Can Ultrasound Guide Return-to-Play? A Deep Dive into Muscle Architecture After Strain

Muscle strain injuries are among the most common issues seen across sport. They sideline sprinters, footballers, rugby players, endurance athletes, and weekend warriors alike. Clinicians often focus on restoring strength, reducing symptoms, and progressing running loads. But in recent years, advanced imaging and access to especially musculoskeletal ultrasound has revealed a more complicated truth:

Large-scale injury surveillance data from professional soccer highlight the magnitude of muscle strain injuries in elite sport.

Injury Incidence in Professional Soccer: A 6-Year MLS Cohort Study

Muscles don’t simply “heal back to normal.” They remodel.

And ultrasound is the only tool that allows us to see that remodeling in real time.

Between 2020 and 2025, several high-impact studies have changed how we think about recovery after muscle strain. They show that even when an athlete feels 100% pain-free, strong, fast structural changes deep within the muscle–tendon unit may persist for months or years.

This blog breaks down what modern research tells us, and more importantly, how ultrasound should (and shouldn’t) be used in return-to-play (RTP) decisions.

1. Muscle Strains Leave Long-Term Architectural Footprints

Historically, we viewed muscle strains as soft-tissue injuries that resolve fully with time, load management, and strengthening. But a series of studies out of Copenhagen led by researchers like Bayer, Nielsen, Højfeldt, and Kjaer have shown that the aftermath is far more nuanced.

What changes persist long-term?

Scar tissue at the myotendinous junction (MTJ)

Ultrasound routinely shows a hyperechoic, compacted region at the MTJ, even long after symptoms resolve. This scar represents altered collagen organization and mechanical continuity.

Persistent Aponeurotic Remodeling After Muscle Strain Injury

Shorter fascicle lengths	Altered pennation angle and muscle thickness	Aponeurosis thickening or waviness	Persistent abnormalities despite good strength
Hamstring studies (Timmins 2015; Porter 2012) showed that previously injured biceps femoris fascicles may remain significantly shorter than the uninjured side. Nielsen et al. (2023) later confirmed this pattern in the calf. Shorter fascicles = less tissue available to absorb lengthening forces → higher strain on the connective tissue interface.	Fascicles may insert at steeper angles post-injury. This reflects remodeling rather than true “normalization.”	One of the breakthrough findings (Nielsen 2023) was that the aponeurosis previously considered a passive sheet can remain thicker, irregular, or more curved after injury. This is important because the aponeurosis plays a major role in force transmission and muscle gearing. Shape changes may influence how the entire muscle behaves under load.	The most important takeaway from Bayer et al. (2021): Even after heavy resistance training, these structural changes do not fully normalize, despite excellent functional improvements.

This is why ultrasound is so valuable:

It shows us the architecture athletes compete with, not just their symptoms or strength.

2. So What Can Ultrasound Truly Measure After a Strain?

Ultrasound gives a uniquely detailed look at how tissue remodels something MRI cannot capture dynamically.

A. Morphology (Structure)Ultrasound can quantify:

• Scar tissue

• Fiber disruption vs continuity

• Muscle thickness

• Fascicle orientation

• Pennation angle

• Aponeurosis structure

B. Architecture

This is where ultrasound shines compared to MRI.

We can measure:

• Fascicle length (BFlh, GM, soleus most reliable)

• Pennation angle changes

• Relative thickening of the central tendon or aponeurosis

• Side-to-side asymmetries

This is the layer of injury we often cannot see clinically.

Ultrasound Measurement of Muscle Architecture: Thickness, Pennation Angle, and Fascicle Length

C. Dynamic Behavior

Dynamic ultrasound allows real-time assessment of:

• Scar deformation during contraction

• MTJ motion

• Tendon elongation

• Aponeurosis sliding

• Force coupling between muscle compartments

In athletes returning to high-speed running, this dynamic information matters.

For example:

A scar that appears stable at rest may gap or deform abnormally under load → a sign to delay sprint exposure or adjust loading progressions.

But Should Ultrasound Be Used as a Return-to-Play Gatekeeper?

This is the most common question I get when teaching RMSK courses or consulting with sports teams:

“Should RTP decisions require a normal-looking ultrasound?”

No. And the evidence strongly supports this.

Here’s why.

A. Imaging rarely normalizes even in athletes who never reinjure

In Bayer’s 2021 randomized controlled trial:

• Athletes got stronger

• Functional tests improved

• Pain resolved

• They returned to sport

But:

• Fascicle length did not fully normalize

• Scar morphology persisted

• Aponeurosis shape remained altered

If we demanded a “clean” ultrasound before RTP,

no one would return on time.

