The Hip and Groin

01What the surveillance shows

The Numbers on Groin Injury, Hip Shape and Load Sharing

Eight findings that place the groin at the point where pelvic load sharing fails first.

02The joint under load

The Hip Is Built Deep, and Its Geometry Sets the Terms

The hip is a deep ball-and-socket joint built to carry the full force of athletic movement, and its geometry decides how much load it tolerates before something gives. The femoral head seats into the acetabulum. A fibrocartilage labrum rings the socket, deepens it, seals the joint, and spreads pressure across the surface rather than concentrating it at a rim.

How completely the socket covers the head is measurable, and the normal figure is lower than the textbook impression. Computed tomography of 409 asymptomatic hips found mean coverage of 40 percent anteriorly, 61 percent superiorly and 48 percent posteriorly, with lateral coverage corresponding to a mean center-edge angle of 31 degrees Larson 2015.

The femoral side has been remeasured too. Magnetic resonance imaging of 3,226 adults from a population-based cohort put the mean angle of femoral inclination at 127 degrees, with a reference range of 114 to 140 degrees Fischer 2020. The authors state that the historical lower limit of 120 degrees is probably too high, which would mean radiological hip pathology has been overcounted.

These are not trivia. They are the structural fingerprints that explain why one athlete is built for durability and another for impingement.

Why one hip shape suits one sport and betrays another

Not all hips are equal. A sprinter, a hockey player, and a dancer each load the joint along a different vector, and the bony shape that protects one sport can betray another. The depth of the socket, the angle of the neck, and the version of both head and acetabulum combine into a personal load profile.

Version is the rotation of the socket and the femoral neck in the horizontal plane, and it varies widely among people who have never had a symptom. In that same asymptomatic cohort, 15 percent of hips showed the cranial retroversion that produces a crossover sign on a plain film, and 30 percent had posterior coverage under 50 percent Larson 2015.

This is why a precise hip exam reads the architecture before it reads the symptom, asking why this joint, in this athlete, broke down now. The same finding on two films means two different things depending on the pelvis it sits in.

03How the pelvis carries force

The Pelvis Transmits Load by Sharing It Rather Than by Bracing

The pelvis holds together under athletic load because tension is distributed across the whole ring, not because one side braces against another. The conventional account is an arch, with the sacrum wedged between the ilia. On that reading, the greater the vertical shear at the sacroiliac joints, the more the structure leans on self-bracing by obliquely oriented muscles such as the internal oblique.

That account was tested directly. Thirty men held a bucket at 0, 5, 10 and 15 percent of body weight in three stances, two-legged and two one-legged, while electromyography recorded eight lumbopelvic muscles Pardehshenas 2014. Standing on one leg removes an impost, and an arch without an impost has nothing to push against.

The internal obliques co-activated as task demand rose, and no unilateral pattern appeared in any trial. Latissimus dorsi and erector spinae fired against the direction of load. The lumbopelvic region used the same strategy on one leg as on two, which is not what the arch analogy predicts.

The authors read their data as a suspensory system instead, a wheel whose rim stays true because tension is adjusted across every spoke at once. Load transfer through the pelvis is a distributed problem, and a local change in tension does not stay local. The kinetic chain argument behind that claim belongs to The Kinetic Chain.

Where the adductors sit in the ring

The adductor group is one set of spokes, and it is the set that fails most often. Adductor longus, brevis and magnus, along with pectineus and gracilis, pull the thigh inward and steady the pelvis through every plant and pivot. The adductor longus origin at the pubis is the classic strain site.

What a healthy ring produces is a balance, not a maximum. In 394 asymptomatic male professionals, eccentric hip adduction measured 3.0 Nm/kg against 2.6 Nm/kg for abduction, an adduction to abduction ratio of 1.2, with the supine squeeze test at 3.6 N/kg Mosler 2017.

The risk literature points at the same ratio. Reduced hip adductor strength relative to the abductors carries a risk signal for groin injury alongside previous injury, level of play and low sport-specific training Whittaker 2015. A pelvis in trouble is one whose spokes have stopped pulling in proportion.

04Impingement and hip shape

Cam Morphology Is Acquired While the Growth Plate Is Still Open

Femoroacetabular impingement is abnormal contact between the femoral neck and the acetabular rim, and it takes three shapes. The cam type is an aspherical femoral head that jams into the socket on flexion and internal rotation. The pincer type is an over-covering rim that pinches the neck. The mixed type carries both.

Cam shape is not something an athlete is simply born with. Sixty-three preprofessional soccer players, mean age 14.4 years, were radiographed at baseline and again 2.4 years later Agricola 2014. Among the boys aged 12 and 13 at entry, a flattened head-neck junction rose in prevalence from 13.6 percent to 50.0 percent.

