With many recreational runners ramping up their training in hopes of getting a personal best, a new measure of stress in the body demonstrates that more isn’t better when it comes to endurance sport training.

A University of Guelph study is the first to show that overload training may alter firing in the body’s sympathetic nerve fibres, which could hinder performance.

The study revealed that muscle sympathetic nerve activity, which constricts the muscle’s blood vessels and indicates stress in the body, increased in over-trained athletes.

Athletes who follow a consistent training regime don’t have the same overload stress and demonstrate improvements in their overall fitness and other markers of cardiovascular health.

Published in the journal Medicine and Science in Sports and Exercise, this is the first study to investigate the impact of overload training on muscle sympathetic nerve activity.

Previous studies have measured indirect physiological factors, such as heart rate variability, but examining muscle nerve fibre activity provides a direct measure of the nervous system’s response.

The researchers discovered sympathetic nerve activity increased in the overtrained athletes after the three-week period of overload training. Generally, sympathetic nerve activity stays pretty consistent day to day.

Recreational athletes who follow a regular balanced training programme showed no jump in nerve activity. Instead, they demonstrated improved cardiac reflex sensitivity and heart rate variability – signs of improved physical health.

Athletes who do overload training do not experience this same level of improved health. They don’t get worse, but they don’t get any better either, and their sympathetic nerve activity went up. It appears the overtraining negate some of the beneficial effects of regular training.