# Back Muscle Strength Is Associated with Self-Reported Morning-Erection Frequency in Apparently Healthy Japanese Male University Students: A Cross-Sectional Study

## TL;DR

Stronger back muscles in young Japanese men are linked to more frequent morning erections, suggesting a possible connection between physical health and sexual function.

## Contribution

This study identifies back muscle strength as a novel, independent predictor of morning-erection frequency in healthy young men.

## Key findings

- Back muscle strength is independently associated with higher morning-erection frequency after adjusting for BMI and handgrip strength.
- Over 59% of participants reported low morning-erection frequency, indicating potential early adult sexual health concerns.
- Clustering analysis showed a higher proportion of high morning-erection frequency in the group with higher BMI and strength.

## Abstract

What are the main findings?
Back muscle strength, unlike handgrip strength or body mass index, is independently associated with increased morning-erection frequency in apparently healthy young men.Over 50% of healthy Japanese male university students report infrequent morning erections, suggesting that low nocturnal erectile indicators may be prevalent even in early adulthood.

Back muscle strength, unlike handgrip strength or body mass index, is independently associated with increased morning-erection frequency in apparently healthy young men.

Over 50% of healthy Japanese male university students report infrequent morning erections, suggesting that low nocturnal erectile indicators may be prevalent even in early adulthood.

What are the implications of the main findings?
The frequency of morning erections serves as an intercourse-independent, low-burden indicator that could be useful for research and for initiating prevention-oriented conversations in university health services and primary care.Because this was a cross-sectional study and utilized a single-item self-reported outcome with modest discrimination, further longitudinal studies are needed before considering any screening or routine assessment applications.

The frequency of morning erections serves as an intercourse-independent, low-burden indicator that could be useful for research and for initiating prevention-oriented conversations in university health services and primary care.

Because this was a cross-sectional study and utilized a single-item self-reported outcome with modest discrimination, further longitudinal studies are needed before considering any screening or routine assessment applications.

Background/Objectives: Morning erections provide an intercourse-independent indicator of nocturnal erectile physiology. We aimed to examine whether body mass index (BMI) and muscle strength are associated with morning-erection frequency in apparently healthy Japanese male university students. Methods: This cross-sectional study analyzed 125 men with complete data (170 assessed; 45 excluded). Handgrip and back muscle strength were measured using dynamometry; BMI was calculated from height and weight. Morning-erection frequency was assessed using a single 6-category item and was dichotomized as low vs. high. Univariable and multivariable logistic regression models were fitted. Exploratory principal component analysis (PCA) and k-means clustering (k = 2, silhouette-supported) were performed. Results: Seventy-four participants (59.2%) were classified as low frequency. Back muscle strength was associated with high frequency (univariable odds ratio [OR] 1.61; 95% confidence interval [CI] 1.07–2.42; and p = 0.021) and remained significant after adjustment for BMI and handgrip strength (OR 1.88; 95% CI 1.02–3.47; and p = 0.045), whereas BMI and handgrip strength were not significant. Clustering identified two clusters (n = 41 and n = 84); Cluster 2 (higher BMI/strength) had a higher proportion of high morning-erection frequency (48% vs. 27%). Conclusions: In apparently healthy young men, greater back muscle strength was independently associated with higher self-reported morning-erection frequency. In this cohort, 59.2% reported infrequent morning erections, suggesting potential relevance even in early adulthood. Given the exploratory clustering, the single-item outcome, and likely residual confounding, these findings are hypothesis-generating and warrant longitudinal validation.