In every hip with an open growth plate at baseline, a frank prominence rose from 2.1 percent to 17.7 percent, and the alpha angle rose from 59.4 to 61.3 degrees. After the proximal femoral growth plate closed, neither prevalence nor severity increased further. The shape is laid down inside a narrow window of skeletal growth and then holds.

Two features predicted which hips would go on to form a cam. A small neck-shaft angle and limited internal rotation at baseline both associated with cam deformity and predicted an alpha angle above 60 degrees at follow-up. That makes the adolescent hip, not the adult one, the place where hip shape is still an open question.

How common the finished shape is in adults remains unsettled. Thirty studies of cam morphology returned prevalence estimates from 5 to 75 percent, all of them at high risk of bias Dickenson 2016. The review could not demonstrate a higher rate in athletes than in anyone else.

What the FADIR test can and cannot settle

The athlete reports a pinch at the bottom of a squat, a loss of internal rotation, a hip that feels blocked rather than weak. The maneuver that reproduces it is FADIR, flexion with adduction and internal rotation, which drives the neck into the anterosuperior rim.

FADIR is a screen, not a verdict. Pooled across the studies that qualified for meta-analysis, flexion-adduction-internal rotation reached a sensitivity between 0.94 and 0.99, and flexion with internal rotation reached 0.96 Reiman 2015. The diagnostic odds ratios ran from 5.71, at a confidence interval of 0.84 to 38.86, to 7.82, at 1.06 to 57.84.

The authors concluded that these tests possess only screening accuracy. A negative FADIR is worth something, because a hip that tolerates that position is unlikely to be impinging. A positive FADIR is a reason to keep looking. The scour test, hip flexion with compression and circumduction, is a provocation maneuver on the same footing.

Range of motion is a weaker marker than the clinic assumes. Across 22 studies of symptomatic impingement, hip joint range did not differ from healthy controls Freke 2016. What did differ was hip muscle strength and single-leg dynamic balance, which are load sharing measures rather than shape measures.

Reading hip shape early, while the athlete is still growing, is a different task from chasing damage that has already accumulated inside the joint. The Agricola authors go further and suggest that adjusting athletic load during that short growth window could change how many cam hips exist at all.

05The labrum as a sensor

The Labrum Seals the Hip Joint and Reports From Inside It

The acetabular labrum is the fibrocartilage ring that seals and stabilizes the hip, and it is also a sense organ. Twenty human labra examined histologically carried free nerve endings in every specimen, sitting mostly in the superficial layer and on the chondral side Alzaharani 2014.

Three types of nerve end organ appeared in all twenty specimens as well. Both the free endings and the end organs concentrated in the anterosuperior and posterosuperior zones. The authors concluded that the labrum can mediate pain and joint position sense, which makes a torn labrum a sensory problem as well as a mechanical one.

The striking part is how quiet the tissue can be. Labral defects appeared on magnetic resonance imaging in 44 percent of 63 asymptomatic volunteers, against 61 percent of matched patients with symptomatic impingement, a difference that did not reach significance Tresch 2017.

Acetabular cartilage separated the two groups far more sharply, at 14 percent of volunteers against 47 percent of patients. So the labrum tears widely and quietly, while the cartilage behind it tracks symptoms much more closely.

The wider record on imaging findings in people without pain belongs to Findings in People Without Pain, and Imaging the Athlete carries the athlete-specific version of the same problem.

Why a torn seal costs more than a seal

A torn labrum changes the physics of the joint. The seal that once held a film of fluid and shared the load now leaks and catches. The athlete feels clicking, a deep ache, a sense that the hip can give way under a plant it used to take without thought.

It changes the reporting too. Because the labrum is richly innervated, injury degrades the joint position sense the nervous system relies on to protect the hip in real time. As it frays, the joint sends a thinner report from the exact zone that carries the densest receptor supply.

A joint that moves cleanly feeds the nervous system an accurate map. A nervous system that trusts the map turns that power loose without hesitation, and clean afferent input from the hip feeds the athlete's central integrative state.

Joint position sense error in degrees is a measurable quantity, and Proprioception and Joint Position Sense owns that measurement across the athlete's joints. Hesitation on the plant is what an unmapped joint looks like from the sideline. The aim of care at the hip is confident, well-mapped motion rather than a cleaner image.

06Adductor related groin pain

Two Thirds of Every Groin Injury in Professional Football Is Adductor Related

Groin pain in the athlete is most often the adductor complex failing under cutting, kicking and change of direction. In a men's professional league followed for two seasons, 206 of 1,145 time loss injuries were groin injuries, and 68 percent of those were adductor related Mosler 2018. Iliopsoas took 12 percent and pubic related pain 9 percent.

The absence is real. One player in five at a club sustained a groin injury each season, at an incidence of 1.0 per 1,000 playing hours and a median absence of 10 days. Seven seasons of European professional football tell the same story from a larger sample Werner 2009.

