Anatomy / biomechanics / clinical reasoning
A movement is visible.
Its cause is not.
Scapular winging, a dropping pelvis and an inward-moving knee can begin an assessment. None completes it.
Are we describing a movement, measuring a capacity, identifying a condition or predicting an outcome? A useful observation can answer the first question while leaving the others open.
Why a short video can be persuasive.
A visible pattern feels like a complete explanation: the shoulder blade sticks out, so a muscle must be weak; the pelvis drops, so the gluteus medius must be the cause; the knee moves inward, so an injury must be approaching. Each sentence quietly moves from observation to mechanism and then to prediction. Research has to test those steps separately.
This matters in training as much as in rehabilitation. If the working explanation is wrong, the exercise may address the wrong limitation. It can also make a harmless difference feel threatening. Equally, dismissing every visible change as normal can miss a meaningful loss of function. The useful middle ground is a specific hypothesis that can be checked against symptoms, history, examination and repeated performance.
The shoulder blade has several partners.
The scapula moves over the rib cage as the arm moves. Serratus anterior and the different portions of trapezius contribute to its control; rhomboids also influence its position. The movements include rotation, tilt and translation. A posterior view shows only part of this three-dimensional behaviour.
Prominent winging deserves more precision than the broader label scapular dyskinesis , which describes altered scapular motion. Neurological winging can follow long thoracic nerve dysfunction affecting serratus anterior, or spinal accessory nerve dysfunction affecting trapezius. Muscle detachment and other causes also exist. The examination must distinguish these possibilities; an exercise video cannot establish which structure is involved. [1]
01 / Locate the structure before naming the cause
Serratus anterior
Runs from ribs to the anterior surface of the medial scapular border. It helps keep the scapula associated with the chest wall and contributes to upward rotation. Long thoracic nerve dysfunction is one possible cause of winging.
How often does altered motion occur without pain?
A 2023 systematic review retained 34 studies containing 2,365 adults. Scapular dyskinesis was reported in about 48% of asymptomatic participants and 60% of symptomatic participants across the combined groups. Methods for classifying it were inconsistent. The overlap makes a crucial point: an observed movement pattern is neither necessary nor sufficient for pain. The review describes prevalence; it does not prove that every finding is harmless. [2]
Prospective evidence asks a different question. A review of five studies followed 419 initially asymptomatic athletes for 9–24 months. Shoulder pain occurred in 56 of 160 athletes classified with dyskinesis and 65 of 259 without it. The pooled risk ratio was 1.43, with a 95% confidence interval of 1.05–1.93. This supports an association in those sporting settings, but not a diagnosis or proof that correcting the observed motion prevents injury. Different exposure, strength and previous history can matter. [3]
These findings can coexist. A feature can be common without symptoms and still be associated with a future outcome in a particular population. Clinical value depends on how much it adds to the rest of the assessment.
The pelvis is a problem of forces and options.
During single-leg support, body weight can create an external turning effect around the stance hip. Hip abductor muscles contribute an opposing moment. A moment is force multiplied by its perpendicular distance from an axis. The gluteus medius is important, but the task also involves gluteus minimus, other muscles, trunk position and coordination. Leaning the trunk changes the geometry of the demand.
A Trendelenburg sign usually describes a drop of the opposite side of the pelvis during single-leg support. A related walking pattern can include compensations. It does not uniquely identify gluteus medius weakness or its cause. The distinction matters because a nerve lesion, pain-related inhibition and a task strategy are different hypotheses, even if a short clip appears similar.
02 / Change the demand without changing the muscle
The same load.
A different moment.
External moment:
N·m
Modelled opposing muscle force:
N
Does the test measure strength?
A 2020 study measured 18 participants with a dynamometer and three-dimensional motion capture. Both hips were assessed, giving 36 side-specific observations rather than 36 independent people. The study compared peak hip-abductor torque with pelvic movement during the Trendelenburg test. It found no significant overall relationship in this healthy young sample, and agreement between the practitioner’s classification and motion analysis was weak: κ approximately 0.22–0.25. The small sample and absence of intra-articular hip disease limit generalisation. It nevertheless challenges using the visible test as a substitute for a strength measurement. [4]
An earlier experiment went further by temporarily reducing hip-abductor function with a superior gluteal nerve block in nine healthy men. Average measured strength fell by about 52%, yet pelvic-drop measures did not change systematically; only one participant showed the expected compensation pattern. This is a small, artificial experiment rather than a model of every clinical disorder. It shows why a mechanically plausible explanation can still make an unreliable screening rule. [5]
A practical assessment can therefore retain the observation while testing strength separately, considering pain and comparing the task under changed conditions. “The pelvis drops” is a defensible description. “This proves the gluteus medius needs this one exercise” is a stronger claim.
