Clinical assessment of nail and skin lesions in indoor climbers: A clinic-based cross-sectional study

Abstract

BACKGROUND

Indoor climbing has grown substantially in popularity; however, clinically assessed data on climbing-related nail and skin conditions remain limited. Tight, performance-oriented climbing shoes and repetitive training exposure may contribute to mechanical stress affecting the toes and forefoot, but evidence derived from direct clinical examination is scarce.

AIM

To characterize the frequency, typology, and anatomical distribution of nail and skin lesions in a clinic-based sample of indoor climbers, and to assess associations with footwear compression and training exposure.

METHODS

A retrospective cross-sectional analysis was conducted from data on 156 indoor climbers attending a single outpatient podiatric clinic. Lesions were classified by type, anatomical location, laterality, and affected digit. Demographic, anthropometric, footwear-related (shoe size discrepancy), and training-related variables were also recorded. Associations between lesion burden and continuous variables were assessed using Spearman’s rank correlation and non-parametric tests, with statistical significance set at P < 0.05.

RESULTS

Patients showed a mean of 6.6 lesions per individual (range 0-21), most exhibiting multiple nail and skin lesions. Hyperkeratosis accounted for approximately two-thirds of recorded lesions and was mostly localized to the first toe and periungual regions. Traumatic nail disorders, including Beau’s lines, longitudinal ridges, onycholysis, and subungual hematomas, were predominantly hallux-related. Shoe size discrepancy showed a weak but statistically significant positive correlation with total lesion burden (ρ = 0.195, P = 0.015), as did weekly training frequency (ρ = 0.180, P = 0.025). No significant associations were observed for age, sex, body mass index, or session duration.

CONCLUSION

In the selected population of the study, footwear compression and training frequency showed weak but statistically significant associations with lesion burden, while demographic variables showed no significant correlation. Although causal inference is not possible from this cross-sectional design, findings highlight the potential clinical relevance of modifiable sport-specific factors and warrant longitudinal investigation

Key Words: Indoor climbing; Nail disorders; Hyperkeratosis; Foot injuries; Climbing footwear; Podiatry

Core Tip: Indoor sport climbing requires tight technical footwear that increases mechanical stress on the toes and forefoot, yet clinically assessed evidence on climbing-related nail and skin conditions remains limited. In this clinic-based cross-sectional study of 156 indoor climbers, hyperkeratosis and traumatic nail disorders were the most frequent findings, often involving the hallux and periungual regions. Greater climbing shoe downsizing and higher weekly training frequency showed weak but significant associations with lesion burden. Although observed effect sizes were small and causal inference is not possible from this design, findings highlight the potential role of modifiable sport-related factors and suggest the value of preventive podiatric assessment in climbers.

INTRODUCTION

Indoor sport climbing has increasingly grown in recent years, underlining the necessity to understand sport-specific health conditions for injury prevention and long-term participation[1,2]. The foot, as the primary interface with the climbing surface, plays a key role in load transmission, stability, and movement precision so foot-related morbidity may therefore impair both comfort and performance[2].

The use of technical climbing shoes is believed to enhance precision and force transfer through a tight, asymmetrical, and downturned design, but it can expose the forefoot to repetitive compressive and shear stresses. Specifically, the sustained clawing postures, toe loading, and prolonged footwear compression may contribute to pressure-related hyperkeratosis and traumatic nail alterations[3,4].

Despite the growing prevalence of indoor climbing, clinically verified and anatomically detailed assessments of foot pathologies in this population remain scarce. Existing studies have largely concentrated on upper extremity and shoulder injuries, and investigations of foot health frequently rely on self-administered questionnaires, which may underestimate lesion frequency and fail to capture subclinical manifestations[4,5]. Evidence exploring associations between podiatric lesions and modifiable factors such as footwear fit and training exposure is particularly limited, restricting the development of evidence-based preventive strategies[6].

This study aimed to address this gap by characterizing the frequency and distribution of nail disorders and hyperkeratosis in indoor climbers using standardized and potentially reproducible podiatric examination protocol. The secondary objective was to explore associations between lesion burden and modifiable sport-related factors, specifically climbing shoe size discrepancy and training exposure.

