruit muscles that move the limbs toward or away from the midline, like the deltoids (during shoulder abduction) or the adductors (involved in adduction of the legs).

Understanding joint actions also helps in selecting the appropriate exercises for rehabilitation or strengthening, as specific muscle groups are targeted depending on the desired outcome.

Conclusion: Why Joint Actions Matter

In summary, joint actions are a fundamental aspect of movement, helping us describe how bones and muscles work together during exercise, sports, and daily activities. Whether you’re a fitness professional designing a workout or a therapist working on a rehabilitation plan, understanding joint actions enhances your ability to communicate, analyze movement, and optimize performance. This knowledge is essential for anyone involved in movement sciences or health-related fields, as it provides a deeper understanding of how the body functions and how to best support its well-being.

In your journey to improved health, wellness, and fitness, remember that joint actions are more than just terms—they’re a guide to more effective, mindful movement!