Mastering Shoulder Function: Exploring the Relationship between Load and Overhead Kinematics

When it comes to shoulder function, there's a lot more going on than meets the eye. It's not just about lifting your arm; it's about the intricate dance between your scapula (the shoulder blade) and humerus (the upper arm bone) during arm elevation. This intricate relationship is known as scapulohumeral rhythm, and understanding it is crucial for comprehending the true essence of shoulder function. While we've long been familiar with how the shoulder behaves without any added weight, recent research has uncovered fascinating insights into how load impacts shoulder kinematics. In this blog post, we'll dive deep into these discoveries and explore their implications for rehabilitation and exercise planning. So, let's get started!

The Unloaded Arm Elevation Journey: Let's start by unpacking the journey of arm elevation without any added weight. It can be broken down into three distinct phases:

1. Setting Phase: This is where it all begins. As you actively lift your arm up to 30 degrees, your scapula seeks a stable position. Think of it as your shoulder blade finding its "happy place" to start the movement.

2. 30-90 Degrees of Arm Elevation: As your arm continues its upward journey, the scapula starts to rotate laterally. It stabilizes against the thoracic wall, preventing excessive superior or inferior migration. In this phase, the classic ratio of humeral elevation to scapula upward rotation is around 2:1.

3. 90-180 Degrees of Arm Elevation: Now we're reaching the summit! During this final phase of arm elevation, the scapula continues its rotation, causing the center of rotation to shift towards the glenoid (the shoulder socket). This results in further superior movement of the glenoid, lateral migration of the scapula's inferior angle, and an anterior orientation of the glenoid fossa. Here, the humeral elevation to scapula upward rotation ratio is approximately 1:1.

The Load Factor: How it Alters Shoulder Kinematics

Now that we've covered the basics, let's shift our focus to the impact of load on shoulder kinematics. Groundbreaking studies conducted by McQuade and Smidt (1) have revealed some intriguing findings. When performing arm elevation with external loads, the scapulohumeral rhythm ratio undergoes a significant change. With light loads, the ratio increases to around 3:1, and with heavy loads, it can soar up to 4.5:1.

What does this mean? Well, it suggests that when you're lifting a load, your scapula takes on a greater role as a stabilizing platform for optimal glenohumeral mobility. In simpler terms, your shoulder blade works harder to provide stability while allowing your shoulder joint to move freely.

Implications for Rehabilitation and Exercise Prescription

Now that we understand the altered scapulohumeral rhythm during loaded arm elevation, we can discuss its implications for rehabilitation and exercise planning. The change in shoulder kinematics raises concerns about potential sub-acromial impingement. In other words, there's a risk that the rotator cuff might not have enough space to clear the acromion, potentially causing discomfort or injury.

To mitigate this risk and ensure a successful rehabilitation journey, it's crucial to prioritize scapula stabilization. Incorporating exercises that strengthen the muscles around the shoulder blade (peri-scapula strengthening exercises) can enhance stability, reduce the risk of impingement, and promote proper shoulder function.

Additionally, exercise prescription becomes key. Carefully managing the volume of work, including the choice of exercises and repetitions, is vital for progressive rehabilitation without overwhelming the shoulder complex. A well-thought-out program, customized to the individual's needs and capabilities, will yield the best results.

In Conclusion, the intricate relationship between the scapula and humerus during arm elevation is a fascinating topic that continues to unravel new insights. By understanding how load impacts shoulder kinematics, we can optimize rehabilitation outcomes and improve exercise prescription. So, whether you're a clinician or a fitness professional, keeping these factors in mind when designing exercise programs and prescribing loads is crucial.

Remember, strengthening the scapula and considering the altered scapulohumeral rhythm during loaded arm elevation can pave the way for successful rehabilitation and a healthier, happier shoulder joint. Let's continue to explore and apply these discoveries to ensure positive progress and optimal outcomes for our patients and clients. Together, we can unlock the secrets of shoulder function and help individuals reach new heights of physical well-being!

1. McQuade KJ, Smidt GL, “Dynamic scapulohumeral rhythm: the effects of external resistance during

elevation of the arm in the scapula plane”. J Ortho Sports Phys Ther 27:125 – 133, 1998.