MATERIALS AND METHODS

This retrospective cross-sectional study was based on the analysis of fully anonymized data prospectively collected during routine podiatric clinical practice at a single outpatient clinic in Milan, Italy (January 2023 to January 2025). All participants had attended the clinic spontaneously for podiatric consultation and reported regular practice of indoor sport climbing during structured medical history taking. Because all data were irreversibly anonymized prior to analysis and no additional procedures were performed for research purposes, formal Institutional Review Board approval was not required under applicable local regulations, which exempt retrospective analyses of fully anonymized clinical data from mandatory formal review. The study was conducted in accordance with the ethical principles of the Declaration of Helsinki and reported following the STROBE guidelines for cross-sectional studies.

This retrospective cross-sectional analysis of prospectively collected clinical data reflects a clinic-based sample of individuals who actively sought podiatric care. Findings should not be interpreted as population-level prevalence estimates, as individuals presenting to a specialist clinic may have a higher burden of symptoms than the broader climbing population.

A structured overview of the study design, eligibility criteria, data collection procedure, and analytical approach is provided in Figure 1.

Figure 1 Structured overview of study design and methodology - see methods section and separate PowerPoint file. Shoe size delta: Everyday shoe size minus climbing shoe size (European Union sizing). This design reflects a clinic-based sample and does not permit population-level prevalence estimates. IRR: Incidence rate ratio; IQR: Interquartile range; BMI: Body mass index.

Participants

Individuals were included if they met all of the following conditions: Age ≥ 18 years; self-reported regular practice of indoor sport climbing (≥ two sessions per week); habitual use of technical climbing shoes during training; availability of complete clinical, demographic, footwear-related, and sport-related data.

Individuals were excluded if they presented with systemic dermatological diseases (e.g., psoriasis, lichen planus), history of foot or ankle surgery, acute traumatic injuries unrelated to climbing, or other medical conditions known to alter nail or skin morphology. Climbing discipline (bouldering, lead, or mixed) and competitive level were not systematically recorded; residual confounding from unmeasured training characteristics cannot be excluded. A total of 156 individuals met the inclusion criteria. Given the exploratory and descriptive nature of the study, a formal a priori power calculation was not performed; the sample size was determined by the number of eligible cases available during the study period.

Data collection

For each participant, the following variables were recorded: Age (years); sex; self-reported body weight (kg) and height (cm), used to calculate body mass index [body mass index (BMI), kg/m²].

Sport-specific data were collected through a structured questionnaire and included: Years of climbing practice; weekly training frequency (sessions per week); average session duration (hours); total weekly training volume (hour/week).

Footwear variables included everyday shoe size and climbing shoe size (European Union sizing); Shoe size delta (SZD) was calculated as the difference between everyday and climbing shoe size and was used as a proxy measure of footwear constriction despite SZD alone, representing a crude measure of footwear compression, cannot capture other potentially relevant characteristics including shoe stiffness, asymmetry, toe box geometry, or material properties, which may also contribute to localized pressure and tissue response.

Clinical podiatric assessment

Podiatric data originally collected during routine clinical visits were reviewed retrospectively. Lesions were documented photographically, while both visual and manual inspection records were used to identify hyperkeratosis and nail disorders (onycholysis, Beau’s lines, subungual hematomas, and traumatic ridges). Lesions were coded by type, laterality, location, and affected digit, and multiple lesions per participant were permitted and recorded as independent entries. All records, including photographs, were reviewed by one experienced podiatrist with 10 years of clinical practice to classify lesions according to predefined operational criteria. The absence of blinded assessment and prospective data collection is acknowledged as a methodological limitation.

For facilitating the analysis of overall burden, for each patient the primary outcome was total lesion count, in order to simplify the more complex within-subject pattern of co-occurring lesions.

Statistical analysis

Descriptive statistics were used to summarize participant characteristics and lesion distribution. Due to non-normal data distribution, continuous variables were reported as medians and interquartile ranges, and categorical variables as n (%). Associations between lesion burden and continuous variables were assessed using Spearman’s rank correlation coefficient. Between-group comparisons were performed using the Mann-Whitney U test or Kruskal-Wallis test as appropriate. An exploratory multivariable negative binomial regression model evaluated the independent association between lesion count and selected variables (SZD, training exposure, age, sex, and BMI), with results reported as incidence rate ratios (IRR) with 95% confidence interval (CI). All tests were two-tailed; statistical significance was set at P < 0.05. Analyses were performed using IBM SPSS Statistics (version 30.0).

RESULTS

A total of 156 indoor climbers were included. The sample was predominantly male, with a mean age of approximately 28 years (range 18-49) and a mean BMI of approximately 22 kg/m². Participants reported a mean training frequency of two sessions per week and a mean session duration of 2.4 hours. The SZD showed a mean value of 1.3 shoe sizes. The mean number of podiatric lesions per participant was 6.6 (range 0-21), indicating marked inter-individual variability.

