Groin Injury and Hip Adductor Strength: What the Research Shows

Groin injuries account for 10–18% of all time-loss injuries in professional soccer — making them the most prevalent lower-extremity muscle injury in the sport, ahead of hamstring strains (Ekstrand et al., 2011, UEFA Elite Club Injury Study). In ice hockey, field hockey, and Australian Rules football the proportions are similar. What makes these injuries particularly costly is their stubborn recurrence rate: adductor strains re-injure at 14–17% within the same season when players return before adequate strength recovery. The research is unambiguous — hip adductor strength deficit is the single most modifiable risk factor, and the evidence for targeted adductor loading as prevention is among the strongest in sports injury science.

Epidemiology: How Common Are Groin Injuries?

The UEFA Elite Club Injury Study, running across 51 European clubs over 17 seasons, recorded 2,275 groin injuries resulting in more than 17,000 missed training days. Mean absence per injury is 12–23 days for adductor strains, but athletic pubalgia — the chronic overuse variant — can sideline athletes for 3–6 months. Men are disproportionately affected at a 3:1 ratio compared to women, likely because male adductors operate closer to their structural load limits in explosive lateral-change-of-direction tasks.

Seasonal distribution is not random. Injury rates spike in the first 4 weeks of pre-season — when training load rises sharply but adductor conditioning lags — and again mid-season around weeks 20–24 when cumulative fatigue is highest. This time-clustering provides a direct argument for frontloading adductor training before the pre-season load spike begins.

Adductor Mechanics and Injury Mechanisms

The hip adductor group — adductor longus, brevis, magnus, gracilis, and pectineus — spans both hip flexion-extension and the hip adduction plane. Adductor longus is the most frequently strained, in part because it has a proportionally small cross-sectional area relative to the force demands placed on it during maximal stride, kick, or change-of-direction tasks.

Injury typically occurs in one of two kinematic scenarios:

• Eccentric overload during stride: At toe-off, the contralateral adductor undergoes a rapid high-force eccentric contraction to stabilise the pelvis. If eccentric adductor strength is insufficient — particularly in the lengthened range beyond 20° abduction — the muscle-tendon unit exceeds its load tolerance.
• Isometric-to-eccentric transition in cutting: During lateral cuts, the adductor switches from isometric stabilisation to rapid eccentric loading within <100 ms. This fast eccentric demand requires both high peak force capacity and high rate of force development (RFD) — qualities that standard bilateral adductor squeezes do not train adequately.

Neuroimaging studies (MRI grading) show that >85% of acute adductor strains occur at the proximal myotendinous junction of adductor longus — precisely the region loaded in maximal-length eccentric positions, supporting the prescription of exercises that stress the lengthened range.

Adductor-to-Abductor Ratio: What the Numbers Mean

The most widely cited risk threshold comes from Tyler et al. (2001), who prospectively followed 47 professional ice hockey players and found that athletes with an adductor-to-abductor strength ratio below 0.80 (measured isometrically) were 17 times more likely to sustain a groin strain during the subsequent season. This finding has been replicated in soccer cohorts (Engebretsen et al., 2010) with a similar odds ratio of 4.4 for athletes below 0.85.

Note: these ratios are derived from isokinetic dynamometry at 60°/s. Handheld dynamometry field testing yields systematically lower absolute values but preserves the relative threshold framework if testing conditions are standardised.

Copenhagen Adduction: The Research Case

The Copenhagen Adduction Exercise (CAE) — a partner-assisted lateral plank that loads the adductor eccentrically in the lengthened range — has accumulated the strongest evidence base of any groin injury prevention exercise. Harøy et al. (2019) conducted a cluster-randomised controlled trial across 35 Norwegian soccer teams (n = 461 players) and found that a 16-week CAE protocol reduced groin injury incidence by 41% compared to controls. This is comparable in magnitude to the Nordic Hamstring Exercise's effect on hamstring injuries.

What makes CAE mechanistically sound is that it specifically trains the proximal adductor longus in the eccentric-lengthened range — the exact tissue and loading condition responsible for >85% of strains. Isometric adductor squeeze variations, by contrast, train the muscle in a shortened position that produces minimal protective benefit against stride-length eccentric demands.

Progression of CAE involves four stages: (1) isometric holds, (2) limited range eccentric lowering with partner assistance, (3) full range unassisted lowering, (4) loaded full range. Moving through all four stages over 6–8 weeks ensures progressive tissue stress without exceeding recovery capacity.

Evidence-Based Prevention Protocol

The following protocol consolidates the Tyler et al. pre-season loading study (2002) and the Harøy et al. CAE programme into a practical in-season structure:

Dosing the Copenhagen Adduction Exercise

Begin with Stage 1 isometric holds: 3 sets × 8 s holds, progressing to 3 × 5 reps lowering (3 s eccentric) at Stage 2–3, and 3 × 8 full reps at Stage 4. Research suggests the adductor longus proximal tendon adapts structurally (increased tendon stiffness on ultrasound) after 8 weeks of progressive CAE, which is the structural target underlying injury-rate reduction.

Return-to-Sport Criteria After Groin Injury

Premature return is the primary driver of the 14–17% same-season recurrence rate. Published return-to-sport criteria from the Doha Agreement on groin pain (Weir et al., 2015) include three pillars:

1. Pain-free resisted hip adduction at 0° and 45°: Tested isometrically with handheld dynamometer; must match the uninjured side within 10%.
2. Full pain-free range of motion: Hip abduction ≥40° with no end-range pain; hip flexion ≥110°.
3. Sport-specific functional tests: Ability to perform single-leg hop, side-step, and running direction changes at ≥90% of pre-injury speed without pain.

A composite score combining these three domains predicts successful return with 78% sensitivity in soccer and hockey populations. Adductor:abductor ratio should reach ≥0.80 before full training resumption; ratio ≥0.90 before competitive match play.

Monitoring Adductor Strength In Season

Longitudinal monitoring catches fatigue-driven strength deficits before they become injuries. The practical monitoring menu:

• Handheld dynamometry (HHD) squeeze test: 30-second bilateral isometric squeeze at 45° hip flexion, 0° abduction; record N and compute ratio. Retest every 3–4 weeks in season.
• Adductor squeeze pain VAS: Self-reported pain 0–10 during maximal squeeze. Score ≥3 triggers a 48 h rest and reassessment.
• Single-leg CMJ asymmetry via IMU: Bilateral jump height asymmetry >10% correlates with adductor loading asymmetry in soccer players (Bishop et al., 2021), providing an indirect weekly screen without tissue-specific testing.

Combining HHD every month with weekly CMJ asymmetry via PoinT GO gives a practical two-tier monitoring system: the fast proxy (jump asymmetry) flags sessions requiring reduced loading; the precise proxy (HHD) tracks true structural adaptation over weeks.