The Achilles Stiffness Edge: Promoting Young’s Modulus in Power and Endurance Athletes with Dry Needling & Electrical Stimulation

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Introduction

Achilles tendon health is crucial for both performance and injury prevention in athletes. The Achilles tendon, connects the calf muscles to the heel bone, and is subjected to significant stress, particularly in power and endurance athletes due to the respective volume and magnitude required to push away from the Earth. Recent technique advances in dry needling (DN) and electrical stimulation (ES) have shown promising results in enhancing tendon properties such as, stiffness and elasticity, through stimulating the tissue’s natural piezoelectric and topographical reorganization. This blog explores some of the scientific principles behind these interventions, compares Achilles tendon properties in different types of athletes, and provides evidence-based protocols for clinicians, coaches, and athletes interested in preventing Achilles injuries and promoting optimal stiffness and elasticity respective to sport demands.

Scientific Definitions and Concepts

Piezoelectric Properties: In regards to biological material, piezoelectric properties refer to the ability of certain tissues, including tendons, collagen, and bone, to generate an electric charge in response to mechanical stress. This phenomenon is significant in tendons and collagen because it helps in energy dissipation and load transfer (Gautschi, 2002). For instance, when a tendon is pulled under tension, it’s natural piezoelectric properties store energy and tend to pull back in a similar fashion to spring.

Young’s Modulus: Young’s modulus is a measure of the stiffness of a material. In tendons, it reflects their ability to resist deformation under load. Higher Young’s modulus values indicate stiffer tendons, which are essential for effective force transmission (Kubo et al., 2010; Maganaris et al., 2002). This can be seen in power athletes with a larger aggregate Type I collagen fibers in the Achilles when compared with the general public and recreational athletes. 

Topographical Reorganization: Topographical reorganization is the structural adaptation of tissues in response to mechanical loading, tension, or changes in their environment. In tendons and collagen, this involves the alignment and realignment of fibers to better handle applied forces and improve mechanical function (Fratzl & Collagen, 2008; Scherer et al., 2014). We see these changes in the thickness of tendons and the bony prominences in bones, such as the ischial tuberosity where the hamstrings pull from, during hip extension; and the posterior calcaneal tuberosity where the Achilles pulls from, during plantar flexion. 

Dry Needling (DN): In short, dry needling is a technique involving the insertion of fine, acupuncture needles into specific locations in muscles, tendons, and tissues to relieve pain and improve function. The goal is to stimulate a local twitch response, helping to release muscle tightness and enhance blood flow to the targeted area (Dommerholt et al., 2013; Kietrys et al., 2013). DN can effectively reduce pain and improve muscle activation, which is beneficial for athletic performance (Fernandez-de-las-Penas et al., 2014) and enhanced nutrition and oxygen drop off at the targeted area.

Acupuncture: Acupuncture is a traditional Chinese medicine practice that involves inserting needles into specific points on the body to balance the body’s energy flow or “Qi.” It is used to treat various conditions, including musculoskeletal pain and tendon issues (Zhang et al., 2018; Vickers et al., 2018). Acupuncture influences the release of neurochemicals that modulate pain perception and promote healing (Han, 2011).

Electrical Stimulation (ES): Electrical stimulation uses electrical currents to stimulate nerves and muscles, enhancing blood flow, reducing pain, and promoting healing in injured tissues. Different frequencies and intensities of ES target various aspects of muscle and tendon recovery (Shen et al., 2022; Baker et al., 2017). ES increases the production of growth factors essential for tendon repair and regeneration (Cui et al., 2019).

Flexural Modulus: This measures a material’s ability to resist bending deformation. For tendons, it reflects flexibility and resistance to bending forces, crucial for activities involving significant limb movements (Zhang et al., 2020; Kjaer et al., 2006), such as the repetitive plyometric demands of long distance running. 

Creep: Creep is the tendency to deform gradually under constant stress. Understanding creep behavior helps in designing rehabilitation protocols to prevent overstretching and ensure proper tendon function (Kjaer et al., 2006; Wang et al., 2013).

Stiffness: Stiffness is the resistance to deformation. In tendons, stiffness is crucial for efficient force transmission from muscles to bones, affecting athletic performance and injury risk (Muraoka et al., 2005; Kubo et al., 2010).

Types of Collagen: Collagen is a primary structural protein found in connective tissues. The most common types are:

  • Type I Collagen: The most abundant collagen in tendons and bones, providing tensile strength (Kjaer, 2004).
  • Type III Collagen: Present in early wound healing and in more flexible tissues, contributing to elasticity and tissue pliability (Gelse et al., 2003).

Definitions of Neurotrophic and Growth Factors

Nerve Growth Factor (NGF): NGF is a neurotrophic factor involved in the growth, maintenance, and survival of neurons. It promotes the regeneration of peripheral nerves and can reduce pain by modulating nerve sensitivity (Skaper, 2012).

Brain-Derived Neurotrophic Factor (BDNF): BDNF is a neurotrophic factor that supports the survival and differentiation of neurons in the central nervous system. It is involved in pain modulation and can be influenced by physical activity and electrical stimulation (Liu et al., 2012).

