STUDY RATIONALE:
Delayed-onset muscle soreness (DOMS) develops 12-72 hours (h) after unaccustomed eccentric exercise and manifests as localized pain, swelling, and functional impairment. This is particularly significant for elite volleyball players, whose performance depends on explosive vertical jumping and sustained lower-body power. A single volleyball match may involve more than 100 maximal vertical jumps combined with rapid lateral movements, making the quadriceps, hamstrings, and gastrocnemius muscles susceptible to repeated eccentric loading during landing phases.
Transcutaneous electrical nerve stimulation (TENS) enhances local blood flow and activates large-diameter sensory fibers, which may accelerate metabolite clearance and modulate pain perception. Foam rolling (FR) applies bodyweight-driven compressive forces using a cylindrical device, with reported benefits including improved tissue compliance and reduced fascial restrictions. Despite growing interest in these modalities, direct head-to-head comparisons in elite female volleyball players are lacking.
STUDY DESIGN:
This was a prospective, randomized, controlled, parallel-group trial conducted at the Zahedan Olympic Village, Iran, between September and December 2020. The study employed a three-arm design comparing TENS, FR, and passive rest (control).
RANDOMIZATION AND ALLOCATION CONCEALMENT:
A researcher who was not involved in data collection generated the randomization sequence using computer software (Random Allocation Software, version 2.0). The sequence was generated using a simple randomization method with a 1:1:1 allocation ratio. Group assignments were placed in sequentially numbered, opaque, sealed envelopes, which participants opened only after completing all baseline assessments. This procedure ensured that neither participants nor investigators could predict group allocation prior to enrollment.
BLINDING:
Due to the nature of the interventions, participant blinding was not possible. However, the outcome assessor who conducted all functional performance tests and processed blood samples was blinded to group allocation throughout the study. Data analysis was performed by a statistician who was blinded to group codes until the analysis plan was finalized.
PARTICIPANT SELECTION:
Inclusion criteria:
Age 18-28 years At least five years of competitive volleyball experience Current participation in Iran's national Premier League Training frequency of at least five sessions per week Absence of musculoskeletal injury during the preceding six months
Exclusion criteria:
Use of anti-inflammatory medications or supplements Contraindications to TENS (cardiac pacemaker, epilepsy, pregnancy) Exposure to study interventions within the preceding four weeks Lower-body strength training within 72 hours prior to testing
SAMPLE SIZE:
Sample size was calculated a priori using G\*Power software (version 3.1.9.7). Based on a repeated-measures analysis of variance (ANOVA) design with three groups, four time points, an expected effect size of f = 0.28, alpha = 0.05, power = 0.80, correlation among repeated measures of 0.65, and nonsphericity correction of 0.89, a minimum of 30 participants (10 per group) was required.
EXERCISE PROTOCOL:
All participants completed an eccentric exercise protocol consisting of five sets of 15 eccentric-focused repetitions on a 45° angled leg press, with the load fixed at 110% of the concentric one-repetition maximum (1RM). A 4-second controlled eccentric phase was paced by a digital metronome. This protocol reliably induces DOMS that peaks within 24-48 hours.
TENS INTERVENTION:
TENS was applied to the quadriceps and hamstrings of the dominant leg using a clinical-grade stimulator. Four self-adhesive electrodes were positioned over the rectus femoris and biceps femoris. Stimulation parameters: biphasic symmetric waveform; frequency 120-150 Hertz (Hz); pulse width 100 microseconds (μs); intensity 10-30 milliamperes (mA) adjusted to produce strong, comfortable paresthesia without visible muscle contraction; duration 20 minutes per session.
FOAM ROLLING INTERVENTION:
Participants used a medium-density foam roller (15 centimeters \[cm\] diameter × 45 cm length) following a standardized bilateral sequence targeting the quadriceps, hamstrings, iliotibial band, gastrocnemius-soleus complex, and gluteal muscles. Each muscle group was rolled for 45 seconds per side. Three complete cycles were performed, totaling approximately 20 minutes per session.
CONTROL CONDITION:
Participants in the control (CON) group rested quietly in a supine position for 20 minutes without receiving any therapeutic intervention.
TIMING:
Interventions began 30 minutes after completion of the eccentric protocol and were repeated 24 hours later, resulting in two intervention sessions.
OUTCOME ASSESSMENT:
All assessments were conducted at baseline (pre-exercise) and at 1, 24, and 48 hours postexercise by a single trained assessor blinded to group allocation.
Primary outcomes:
Serum creatine kinase (CK)-measured using enzymatic colorimetric assay Vertical jump height-assessed using Sargent jump test (best of three trials) Anaerobic peak power-derived from Running-based Anaerobic Sprint Test (RAST) (six maximal 35-meter sprints)
Secondary outcome:
4\. Muscle soreness-quantified using 100-millimeter (mm) visual analog scale (VAS) during standardized bodyweight squat
STATISTICAL APPROACH:
Mixed-design repeated-measures ANOVA with Group (TENS, FR, CON) as the between-subject factor and Time (baseline, 1, 24, and 48 hours) as the within-subject factor. Greenhouse-Geisser corrections were applied when sphericity assumptions were violated. When significant interactions were observed, simple effects comparisons were conducted at each time point using estimated marginal means with the pooled within-cell error term. Bonferroni adjustments were applied for multiple pairwise comparisons. Effect sizes are reported as partial eta squared (η²p) for omnibus effects and Cohen's d for pairwise comparisons, accompanied by 95% confidence intervals (CI). Statistical significance was established at p \< 0.05.
SAFETY MONITORING:
Participants were monitored for adverse events, including skin irritation at electrode sites, excessive soreness, or vasovagal responses during blood collection. A standardized adverse event form was completed after each session.
COMPLIANCE:
Participants were instructed to maintain habitual dietary and hydration practices and to abstain from analgesic or anti-inflammatory medications, alcohol, caffeine, and any recovery modalities outside the study protocol for 72 hours after the exercise bout. Compliance was monitored using daily self-report checklists.