There, 628 hip and groin injuries arrived at 1.1 per 1,000 hours, split as 3.5 per 1,000 match hours against 0.6 in training. Adductor related injury accounted for 399 of them and iliopsoas for 52. Over half were moderate or severe, mean absence ran 15 days, and reinjuries made up 15 percent of the total.

Naming the entity matters, because the region holds several. Twenty-four experts from 14 countries agreed a taxonomy with three headings: defined clinical entities, meaning adductor, iliopsoas, inguinal and pubic related groin pain, then hip related groin pain, then other causes Weir 2015.

That classification runs on history and physical examination rather than on imaging. Manual muscle testing teases the group apart, isolating each muscle so rehabilitation targets the true failure rather than the region. The FABER test, flexion with abduction and external rotation, and palpation of the adductor longus origin at the pubis separate adductor injury from hip joint and pelvic sources.

Surveillance also undercounts the problem badly. Across 39 weeks in 407 Spanish players, weekly prevalence of all groin problems ran 11.7 percent, of which 1.3 percent involved time loss and 10.4 percent did not Esteve 2020. Players carrying groin problems scored 19.5 points lower on hip and groin sporting function, whether or not they had missed a session.

The groin rarely fails on its own

What separates athletes with long-standing groin pain from those without is partly a matter of timing. Ten elite and subelite Australian football players with long-standing groin pain and 12 asymptomatic controls performed an active straight leg raise while fine-wire electromyography recorded the abdominal wall Cowan 2004.

In the controls, transversus abdominis contracted in a feedforward manner. In the players with groin pain, its onset was delayed. Internal oblique, external oblique and rectus abdominis onsets did not differ between the groups. One link in the ring lost its schedule while the others kept theirs.

So the mapping runs wider than the sore spot: hip joint, adductors, abdominal wall and pubic bone. Treating the strain while leaving the rest of the ring unread is how an athlete comes back with the same pain a month later. Power flows through this region. So does breakdown.

07What one exercise moved

One Adductor Exercise Cut Reported Groin Problems by 41 Percent

A single exercise, trained across a season, cut the reported prevalence of groin problems. Thirty-five semiprofessional Norwegian teams were randomized by cluster, 18 teams and 339 players to an Adductor Strengthening Programme and 17 teams and 313 players to normal training Harøy 2019.

The programme ran three times a week through a 6 to 8 week preseason and once a week across 28 weeks of competition. Average prevalence of groin problems was 13.5 percent in the intervention teams against 21.3 percent in the controls. The risk of reporting a groin problem was 41 percent lower, at an odds ratio of 0.59.

That result stands out because earlier groin-specific prevention programmes had not moved groin injury rates at all. What changed was the exercise selection, and the selection came from a measurement.

Surface electromyography in 40 healthy male elite soccer players graded eight hip adduction exercises by adductor longus intensity Serner 2014. Peak activity ranged from 14 percent to 108 percent of a maximal voluntary contraction across the eight. Three of them produced a 35 to 48 percent difference between the two legs.

The ratio moved more than the muscle

The strength result is the one that carries the argument. Twenty-four under-19 subelite players were randomized to eight weeks of supervised progressive Copenhagen Adduction alongside usual training, or to usual training alone Ishøi 2016.

Eccentric hip adduction strength rose 35.7 percent. Eccentric hip abduction strength rose 20.3 percent, in a group that trained neither. The ratio between them rose 12.3 percent, and the control group moved on none of the three. Compliance ran at 91.25 percent with median muscle soreness between 0 and 2.

Read that as a sharing result rather than a strength result. The abductors gained without being loaded, and the proportion between the two sides of the ring shifted. The risk term in the groin literature is the relative one, adductor strength measured against abductor strength Whittaker 2015, so the ratio is the number that moved in the direction risk cares about.

The treatment record points the same way. Sixty-eight athletes carried adductor related groin pain of median 40 weeks' standing Hölmich 1999. They were randomized to an active training programme aimed at the strength and coordination of the muscles acting on the pelvis, or to physiotherapy without active training.

Twenty-three athletes in the active group returned to sport without groin pain, against four in the other, at an odds ratio of 12.7. What worked was training strength and coordination across the muscles acting on the pelvis. Passive treatment of the painful region returned four athletes.

Where the adductor tendon itself is the pain source, the continuum model of tendon pathology and the heavy slow resistance evidence sit with Tendon Pain. The staged progression an athlete moves through, the hop test batteries and the eccentric hamstring record belong to Lower-Extremity Rehab. The right return-to-sport standard is not the absence of pain but the presence of control, and Return to Play owns that decision.