Platelet-Derived Growth Factor (PDGF): PDGF is a growth factor that plays a role in cell proliferation and wound healing. It is critical for tendon repair and regeneration by promoting collagen synthesis and tissue remodeling (Raines, 2004).

Transforming Growth Factor Beta (TGF-β):  TGF-β is a growth factor involved in tissue repair and fibrosis. It regulates collagen production and matrix remodeling, which is essential for tendon healing and adaptation (Kang et al., 2017).

Comparing Achilles Tendon Properties in Power vs. Endurance Athletes

Differences in Collagen Composition: In regards to collagen, form follows function. Power athletes tend to have a higher proportion of Type I collagen, due to the demands of explosive activity in their sport. The type I collagen contributes to the increased stiffness and strength required for said explosive movements. In contrast, endurance athletes often have a higher ratio of Type III collagen, which provides greater elasticity and the ability to withstand repetitive, lower-intensity loads (Kjaer, 2004).

Differences in Stiffness and Young’s Modulus: Studies have shown that power athletes tend to have stiffer Achilles tendons with higher Young’s modulus values compared to endurance athletes (Muraoka et al., 2005; Kubo et al., 2010; Couppe et al., 2008). This increased stiffness allows for more efficient force transmission during explosive activities such as sprinting, and jumping, and power lifting. Conversely, endurance athletes often have more compliant tendons, which can absorb and release energy more efficiently during prolonged activities such as running and cycling (Muraoka et al., 2005; Arampatzis et al., 2007).

Impact on Performance and Injury Risk: Stiffer tendons in power athletes contribute to enhanced performance by minimizing energy loss during rapid movements (Kubo et al., 2010). However, they may also be at a higher risk for acute injuries such as tendon ruptures due to the high forces involved (Muraoka et al., 2005). On the other hand, more compliant tendons in endurance athletes can help in reducing repetitive strain injuries but may compromise performance in high-intensity activities (Arampatzis et al., 2007).

Impact of Dry Needling and Electrical Stimulation on Tendon Health

Alteration of Piezoelectric Properties and Topographical Reorganization:

  • Enhanced Charge Generation: Electrical stimulation enhances the piezoelectric response of collagen and tendons by influencing collagen fiber alignment. The application of an external electrical field helps align collagen fibers more effectively, which in turn increases the piezoelectric charge generated when mechanical stress is applied (Magar et al., 2004; Zhang et al., 2016). This realignment is a form of topographical reorganization, where the arrangement of collagen fibers adapts to better handle mechanical loads imposed on the tendon. As mentioned before, you can see this at the ishial tuberosity, and posterior calcaneal tuberosity due to the tensile forces from the hamstrings and Achilles respectively. 
  • Alignment of Collagen Fibers: Electrical stimulation affects the orientation and alignment of collagen fibers, a key aspect of topographical reorganization. Proper alignment is crucial for optimizing piezoelectric effects and enhancing mechanical strength. By promoting the organization of collagen fibers into a more structured arrangement, electrical stimulation contributes to improved piezoelectric properties and overall tendon function (Chen et al., 2011). This process helps in achieving an efficient Young’s modulus that is crucial for both power and endurance athletes.

Impact on Healing and Repair:

  • Promotion of Cellular Activity: Electrical stimulation with dry needling boosts cellular activities such as fibroblast proliferation and collagen synthesis. These processes are essential for the production and organization of collagen fibers. Enhanced cellular activity supports topographical reorganization by ensuring that collagen fibers are realigned in response to mechanical stress, which improves the piezoelectric properties and mechanical properties of tendons (Morrison & Williams, 2004). For power athletes, this means increased tendon stiffness and efficient force transmission, thus increased Young’s Modulus. For endurance athletes, it leads to improved tendon elasticity and resilience, thus increased Young’s Modulus and Flexural Modulus. 
  • Tissue Remodeling: Electrical stimulation with dry needling aids in tissue remodeling by altering the mechanical and electrical environment of tendons. This remodeling is a key aspect of topographical reorganization, where tendon tissues adapt structurally to improve function/transfer of energy under tension. Improved collagen alignment and density through stimulation contribute to enhanced piezoelectric response and mechanical function of tendons (Gur et al., 2008). This adaptation helps power athletes achieve greater stiffness and endurance athletes gain better elasticity, thus optimizing their performance in each respective sport type.

Effects on Tendon Strength and Function:

  • Improved Mechanical Properties: The changes induced by electrical stimulation and topographical reorganization lead to improvements in tendon strength and elasticity. Enhanced piezoelectric properties contribute to better energy dissipation and force transmission through the tendon. For power athletes, this results in increased tendon stiffness (Young’s Modulus), allowing for more effective force transmission during high-intensity activities (Chen et al., 2011). Endurance athletes benefit from improved tendon elasticity, which helps in absorbing and releasing energy efficiently during prolonged activities.
  • Reduction in Scar Tissue: Electrical stimulation with dry needling can help reduce scar tissue formation and promote more organized collagen deposition. This organized deposition is a form of topographical reorganization that enhances the piezoelectric response and functional performance of tendons. Reduced scar tissue improves the structural integrity of the tendon, allowing for dissipation of loads through tensile strength and properties, which is crucial for both power and endurance athletes (Magar et al., 2004). Power athletes experience improved force transmission, while endurance athletes benefit from increased flexibility, elasticity, and reduced injury risk.