Overall frequency of podiatric lesions

The majority of climbers presented with at least one podiatric lesion at clinical examination. Multiple lesions were frequently observed in the same individual; consequently, the total number of recorded lesions exceeded the number of participants included in the study. Across the entire sample, hyperkeratosis accounted for approximately two-thirds of all recorded lesions and was the most frequent podiatric finding. Nail disorders were predominantly traumatic, while infectious conditions accounted only for minimal part of the total findings. Patients cohort showed predominantly hyperkeratosis that was more common in men, despite clinically relevant frequencies were observed also in the women cohort. Lesions occurred with a mean frequency of 6.6 per participant, ranging from 0 to 21 (Table 1).

Table 1 Distribution of podiatric lesions by sex (absolute number of lesions).

Podiatric lesion	Female (n)	Male (n)	Total (n)
Hyperkeratosis	323	502	825
Beau’s lines	7	50	57
Longitudinal traumatic ridges	1	30	31
Post-traumatic onycholysis	7	22	29
Subungual hematoma	3	23	26
Splinter hemorrhages	4	14	18
Onychodystrophy	4	7	11
Haglund’s disease	3	9	12
Onychomycosis	0	7	7
Onychomadesis	0	3	3
Onychocryptosis	0	2	2
Warts	0	2	2
Tinea pedis	0	1	1

Data are reported as absolute number of recorded lesions. Multiple lesions per participant were permitted; lesion counts exceed the total number of participants.

Hyperkeratosis

The principal hyperkeratosis subtypes and their distribution are reported in Table 2. Periungual hyperkeratosis was the most common finding. Pinch calluses (n = 95; 11.5%) (Figure 2A) and submetatarsal head hyperkeratosis (n = 87; 10.6%) (Figure 2B) were the next most frequent subtypes. The remaining subtypes, calcaneal xerosis, apical toe lesions, dorsal hyperkeratosis at the proximal interphalangeal joint (Figure 2C), interdigital helomas, plantar toe lesions, retrocalcaneal xerosis, and lateral fifth-toe skin lesions, accounted for the remainder and are detailed in Table 2. The distribution of medial and lateral periungual hyperkeratosis by digit is provided in Table 3; the first nail plate was most frequently affected in both subtypes (Figure 3A). The occurrence of each hyperkeratosis form was higher in males (Table 1).

Figure 2 Hyperkeratosis. A: Pinch callus, first toe; B: Plantar hyperkeratosis; C: Dorsal hyperkeratosis of first toe.

Figure 3 Nail disorders. A: Medial and lateral periungual hyperkeratosis, first toe; B: Beau’s lines on the first nail; C: First nail traumatic ridges; D: First nail subungual hematoma.

Table 2 Distribution of hyperkeratosis subtypes (absolute number of lesions).

Hyperkeratosis subtype	n (%)
Medial periungual hyperkeratosis	260 (31.5)
Lateral periungual hyperkeratosis	192 (23.3)
Pinch callus	95 (11.5)
Submetatarsal head hyperkeratosis	87 (10.6)
Calcaneal xerosis	52 (6.3)
Apical toe lesion	52 (6.3)
Dorsal hyperkeratosis (PIP joint)	32 (3.9)
Interdigital heloma	29 (3.5)
Plantar toe lesion	15 (1.8)
Retrocalcaneal xerosis	6 (0.7)
Lateral skin lesion, fifth toe	5 (0.6)

Digit-level breakdowns for periungual hyperkeratosis are provided in Table 3. PIP: Proximal interphalangeal.

Table 3 Distribution of medial and lateral periungual hyperkeratosis by digit, n (%).

Digit	Medial periungual	Lateral periungual
First nail	85 (32.7)	108 (56.2)
Second nail	58 (22.3)	51 (26.6)
Third nail	53 (20.4)	22 (11.5)
Fourth nail	40 (15.4)	6 (3.1)
Fifth nail	24 (9.2)	5 (2.6)

Nail disorders

The most frequently observed nail disorder was Beau’s lines (n = 57) (Figure 3B), followed by longitudinal traumatic ridges (Figure 3C) (n = 31), post-traumatic onycholysis (n = 29), and subungual hematomas (n = 26) (Figure 3D) (Tables 4 and 5). Beau’s lines, longitudinal traumatic ridges, and onycholysis predominantly affected the first toenail, with progressively decreasing involvement of more lateral digits, consistent with the central biomechanical role of the hallux in climbing. Splinter hemorrhages were less frequent but followed a similar distribution pattern. Infectious nail conditions, including onychomycosis, were relatively uncommon. Overall, nail disorders were characterized by a clear predominance of traumatic over infectious etiologies, with marked concentration on the first toe.

