Normative Values of Nerve Conduction Parameters for Median and Ulnar Nerves Among Adults in a South Indian Population: A Cross-Sectional Study

Introduction

Nerve conduction studies (NCS) are the cornerstone of electrodiagnostic evaluation of peripheral nerve function, providing objective and reproducible data on the integrity of motor and sensory nerve pathways [1]. Accurate clinical interpretation of NCS findings is fundamentally dependent on the availability of reliable, population-specific normative reference values.

Normative values for NCS parameters are not universal. They are significantly influenced by demographic and physiological variables including age, sex, height, weight, and body mass index (BMI), as well as by ethnicity and geographic region. Age-related decline in nerve conduction velocity and sensory nerve action potential (SNAP) amplitude is well-established,[4] while sex-based differences in distal latencies and SNAP amplitudes have been consistently reported across multiple populations [2,3,5]. Height, through its influence on nerve segment length and the temperature gradient along the limb, is recognised as one of the strongest physiological determinants of distal latency and conduction velocity.

Despite this, most normative reference values currently used in Indian clinical neurophysiology practice are derived from Western populations or studies conducted in North India, which may not accurately represent the physiological profile of South Indian adults [3]. Reliance on non-representative normative data risks misclassifying normal physiological variants as pathological or missing early neuropathic change, thereby compromising diagnostic accuracy.

The median and ulnar nerves are the most frequently assessed upper limb nerves in clinical neurophysiology. They are involved in the two most common peripheral nerve entrapment syndromes carpal tunnel syndrome (median nerve) and cubital tunnel syndrome (ulnar nerve) and in systemic neuropathies such as diabetic peripheral neuropathy [1]. Accurate normative data for these nerves therefore has direct and immediate clinical utility.

Studies evaluating NCS normative parameters specific to South Indian or Kerala populations are limited. The study by Ram Kumar and Jose (2024) established normative NCS values for North Kerala,[3] but no comparable data exist for South Kerala. The present study was therefore designed to establish comprehensive, institution-specific normative reference values for bilateral median and ulnar nerve conduction parameters including motor (CMAP), sensory (SNAP), and F-wave variables in healthy adults attending a tertiary care centre in Thiruvananthapuram, Kerala, and to quantify the effects of age, sex, height, weight, and BMI on these parameters.

Materials and Methods

Study Design and Setting

A cross-sectional study was conducted at the Clinical Physiology Unit OPD, Department of Physiology, Government Medical College, Thiruvananthapuram. The study was approved by the Institutional Human Ethics Committee, Government Medical College, Thiruvananthapuram. Written informed consent was obtained from all participants prior to enrolment, in accordance with the ethical principles of the Declaration of Helsinki.

Participants

Healthy adult volunteers accompanying patients attending the Clinical Physiology Unit OPD, aged 20-70 years, were consecutively recruited until the required sample size was achieved. Inclusion criteria: adults aged 20-70 years who provided written informed consent for NCS. Exclusion criteria: known diabetes mellitus, systemic hypertension, hypothyroidism, peripheral neuropathy or myopathy; history of upper limb injury; cardiac pacemakers or rhythm abnormalities; alcoholism; malnutrition; smoking; and use of medications known to affect nerve conduction (e.g., antidepressants). Random capillary blood glucose (GRBS) and blood pressure were verified prior to NCS to exclude undiagnosed diabetes or hypertension.

Sample Size

Sample size was calculated using the formula n = (Z1-α/2² × σ²) / d², where Z1-α/2 = 1.96 (α=0.05), σ = 6.15 m/s (standard deviation of median nerve motor conduction velocity from Ram Kumar et al., 2024[3]), and d = 1 m/s (absolute precision), yielding a minimum required sample size of 145.

Anthropometric Measurements

Height was measured to the nearest centimetre using a stadiometer. Weight was recorded to the nearest kilogram using a calibrated electronic weighing machine. BMI was calculated as weight (kg) divided by height squared (m²).

