Generally speaking, if you are a regular recreational cyclist, you will have a good sense of whether or not you are fit to ride. However, there are a number of areas which should be taken into account in determining how well you are, and if that may influence your decision.
Firstly, systemic illness. This essentially refers to an intercurrent illness, for example an upper respiratory infection, which may result in a fever, a reduction in appetite and sweating . These symptoms are more generalised, and potentially have more severe implications than more localised features, such as a sore throat.
The phrase commonly used is a ‘neck check’. In other words, if your symptoms are below the neck (cough, shortness of breath, chest pain) or more generalised (fever, off food, nausea) then that would be an indication NOT to ride. There are rare, but potentially serious, complications of significant exertion in these circumstances, including some cardiac issues.
Hydration may also reflect your general health status, or simply imply that your fluid intake is too low. Commonly available urine colour charts are easily accessible online, and offer an idea of your hydration status. It is important to note that other factors may alter urine colour, for example beetroot or certain medications, including multivitamins.
Perhaps a more accurate assessment of fluid loss is weight measurement. Clearly you can, and should, hydrate during a ride, but weighing pre- and post-ride indicates your weight loss in kg, and each kg reduction represents a one litre deficit. Due to ongoing losses (raised metabolic rate, ongoing sweat losses) a guide to fluid replacement is calculated at 1.5l per kg of weight lost.
It is recognised that a modest 3% reduction in hydration (ie approximately 2kg in an individual weighing 75kg) is sufficient to impact on physical and cognitive performance, and potentially increase injury risk.
Hydration and nutrition are key components of recovery, and it is also very important not to overtrain. This can be difficult to recognise as the symptoms may be vague and have a multifactorial cause. Overtraining is a term which has largely ben superseded by Unexplained Underperformance Syndrome (UPS). This is a common issue in athletes (incidence is in the region of 10%) and symptoms may include: fatigue and unexpected sense of effort during training, history of a heavy training load, frequent minor infections, mood disturbance, impaired sleep quality, loss of libido, appetite, energy or competitive drive.
Programming a rest day into your exercise diary may provide some insurance against this developing. Keeping an eye on your resting heart rate (in bed, first thing on waking) and if there is any upward trend or you develop any of the above symptoms which don’t settle with appropriate reduction in exercise load then it would be advisable to seek advice from your doctor to exclude an underlying medical cause.