Table 4 Anatomical distribution of nail disorders by affected digit (percentage within each disorder).

Nail disorder	Total (n)	1st toe (%)	2nd toe (%)	3rd toe (%)	4th toe (%)	5th toe (%)
Beau’s lines	57	48.2	21.4	19.6	7.1	3.7
Post-traumatic onycholysis	29	58.6	13.8	6.9	17.2	3.5
Longitudinal traumatic ridges	31	67.7	12.9	9.7	0.0	9.7
Subungual hematoma	26	80.8	11.5	0.0	3.8	3.8
Splinter hemorrhages	18	38.9	5.6	22.2	22.2	11.1
Onychomycosis	7	50.0	33.3	16.7	0.0	0.0
Onychodystrophy	11	20.0	0.0	10.0	20.0	50.0

Percentages refer to distribution across digits within each nail disorder. Onychomadesis (n = 3) and onychocryptosis (n = 2) affected exclusively the first nail and are not shown separately.

Table 5 Distribution of submetatarsal head hyperkeratosis by metatarsal.

Metatarsal head	Total, n (%)
Fifth	48 (55.2)
First	25 (28.7)
Second	9 (10.3)
Third	4 (4.6)
Fourth	1 (1.2)

Associations between footwear compression, training exposure, and lesion burden

Spearman’s rank correlation analysis showed a weak but statistically significant positive association between SZD and total lesion burden (ρ = 0.195, 95%CI: 0.04-0.34, P = 0.015), indicating that greater footwear downsizing was associated with a modest increase in the number of podiatric lesions. In the multivariable negative binomial regression model, SZD remained independently associated with lesion count (IRR: 1.08, 95%CI: 1.01-1.16, P = 0.028) (Table 6). Weekly training frequency showed a weak positive correlation with lesion burden (ρ = 0.180, 95%CI: 0.02-0.32, P = 0.025), and total weekly training volume showed a comparable association (ρ = 0.172, P = 0.032). Session duration alone was not significantly correlated with lesion count (ρ = 0.09, P = 0.24). In the adjusted regression model, weekly training frequency remained significantly associated with lesion burden (IRR: 1.11, 95%CI: 1.02-1.20, P = 0.014), while total weekly volume showed a borderline association (IRR: 1.05, 95%CI: 0.99-1.11, P = 0.081) (Table 7).

Table 6 Multivariable negative binomial regression: Factors associated with lesion burden.

Variable	IRR	95%CI	P value
Shoe size delta	1.08	1.01-1.16	0.028
Weekly training frequency	1.11	1.02-1.20	0.014
Total weekly training volume	1.05	0.99-1.11	0.081
Age	0.99	0.97-1.01	0.42
Sex (male vs female)	1.06	0.91-1.22	0.45
Body mass index	1.01	0.97-1.05	0.62

All variables entered simultaneously. Variance inflation factor < 2 (no multicollinearity). All statistically significant incidence rate ratios are close to 1.0; findings should be interpreted as exploratory. IRR: Incidence rate ratio; CI: Confidence interval.

Table 7 Summary of key associations between exposure variables and total lesion burden.

Variable	Analysis	Effect size	95%CI	P value
Shoe size delta	Spearman ρ	0.195	0.04-0.34	0.015
Shoe size delta	IRR (multivariable)	1.08	1.01-1.16	0.028
Weekly training frequency	Spearman ρ	0.180	0.02-0.32	0.025
Weekly training frequency	IRR (multivariable)	1.11	1.02-1.20	0.014
Total weekly volume	Spearman ρ	0.172	0.01-0.31	0.032
Total weekly volume	IRR (multivariable)	1.05	0.99-1.11	0.081
Session duration	Spearman ρ	0.09	-	0.24
Age	Spearman ρ	-0.03	-	0.68
Sex	Mann-Whitney U	r = 0.06	-	0.41
BMI	Spearman ρ	Approximately 0	-	> 0.05

All ρ values < 0.20 and significant incidence rate ratios close to 1.0 confirm weak effect sizes throughout. IRR: Incidence rate ratio from multivariable negative binomial regression; BMI: Body mass index; CI: Confidence interval.