Nerve Conduction Study Protocol

All NCS recordings were performed using the NEUROSTIM-NS4 system in a quiet, well-lit room at warm ambient temperature. The black electrode served as the active electrode (G1) and the red electrode as the reference electrode (G2). All recordings were obtained with the muscle fully relaxed.

Motor studies (CMAP): For the median nerve, the active electrode was placed on the belly of abductor pollicis brevis and the reference electrode 3-4 cm distally on the first metacarpophalangeal joint; stimulation was applied at the wrist and antecubital fossa. For the ulnar nerve, the active electrode was placed on the belly of abductor digiti minimi and the reference electrode on the fifth metacarpophalangeal joint; stimulation was applied at the medial wrist and elbow. F-wave minimum latency was recorded from the same electrode positions with wrist stimulation.

Sensory studies (SNAP): Antidromic technique was used throughout. For the median nerve, electrodes were placed on the index finger (active electrode on the metacarpophalangeal joint, reference 3-4 cm distally on the distal interphalangeal joint) with wrist stimulation. For the ulnar nerve, electrodes were placed analogously on the little finger with medial wrist stimulation. Supramaximal stimulation was used to obtain maximum CMAP and SNAP amplitudes.

Statistical Analysis

Data were entered in Microsoft Excel and analysed using Python (SciPy library). Descriptive statistics (mean, standard deviation, 95% confidence interval) were computed for all parameters. The ±2SD range was calculated as the approximate 95th percentile normative reference range. Sex-based comparisons were performed using the independent samples t-test (p<0.05 = significant). Pearson correlation coefficients (r) were computed for the relationship of age, height, weight, and BMI with each NCS parameter. Age-group subanalysis was performed across four groups (20-30, 31-45, 46-60, 61-73 years) using one-way ANOVA. One data entry error (left median CMAP distal latency >10 ms) was identified as non-physiological and excluded, reducing the effective n to 144 for that specific parameter.

Results

Demographic and Anthropometric Characteristics

A total of 145 participants were enrolled: 40 males (27.6%) and 105 females (72.4%). Mean age was 38.91 ± 13.24 years (range 19–73 years), distributed across four age groups: 20-30 years (n=48), 31-45 years (n=50), 46-60 years (n=38), and 61-73 years (n=9). Males were significantly taller (168.55 ± 6.81 vs 159.64 ± 5.96 cm; p<0.001) and heavier (71.19 ± 12.16 vs 63.35 ± 9.40 kg; p<0.001) than females. BMI was comparable between sexes (24.95 ± 3.15 vs 24.86 ± 3.43 kg/m²; p=0.883). Demographic and anthropometric characteristics are summarised in Table 1.

Values expressed as Mean ± SD unless stated. *p<0.05, independent samples t-test (male vs female).

Normative Nerve Conduction Values

Normative reference values for all 28 NCS parameters are presented in Table 2, expressed as Mean ± SD with 95% CI and the ±2SD range, which represents the approximate 95th percentile reference range for each parameter.

CMAP: compound muscle action potential; SNAP: sensory nerve action potential; DL: distal latency (ms); Amp: amplitude (mV for CMAP, µV for SNAP); CV: conduction velocity (m/s); F-wave latency in ms. CI: 95% confidence interval. ±2SD range = approximate 95th percentile normative reference range. Note: n=144 for LT Median CMAP DL (one outlier excluded as data entry error).

