Photobiomodulation on NMES-Induced Muscle Fatigue and Damage

This study utilizes a randomized, single-blind, sham-controlled crossover design to evaluate the acute effects of photobiomodulation therapy (PBMT) on neuromuscular electrical stimulation (NMES)-induced fatigue in healthy individuals.

Participants undergo testing across two intervention blocks separated by a 7-day washout period between limbs:

Intervention Phase: Five minutes prior to an electrically evoked fatigue protocol, participants receive either active PBMT (multi-diode GaAlAs laser cluster; 810 nm; 200 mW per diode; 30 J per point across 8 quadriceps sites; total energy dose: 240 J) or sham PBMT (identical positioning with the device inactive and acoustic blinding).

NMES Fatigue Protocol: The protocol is performed on an isokinetic dynamometer with the knee fixed at 90° flexion. Rectangular biphasic symmetrical pulses (100 Hz; 1 ms pulse width; 5 s on, 10 s off) are delivered over 20 minutes to evoke 60 isometric contractions adjusted to an initial torque of 20% maximal voluntary isometric contraction (MVIC).

Evaluation Timeline: Neuromuscular and functional assessments are conducted at Baseline, Immediately Post-Fatigue, and at 24, 48, and 72 hours post-protocol. Tests include maximal voluntary isometric torque, resting supramaximal evoked twitches, transverse ultrasound echointensity of the rectus femoris and vastus lateralis, pressure pain thresholds via algometry, Visual Analog Scales for discomfort, and horizontal Single Hop Test performance.

• Photobiomodulation Therapy (PBMT) (Device): Multi-diode cluster probe (GaAlAs laser; 5 diodes; wavelength: 810 nm; power output: 200 mW per diode; continuous mode) applied across 8 points over the quadriceps femoris (3 on rectus femoris, 3 on vastus lateralis, and 2 on vastus medialis) for 30 seconds per point (30 J/point), delivering a total energy dose of 240 J per limb.
• Sham Photobiomodulation (Device): Identical probe positioning over the 8 quadriceps locations for 30 seconds per point with the laser device completely turned off. Auditory headphones are used to blind participants against sound cues.

Primary outcomes

• Maximal Voluntary Isometric Contraction (MVIC) Torque [Baseline, Immediately post-fatigue protocol, 24 hours, 48 hours, and 72 hours post-fatigue.]: Knee extensor peak isometric torque measured on an isokinetic dynamometer (Biodex System 3 Pro) during 5-second maximal isometric contractions with the knee fixed at 90° flexion, expressed in Newton-meters (N.m).

Secondary outcomes

• Muscle Damage via Ultrasound Echointensity [Baseline, Immediately post-fatigue protocol, 24 hours, 48 hours, and 72 hours post-fatigue.]: Quantitative grayscale histogram analysis (0 = black, 255 = white) of transverse ultrasound images (ImageJ software) of the rectus femoris (at 40%, 50%, and 60% of the femur length) and vastus lateralis (at 50% of the femur length) muscles to assess structural muscle damage.
• Supramaximal Evoked Twitch Torque [Baseline, Immediately post-fatigue protocol, 24 hours, 48 hours, and 72 hours post-fatigue.]: Knee extensor evoked torque produced by single supramaximal electrical twitches (1 Hz, 500 µs rectangular biphasic pulses at rest) delivered to the rectus femoris motor point, measured in Newton-meters (N.m) on the isokinetic dynamometer to evaluate peripheral muscle fatigue.
• Evoked Neuromuscular Efficiency [Baseline, Immediately post-fatigue protocol, 24 hours, 48 hours, and 72 hours post-fatigue.]: Ratio between the supramaximal evoked twitch peak torque and the applied current intensity (Nm/mA) during resting single pulses.
• Sitting Muscle Discomfort [Baseline, Immediately post-fatigue protocol, 24 hours, 48 hours, and 72 hours post-fatigue.]: Subjective sensation of discomfort/pain in the quadriceps femoris evaluated while seated at rest using a 10 cm Visual Analog Scale (VAS), where 0 cm represents 'no discomfort' and 10 cm represents 'maximum discomfort'.
• Pain Pressure Threshold (PPT) [Baseline, Immediately post-fatigue protocol, 24 hours, 48 hours, and 72 hours post-fatigue.]: Mechanical pain sensitivity measured using a digital pressure algometer (applied at 90° in kgf/cm²) over the rectus femoris, vastus lateralis, and vastus medialis muscle bellies to identify delayed onset muscle soreness.
• Single Hop Test (SHT) Functional Performance [Baseline, Immediately post-fatigue protocol, 24 hours, 48 hours, and 72 hours post-fatigue.]: Maximal horizontal single-leg hop distance normalized to limb length, calculated as: \[Hop Distance (cm) / Limb Length (cm)\] * 100.
• Discomfort During Stair Ascent and Descent [Baseline, Immediately post-fatigue protocol, 24 hours, 48 hours, and 72 hours post-fatigue.]: Level of discomfort reported on a 10 cm Visual Analog Scale (VAS) immediately after ascending and descending a 10-step staircase.
• Total Mechanical Work During Fatigue Protocol [During the 20-minute NMES fatigue protocol.]: Cumulative mechanical work quantified by the sum of the torque-time integral (TTI) across all 60 evoked contractions during the 20-minute NMES protocol (expressed in N.m.s).
• Fatigue Index (% Work Decline) [During the 20-minute NMES fatigue protocol.]: Percentage reduction in work performed between the first 5 evoked contractions and the last 5 evoked contractions of the NMES fatigue protocol, calculated as: \[(Initial Work - Final Work) / Initial Work\] * 100.

Sex: All, ages: 18 Years to 40 Years, accepts healthy volunteers

Inclusion Criteria:

* Healthy male and female adults aged between 18 and 40 years.
* Engaging in regular physical activity (minimum of twice a week).
* No history of hospitalization for COVID-19 in the past 12 months.
* Demonstrated ability to tolerate neuromuscular electrical stimulation (NMES).
* Capable of fully understanding and complying with the experimental protocol and procedures.

Exclusion Criteria:

* Presence of any sensory impairments or neurological alterations.
* Ingestion of caffeine, coffee, energy drinks, or other stimulants in the 12 hours preceding the testing sessions.
* Body Mass Index (BMI) greater than 30 kg/m².
* Any medical contraindication to performing maximal physical exertion or presenting an abnormal cardiovascular/blood pressure response after maximal effort.

• Marco A Vaz, PhD, Study chair, Federal University of Rio Grande do Sul
• Marco A Vaz, PhD, Principal investigator, Federal University of Rio Grande do Sul