B. Early rehab accelerates RTP even though imaging still looks abnormal

Multiple studies (Bayer 2017, 2018) show:

• Early loading (day 2)

→ faster RTP

→ no increase in reinjury

→ despite persistent abnormalities on US/MRI at time of RTP

Imaging ≠ readiness.

Readiness = load tolerance.

C. Studies show poor correlation between residual imaging findings and reinjury

Narrative reviews (Wulff 2024; Højfeldt 2025) highlight that:

• Residual edema

• Persistent scar

• Aponeurosis thickening

do not reliably predict reinjury on their own.

This is why ultrasound should inform not dictate RTP decisions.

The Role of Ultrasound Across the Return-to-Play Timeline

RTP Phase	Ultrasound Priorities	What to Look For	How It Guides Progression
Phase 1  - Early Injury Diagnose & Classify	• Establish injury pattern • Identify risk features • Provide prognosis	• Injury location (proximal/distal) • Length of involvement • MTJ vs myofascial pattern • Central tendon involvement • Hematoma or fluid pocket	→ Shapes prognosis from day one→ Informs early loading rules and timelines→ Identifies high-risk patterns (e.g., central tendon, proximal MTJ)
Phase 2 - Mid-Rehab Monitoring. (Week 2–4)	• Assess tissue healing • Confirm mechanical continuity • Detect complications	• Scar bridging present? • Hematoma resolution • Fiber continuity • MTJ behavior under light contraction	If dynamic US shows stable continuity:→ Progress high-speed running→ Introduce long-length eccentrics→ Add plyometrics and multidirectionals
Phase 3 - Pre-RTP Context Check	• Evaluate tissue readiness • Identify residual risk • Integrate architecture with performance	1. Scar maturity • Narrow, compact, hyperechoic = stable • Wide, heterogeneous, fluid-filled = caution 2. Dynamic behavior • Smooth aponeurosis/MTJ deformation • No gapping • No paradoxical movement 3. Architecture • Fascicle length reasonably symmetric • Pennation angle stable • Tendon not excessively elongated	→ Ultrasound provides context, not clearance→ Identifies tissue that is adapted, not “normal”→ Guides communication with coaches/athletes→ Helps tailor final progression (e.g., eccentric dose, sprint exposures)
Performance Integration (Final Decision Layer)	• Align imaging with functional capacity	• Eccentric strength • Isokinetic ratios • High-speed running volumes achieved • Repeated sprint ability • Athlete confidence and symptom response	→ RTP is based on load tolerance + functional readiness→ Ultrasound findings support—not override—the decision

5. The Real Takeaway for Sports Medicine in 2025

Muscle strains don’t simply heal they adapt.

They remodel their internal architecture, scar, and force pathways.

Ultrasound lets us see those adaptations.

But imaging does not decide readiness.

Load tolerance does.

The most modern, evidence-based perspective is:

Use ultrasound to understand the tissue,

not to disqualify the athlete.

This gives clinicians a clearer picture, improves communication with coaching staff, and protects athletes without delaying return unnecessarily.

In the high-speed, congested world of modern sport, that’s exactly what we need.

REFERENCES

Muscle Architecture & Chronic Sequelae

1. Nielsen M, Svensson RB, Fredskild M, et al. Chronic changes in muscle architecture and aponeurosis structure following calf muscle strain injuries. Scand J Med Sci Sports. 2023.

2. Bayer ML, Høegberget-Kalisz M, Svensson RB, et al. Chronic sequelae after muscle strain injuries: influence of heavy resistance training. Am J Sports Med. 2021.

Early vs Delayed Rehab

3. Bayer ML, Magnusson SP, et al. Early versus delayed rehabilitation after acute muscle injury. N Engl J Med. 2017.

4. Bayer ML et al. Tissue perfusion, strength recovery, and pain during rehabilitation after acute muscle strain injury. Scand J Med Sci Sports. 2018.

Fascicle Length & Injury Risk

5. Porter K et al. Biceps femoris fascicle length is shorter in previously injured athletes. J Sci Med Sport. 2012.

6. Timmins RG et al. Strength and architectural risk factors for hamstring strain injury. J Sci Med Sport. 2015.

Aponeurosis Mechanics

7. Hulm J, et al. Structure, function, and adaptation of lower-limb aponeuroses. Sports Med Open. 2024.

8. Borsdorf A, et al. Influence of muscle length on three-dimensional muscle architecture and aponeurosis strain. J Appl Physiol. 2024.

Reviews & RTS Synthesis

9. Wulff I et al. Return to sport, reinjury, and tissue changes after muscle strain injuries. 2024.

10. Højfeldt M et al. The repair capacity spectrum of human skeletal muscle injury. 2025.