Sex-Based Differences in Nerve Conduction Parameters

Sex-stratified comparisons are presented in Table 3. Males demonstrated significantly longer distal latencies across multiple parameters, including right median nerve CMAP distal latency (3.28 ± 0.51 vs 3.06 ± 0.49 ms; p=0.019), bilateral ulnar CMAP distal latencies (right: 2.59 ± 0.38 vs 2.23 ± 0.35 ms, p<0.001; left: 2.57 ± 0.38 vs 2.24 ± 0.39 ms, p<0.001), and bilateral ulnar SNAP distal latencies. F-wave minimum latencies were significantly longer in males for right ulnar (27.62 ± 2.92 vs 26.67 ± 1.77 ms; p=0.019) and left ulnar nerves (27.62 ± 2.42 vs 26.53 ± 1.73 ms; p=0.003). Right ulnar SNAP amplitude was significantly higher in females (27.11 ± 6.14 vs 24.36 ± 3.53 µV; p=0.009). Right median SNAP conduction velocity (p=0.043) and left ulnar CMAP conduction velocity (p=0.048) were significantly higher in females.

Values expressed as Mean ± SD. p-values from independent samples t-test. *p<0.05.

Correlations with Anthropometric Parameters

Pearson correlation coefficients for NCS parameters with age, height, weight, and BMI are presented in Table 4. Age showed the strongest and most consistent associations: significant positive correlations with distal latencies were observed for right and left median CMAP distal latency (r=0.466, p<0.001 and r=0.320, p<0.001 respectively), bilateral median and ulnar SNAP distal latencies, and bilateral median F-wave latencies. Age was significantly negatively correlated with SNAP amplitudes (right median: r=−0.247, p=0.003; right ulnar: r=−0.355, p<0.001) and multiple conduction velocities.

Height showed significant positive correlations with ulnar nerve distal latencies (right ulnar CMAP DL: r=0.341, p<0.001; left ulnar CMAP DL: r=0.299, p<0.001; right ulnar SNAP DL: r=0.192, p=0.021; left ulnar SNAP DL: r=0.194, p=0.019). Weight correlated positively with right median CMAP distal latency (r=0.302, p<0.001) and right ulnar SNAP distal latency (r=0.256, p=0.002). BMI demonstrated fewer significant associations, with a significant positive correlation with right median CMAP distal latency (r=0.199, p=0.016) only among the latency parameters.

r: Pearson correlation coefficient. *p<0.05 (significant values shown in bold). Positive r with distal latency indicates latency increases with predictor; negative r with CV/amplitude indicates decrease.

Age-Group Analysis

Age-stratified NCS values for right median and ulnar nerve parameters across four age groups are presented in Table 5. A progressive and statistically significant increase in median nerve CMAP distal latency was observed across age groups (20-30 yrs: 2.87 ± 0.40 ms; 31-45 yrs: 3.04 ± 0.54 ms; 46-60 yrs: 3.39 ± 0.44 ms; 61-73 yrs: 3.49 ± 0.26 ms; ANOVA p<0.001). Right ulnar SNAP amplitude showed a significant progressive decline with age (20-30 yrs: 28.81 ± 6.61 µV to 61-73 yrs: 21.62 ± 1.31 µV; ANOVA p<0.001). Right median SNAP conduction velocity also declined significantly across age groups (ANOVA p=0.007).

Values expressed as Mean ± SD. p: one-way ANOVA across four age groups. *p<0.05.

Discussion

This study provides comprehensive normative NCS reference values for bilateral median and ulnar nerves in healthy South Indian adults, with quantification of the influence of age, sex, height, weight, and BMI on all parameters. The findings demonstrate that age is the strongest determinant of NCS parameters, that males consistently show longer distal latencies and F-wave latencies than females, and that height is significantly correlated with ulnar nerve distal latencies.

The age-related changes observed in this study are consistent with established literature. Thakur et al. (2010) demonstrated declining CMAP and SNAP amplitudes with advancing age across multiple nerves [4], while Fujimaki et al. (2009) reported that age shows strong correlation with SNAP amplitude in upper limb nerves [5]. Ram Kumar and Jose (2024) similarly documented significant positive correlations between age and latency parameters, and negative correlations between age and amplitude and conduction velocity [3]. The biological basis for these age-related changes includes progressive axonal atrophy, reduced myelin sheath thickness, and decreased density of large-diameter myelinated fibres.