All observed correlation coefficients were below 0.20 and all IRRs were close to 1, indicating weak effect sizes, so these associations should be interpreted as indicative of modest relationships rather than strong predictors of lesion burden.

Demographic and anthropometric variables

Age was not associated with lesion burden (Spearman ρ = -0.03, P = 0.68). No significant correlations were observed between lesion count and BMI, weight, or height (all P > 0.05). Sex-based comparisons did not reveal significant differences in lesion burden between male and female climbers (rank-biserial correlation = 0.06, P = 0.41) (Table 7).

DISCUSSION

This cross-sectional study documented a high frequency of both nail and skin lesions in the selected population of indoor climbers presenting for podiatric evaluation and in the majority of cases, multiple and concurrent findings.

Biomechanical interpretation and comparison with existing literature

The spatial distribution of podiatric lesions appears closely linked to the biomechanical demands of the sport and the design of technical footwear[3,7,8]. Hyperkeratotic lesions, particularly at the medial periungual and apical regions of the hallux, may reflect localized stress from edging and smearing maneuvers, where weight is concentrated on minimal surfaces.

This localized pressure and repetitive shear forces are linked to epidermal thickening[9,10] while traumatic nail disorders remain consistent with chronic micro-trauma related to training exposure and the clawing posture imposed by the downturned, asymmetrical climbing shoe design[5,7].

Regarding the association between SZD and lesion burden, it supports mechanical constriction as a plausible contributing factor[3,10-13], consistent with prior literature on incorrectly fitted footwear[14-16].

The absence of association with age or BMI is consistent with prior literature[6,7,17] and may suggest that also in this specific population, sport-specific factors may be more relevant determinants than intrinsic climber demographics[2-4], highlighting the clinical compromise between athletic precision and foot health.

Clinical and preventive implications

The high frequency of nail and skin lesions in this clinical sample underscores the potential value of systematic podiatric assessment in indoor climbers, even in the absence of acute symptoms. The identification of footwear size mismatch as a modest but significant factor associated with lesion burden highlights the potential role of shoe fitting strategies in prevention. Educating climbers about the risks associated with excessive shoe downsizing may help reduce podiatric burden, though this recommendation should be framed as clinically plausible rather than evidence-established given the cross-sectional design[6,7,11,17].

The association between training frequency and lesion burden further suggests that training load management warrants consideration as part of preventive strategies, though the direction of causality remains unclear. Periodic podiatric screening may facilitate early identification of pressure-related or traumatic lesions, allowing timely intervention before conditions worsen or become performance-limiting[10,11].

Limitations

The most important limitation is the clinic-based sampling strategy. All participants attended the clinic spontaneously, which likely overrepresents symptomatic climbers relative to the broader climbing population, limiting generalizability and potentially overestimating lesion frequency. Additional limitations include: The cross-sectional design, which precludes causal inference and cannot exclude reverse causation; single-centre design, restricting generalizability across geographic regions, training cultures, and footwear practices; absence of a control group (e.g., non-climbing athletes), preventing estimation of the climbing-specific contribution to lesion burden; use of self-reported height and weight for BMI calculation; absence of data on climbing discipline and session intensity, which may introduce residual confounding; use of SZD as a crude proxy for footwear compression without capturing shoe design characteristics; and absence of blinded assessment and prospective data collection.

Methodological contribution and future directions

This study demonstrates the feasibility and value of standardized clinical podiatric assessment, combining structured examination, photographic documentation, and systematic lesion coding by type, digit, and laterality, as a reproducible approach for characterizing foot pathologies in climbers. This protocol, illustrated in Figure 1, may serve as a methodological template for future prospective or multicenter studies aimed at establishing population-level estimates and investigating causal pathways.

Future research should consider longitudinal study designs to clarify the temporal relationship between footwear practices, training load, and podiatric lesion development. Intervention studies evaluating the effects of footwear modifications or load management strategies on lesion incidence would be particularly valuable. Further investigation into specific shoe design features and their biomechanical impact on foot health may contribute to evidence-based recommendations for climbers and clinicians.

CONCLUSION

In indoor climbers, footwear compression and frequency of training showed significant associations with overall lesion burden and no specific correlation with demographic variables. Although specific causal inference is not possible due to the study design and small effect sizes, the findings highlight the potential clinical relevance of modifiable sport-specific factors in this clinical sample. These results provide a clinical snapshot of foot conditions in climbers but require confirmation in prospective studies designed to assess causality and preventive strategies.