An inward-moving knee is not one mechanism.
Genu valgum refers to an alignment description. Dynamic knee valgus describes an observed movement pattern, often during landing, squatting or cutting. The appearance can involve contributions from hip motion, knee motion, tibial rotation, the foot and the viewing angle. A still photograph and a demanding landing test answer different questions.
If weak hip abductors were a universal explanation, stronger abductors should consistently accompany less valgus. A systematic review of seven studies in healthy adults did not find that pattern: two studies reported a negative association, two a positive association and three no association. The tests, tasks and two-dimensional measurement methods varied. These were associations, not experiments showing that a strength change caused a movement change. [6]
03 / Seven studies did not tell one story
Hip-abductor strength versus observed knee valgus
A repeatable measurement may still measure the wrong thing.
In a study of 16 healthy adults performing drop jumps, four video-based measures had good-to-excellent repeatability. Yet the two frontal-plane projection-angle methods showed poor-to-moderate agreement with corresponding three-dimensional analysis, while knee-separation measures agreed more closely. Reliability asks whether a result can be repeated; validity asks whether it supports the intended interpretation. The study concerned a specific double-legged task and cannot validate every phone recording or infer ligament loading from one angle. [7]
Video can still be useful for consistent comparisons within a defined task. Keep camera position, speed, load, fatigue and instructions comparable. Combine the image with symptoms and function. A visible change is worth investigating when it helps answer a meaningful question; reducing an angle is not automatically the same as reducing pain or injury risk.
Make the observation earn its place.
| Step | A useful question | What to avoid |
|---|---|---|
| Describe | Which segment changes, during which task? | A diagnosis from a silhouette. |
| Compare | Does it change with speed, load, support or symptoms? | Comparing unrelated tasks. |
| Measure | What does a separate capacity test show? | Treating appearance as strength. |
| Intervene | Does a defined change improve the relevant outcome? | Using cosmetic symmetry as the only success criterion. |
| Review | Is the benefit repeatable and useful to the person? | Explaining every improvement through the original theory. |
A recent loss of shoulder or leg function, progressive weakness or a new walking change deserves an appropriate clinical assessment. The models here help explain reasoning; they cannot perform a neurological examination. For a stable training question, an observed pattern can guide a measured experiment rather than a fixed label.
What remains uncertain?
The studies use different tasks and populations. A result in healthy young adults is not automatically applicable after surgery, in a nerve lesion or in an older person with falls. Better prospective studies must test whether adding a movement classification improves decisions and outcomes beyond a standard assessment. Mechanistic plausibility, measurement quality and treatment usefulness remain separate parts of that work.
From Instagram to this Insight
This article develops four starting points: scapular winging, genu valgum, gluteus medius and gait and the Trendelenburg quiz. The original labels are expanded into differential explanations. The quiz is not treated as proof of the most common cause in every population, and no reader is assigned a diagnosis from the examples.
Read the studies.
- Anatomy, Etiology, and Management of Scapular Winging (2018). Anatomical and clinical review; causes and differential assessment.
- Is it Time to Normalize Scapular Dyskinesis? (2023). Systematic review; symptomatic and asymptomatic prevalence.
- Scapular dyskinesis and future shoulder pain (2017). Prospective-study systematic review and meta-analysis.
- Determining Trendelenburg test validity and reliability (2020). Dynamometry and three-dimensional motion analysis.
- Steps toward the validation of the Trendelenburg test (2012). Experimental nerve block; nine healthy men.
- Is Hip Muscle Strength Associated with Dynamic Knee Valgus? (2021). Systematic review of seven studies.
- Two- and three-dimensional evaluation of knee valgus (2016). Reliability and concurrent-validity study.