The significantly longer distal latencies observed in males across multiple parameters is a widely documented finding, primarily attributed to greater upper limb length rather than a true conduction deficit. Karthikeyan et al. (2025) reported significantly higher median motor and sensory latencies in males [2]. In this study, height showed significant positive correlations with ulnar nerve distal latencies, confirming the physiological basis of this sex difference greater upper limb length in males increases nerve segment length and thus conduction time at a fixed electrode-to-stimulation distance.

Higher SNAP amplitudes in females for the right ulnar nerve, and faster sensory conduction velocities in selected parameters, are consistent with previous reports. Fujimaki et al. (2009) demonstrated that sex and BMI independently affect SNAP amplitude in upper limb nerves, with females showing significantly higher amplitudes [5], likely reflecting sex differences in subcutaneous fat distribution affecting electrode-to-nerve distance and differences in axonal density. Ram Kumar and Jose (2024) also noted faster conduction velocities in females [3].

The mean right median CMAP distal latency in our cohort (3.12 ± 0.50 ms) is comparable to published Indian normative studies. Ram Kumar and Jose (2024) reported right median motor distal latency of 2.82 ± 1.10 ms from North Kerala [3], while Singh et al. (2017) reported 2.9 ± 0.16 ms in males and 2.6 ± 0.43 ms in females from Punjab [6]. Karthikeyan et al. (2025) reported a median motor latency of 2.77 ± 0.45 ms [2], The slightly higher values in our cohort likely reflect the older mean age (38.9 years) and broader age range (19-73 years) of the study population. These inter-laboratory differences, even within India, reinforce the need for institution-specific normative data as emphasised by Preston and Shapiro [1].

Limitations of this study include the unequal sex distribution (27.6% male vs 72.4% female), which reflects the bystander population at the outpatient unit and limits the precision of male-specific reference ranges. The oldest age group (61-73 years) comprised only 9 participants, restricting statistical power for that subgroup. Skin temperature was not objectively measured and recorded with a contact thermometer, which is a recognised standard in NCS research. Occupation data, though collected, was not analysed as a potential modifier of NCS parameters. Future studies with balanced sex recruitment, larger age-stratified cohorts, and standardised temperature recording would further strengthen normative datasets for this population.

Conclusion

This study establishes comprehensive, population-specific normative NCS reference values for bilateral median and ulnar nerves in healthy South Indian adults from Thiruvananthapuram, Kerala. Age is the dominant modifier, with progressive prolongation of distal latencies and decline in amplitudes and conduction velocities with advancing age. Sex significantly influences distal latencies and F-wave latencies, with males showing consistently longer values, while females demonstrate higher ulnar SNAP amplitudes and faster sensory conduction velocities in selected nerves. Height correlates significantly with ulnar nerve distal latencies. These normative data provide a robust, locally derived reference standard for the electrodiagnostic evaluation of peripheral nerve disorders in the South Indian population.

Declarations

Ethics approval and consent to participate

Approved by the Institutional Human Ethics Committee, Government Medical College, Thiruvananthapuram. Written informed consent was obtained from all participants.

Funding

No funding was received for this study.

Conflict of interest

The authors declare no conflict of interest.

Data availability

Data are available from the corresponding author on reasonable request.

Author Contributions

Dr. Lakshmi G: Conceptualisation, Methodology, Formal Analysis, Writing - Original Draft, Supervision.

Dr. Deepthi Damodaran: Conceptualisation, Methodology, Formal Analysis, Data Collection, Writing - Review and Editing, Corresponding Author.

Dr. Swathy S Krishna: Conceptualisation, Methodology, Data Collection, Formal Analysis, Writing - Review and Editing.

Acknowledgements

The authors also extend their gratitude to all participants who willingly provided informed consent and actively participated in the study and thank you Dr Venkataraman A P (Healthium Medtech) for drafting the manuscript.