Acute Effects of LLLT on Pain Threshold and Pain Tolerance

Pain is defined by the International Association for the Study of Pain as "an unpleasant sensory and emotional experience associated with, or resembling that associated with, actual or potential tissue damage." Pain can be classified into several types: nociceptive pain, neuropathic pain, and nociplastic pain. Nociceptive pain results from actual or potential damage to non-neural tissue and is due to the activation of nociceptors. It is typically acute and arises from injury, inflammation, or mechanical stimuli. Neuropathic pain is caused by damage or disease affecting the somatosensory nervous system. Nociplastic pain involves altered nociception despite no clear evidence of tissue damage or nerve injury.

Acute nociceptive pain is one of the most common presentations in musculoskeletal conditions and plays a crucial protective role by alerting the body to tissue damage and prompting healing responses. However, when inadequately controlled, it may induce central sensitisation and predispose patients to chronic pain syndromes. To address acute pain non-pharmacologically, physiotherapists typically employ a multimodal approach, including Transcutaneous Electrical Nerve Stimulation (TENS), which harnesses sensory-level stimulation to activate spinal inhibitory circuits and dampen pain signals; therapeutic ultrasound (US), where mechanical vibration and mild thermal effects accelerate inflammatory resolution and collagen synthesis; cryotherapy and thermotherapy, which modulate local tissue temperature to reduce nociceptor excitability and optimize perfusion; and manual therapy, including joint mobilization, soft tissue techniques, and muscle energy methods, that elicit both mechanical and neurophysiological analgesia, improving range of motion and function.

More recently, laser therapy has emerged as a powerful adjunct for acute pain relief. Laser therapy refers to the medical use of specific light wavelengths to elicit therapeutic effects in tissues. Broadly, there are two main types used in physiotherapy: High-Intensity Laser Therapy (HILT) and Low-Level Laser Therapy (LLLT). HILT operates at higher power outputs and can penetrate deeper tissues, producing photothermal effects. In contrast, LLLT, also termed photobiomodulation or cold laser therapy, uses lower power levels and longer exposure times, primarily inducing photochemical and photobiological changes without significant heating. LLLT is favoured for its safety and gentle stimulation of tissue repair across a wide range of conditions.

LLLT involves the application of red or near-infrared light, typically between 600 and 1100 nm, at low intensities to avoid thermal effects. It has been widely adopted in clinical practice due to its non-invasive nature, minimal side effects, and effectiveness in reducing pain, promoting wound healing, and modulating inflammation.

The mechanisms underlying LLLT are thought to involve the absorption of photons by cellular chromophores, particularly cytochrome c oxidase in the mitochondria. This enhances mitochondrial respiration and ATP production, modulates reactive oxygen species, and induces transcription factors that promote cell survival and tissue repair. These effects may also extend to modulation of the autonomic nervous system, where LLLT has been shown to promote parasympathetic activity and reduce sympathetic dominance, contributing to analgesic and anti-inflammatory outcomes.

In clinical research, participants' expectations can influence perceived outcomes, a phenomenon known as the placebo effect. Placebo responses can elicit real neurophysiological changes, including activation of endogenous opioid and dopamine pathways and modulation of pain-related brain regions . To differentiate the specific effects of LLLT from expectancy-driven improvements, a sham-controlled design is essential, where participants undergo an identical procedure without therapeutic laser emission.

Despite the growing use of LLLT for pain management, there remains a research gap regarding its immediate effects on pain perception in healthy individuals. Much of the existing evidence focuses on chronic pain populations or multiple-session interventions. It remains unclear whether a single application of LLLT can immediately alter pain threshold or pain tolerance. Pain threshold is the lowest intensity at which a stimulus is perceived as painful, while pain tolerance is the maximum intensity an individual can endure. These measures provide complementary and objective indices of analgesic effects.

This study focuses on nociceptive pain in healthy adults, using standardised external stimulation to assess changes in pain threshold and pain tolerance following a single session of LLLT compared to a sham control.

Aim:

To assess the immediate effect of a single session of Low-Level Laser Therapy (LLLT) versus sham control on pain threshold and pain tolerance in healthy adults.

Inclusion criteria

• Age between 18 and 35 years
• Healthy individuals without chronic medical conditions
• Free from chronic or acute pain
• All genders
• Able to provide informed consent

Exclusion criteria

• Menstruation at the time of testing
• Sensory deficits (failure to pass sharp-dull test)
• Pregnancy
• History of epilepsy
• Presence of cardiovascular conditions (including pacemaker)
• Acute hand injuries
• Diagnosed neurological disorders
• Use of analgesics or NSAIDs within the previous 48 hours
• Contraindications to low-level laser therapy or electrical stimulation (e.g., light sensitivity, malignancy)