08What we corrected

Four Figures Corrected and One Quotation Removed From This Page

This page previously gave the center-edge angle as 38 degrees in men and 35 in women. Neither figure could be traced to a source, so both are gone. Computed tomography of 409 asymptomatic hips, screened to exclude frank dysplasia and overcoverage, puts mean lateral coverage at a center-edge angle of 31 degrees Larson 2015.

It also gave the angle of femoral inclination as 125 degrees in the adult. Magnetic resonance imaging of 3,226 adults places the mean at 127 degrees, with a reference range of 114 to 140 Fischer 2020. The same paper puts the historical lower limit of 120 degrees in doubt.

The page stated that a positive FADIR test confirms impingement. The meta-analysis says otherwise: flexion-adduction-internal rotation carries screening accuracy and nothing more Reiman 2015. The sentence has been rewritten to say what a positive and a negative result each buy.

Loss of internal rotation was presented as a signature of impingement. Across 22 studies, hip range of motion in symptomatic impingement did not differ from controls Freke 2016. Limited internal rotation does track cam morphology in the growing hip Agricola 2014, so the page now states both findings and keeps them apart.

The page also carried a quotation attributed to Dr. Jason Dulberg that was not drawn from anything he said or wrote. It has been removed. Claims here are either sourced to the literature or named explicitly as the model's.

09The model's claim

The Groin Is Where a Pelvis Stops Sharing Its Load

Two layers run through this page and they should not be confused. The established science is the epidemiology, the lumbopelvic recordings under graded load, the cam formation cohort, the diagnostic accuracy meta-analysis, the labral histology, and the adductor trials. Each belongs to the investigators who ran it, and each is cited above.

The Unified Model of Tone separates two things the hip literature keeps merging. The manuscript holds that "most standard tests measure quantity rather than organization". It adds that "reference ranges describe populations, and a person can sit comfortably inside a population range while having drifted far from their own functional baseline".

The hip is the sharpest case of that in sport. Cam morphology prevalence estimates run from 5 to 75 percent, labral defects sit in 44 percent of people with no symptoms, and adductor strength in a healthy professional squad has a wide spread. Every one of those is a quantity, and none of them is a share.

Applied to this region, the model reads groin pain as a failure of proportion rather than a failure of tissue. A pelvis carries force by adjusting tension across the whole ring at once. A strength test reads one spoke. A ratio reads two. Neither reads the schedule the ring runs on.

That gives three ways for sharing to fail, and the literature already shows all three. One spoke can go weak, which is reduced adductor strength. The proportion between spokes can drift, which is the adduction to abduction ratio. A link can arrive late, which is transversus abdominis firing after its cue rather than before it.

The prediction this page makes

Groin screening today is a battery of quantities: how much force the adductors make, how far the hip rotates, how sore the pubis feels on palpation. The model says the deciding variables are a proportion and a schedule, and that both belong to the same regulatory state the rest of the athlete runs on.

The measurement fits inside a morning a program already runs. Record the eccentric adduction to abduction ratio against the reference value of 1.2, and adductor squeeze force in newtons per kilogram against 3.6. Add transversus abdominis onset latency on an active straight leg raise, and RMSSD at rest, in the same athletes on the same day.

One pairwise result already sits in the record. Eight weeks of a single adduction exercise raised eccentric abduction strength 20.3 percent in a muscle group that was never trained Ishøi 2016. Measures that were supposed to be independent moved together, which is what a shared underlying state produces.

The model predicts that this set tracks groin problems across a season better than adductor strength alone, because all four are readings of one organization. This is a claim about how a pelvis is regulated rather than a claim about what treatment does.

If the adduction to abduction ratio, adductor squeeze force, transversus abdominis onset latency, and RMSSD are shown to move together within the same athletes across a season, the unification claim is confirmed.

10The tone reading

Load, Coupling and Gain Across the Pelvic Ring

Three signatures of tone show up in how a pelvis shares load, each inside a measurement sports medicine already records.

The rest of the library carries the same logic through its other foundations. Constraint is a hip held rigid to feel safe, buying stability by spending the transitions a cut needs. Time course is the growth window in which cam morphology forms, after which prevalence and severity stop rising. Input quality is what a frayed labrum sends from the anterosuperior zone, which carries the densest receptor supply in the joint. Prediction is the feedforward contraction that should arrive before the leg lifts, and that arrived late in the players who hurt. Set point is the adduction to abduction ratio of 1.2 that a healthy professional squad holds. Oscillation is the rhythm of stride and plant that loads the ring thousands of times in a match. The full framework is set out in the Unified Model of Tone.

11Where this sits

How This Page Relates to the Rest of the Library

Seven places this argument continues, each with the claim that earns the link.

12Questions athletes ask

Questions Athletes Ask

13The sources

References

20 primary sources, each linked to its record. Figures quoted on this page were checked against the published abstract.