Neurotrophic Growth Factors:

  • Role in Healing and Adaptation: Neurotrophic growth factors such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) play a crucial role in tendon healing and adaptation. These factors are involved in nerve regeneration, inflammation regulation, and collagen synthesis, contributing to the overall repair process (Frostick et al., 1998).
  • Stimulation Effects: Electrical stimulation with dry needling can increase the expression of neurotrophic growth factors, enhancing their beneficial effects on tendon healing and adaptation. This can lead to improved nerve function, reduced pain, and better structural integrity of the tendon (Chen et al., 2011).

Considerations for Electrical Stimulation Parameters:

  • Frequency: The frequency of electrical stimulation affects how tendons respond. Low frequencies (1-10 Hz) promote collagen synthesis, while medium frequencies (20-50 Hz) enhance mechanical properties. Optimal frequency settings facilitate effective topographical reorganization and improve piezoelectric properties and repair processes (Johnson & Sisk, 2014). For power athletes, frequencies that promote stiffness are beneficial, while endurance athletes may benefit from frequencies that enhance elasticity.
  • Pulse Width: Medium pulse widths (200-400 µs) are often used to penetrate and stimulate tendon tissues effectively without causing excessive discomfort. Proper pulse width settings support topographical reorganization by ensuring deep tissue stimulation and alignment (Morrison & Williams, 2004).
  • Duration and Intensity: The duration and intensity of electrical stimulation should be tailored to individual responses and therapeutic goals. Overuse or excessive intensity might cause adverse effects or discomfort. For power athletes, higher intensities might be used to promote stiffness, while endurance athletes may require moderate intensities to enhance flexibility (Gur et al., 2008). The right balance supports effective topographical reorganization and optimal tendon performance.

Best Practices and Protocols

For Uninjured Athletes

Power Athletes:

  • Exercise Regimen: Emphasis on eccentric and isometric exercises to increase tendon stiffness and Young’s modulus (Kubo et al., 2010). Examples include controlled heel drops through full available range of motion against load to maximize tensile forces at the bottom 1/4 – 1/3 of the movement. 
  • Dry Needling with Electrical Stimulation: Needles placed along the tendon with low-frequency (2 Hz) stimulation to enhance collagen alignment and promote healing (Shen et al., 2022).
  • Frequency and Duration: Three sessions per week for 4-6 weeks, with each session lasting about 30 minutes (Cui et al., 2019).

Endurance Athletes:

  • Exercise Regimen: Focus on moderate-intensity, high-repetition exercises to improve tendon elasticity and prevent overuse injuries (Muraoka et al., 2005). Activities such as long-distance running at a steady pace and high-repetition, low-weight calf raises are recommended.
  • Dry Needling with Electrical Stimulation: Similar needle placement with moderate-frequency (10 Hz) stimulation to balance collagen synthesis and alignment (Urabe et al., 2023).
  • Frequency and Duration: Two sessions per week for 6-8 weeks, each session lasting about 20 minutes (Ziying et al., 2018).

For Injured Athletes Recovering from Tendonopathy

Rehabilitation Exercises and Protocols:

  • Power Athletes: Incorporate progressive loading exercises starting with isometric contractions and advancing to eccentric and plyometric exercises as pain decreases (Kjaer et al., 2006; Silbernagel et al., 2007). This may include exercises like single-leg calf raises and jump training.
  • Endurance Athletes: Use a combination of low-load, high-repetition exercises to maintain tendon elasticity and gradually reintroduce running or cycling activities (Zhang et al., 2020). Rehabilitation can start with water-based exercises to reduce load on the tendon before progressing to land-based activities.

Dry Needling and Electrical Stimulation Protocols:

  • Needle Placements and ES Settings: For both power and endurance athletes, needles should be placed along the Achilles tendon, focusing on tender points and areas of thickening. Use low-frequency (2-10 Hz) ES to stimulate collagen production and alignment (Urabe et al., 2023).
  • Frequency and Duration: Two to three sessions per week for 6-8 weeks, with each session lasting 20-30 minutes (Cui et al., 2019; Ziying et al., 2018). Adjust based on individual tolerance and progress.

Conclusion

The integration of dry needling and electrical stimulation offers a promising approach to enhancing tendon health, particularly in optimizing Young’s modulus for both power and endurance athletes. By improving piezoelectric properties, collagen alignment, and overall tendon function, these interventions can help athletes achieve better performance and reduce the risk of injuries. Adapting these techniques to the specific needs of power versus endurance athletes and tailoring treatment protocols accordingly can lead to significant improvements in tendon health and athletic performance.

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