The truth hurts...
First, we must acknowledge the truth that we start to lose fitness within just 5-10 days of not training. These changes are mainly cardiovascular in nature (e.g. decreased blood volume, stroke volume, and cardiac output), resulting in a decreased VO2max and an increased heart rate at a given effort.1 However, these changes can be reversed very quickly (as quickly as 5 days) when you return to training, particularly if you have years of training under your belt and your layoff is short in duration (less than 2 weeks). So, a brief break from training should not be a cause of major concern.
"What if my layoff is longer in duration?"
Research shows that aerobic capacity continues to gradually decrease over the span of about 8 weeks of no training, while strength and muscle mass decrease at a much slower rate. However, if you are able to partake in even just a small amount of physical activity each week, you can greatly minimize the effects of detraining in both aerobic capacity and strength. For instance, one study compared a group of world-class kayakers who completely rested for 5 weeks to a group that partook in 40 minutes of moderate intensity endurance training (80% VO2max) just two times per week and one resistance training session (dosed at 3 sets of 10 repetitions at 70-75% of 1 repetition maximum weight) per week for 5 weeks.2 While the group that did not train at all for 5 weeks experienced an 11.3% decrease in VO2max and an 8.9% decrease in 1 repetition maximum strength, the group that performed 3 total training sessions per week experienced a decrease of just 5.6% in VO2max and 3.9% in 1 repetition maximum strength.2 These athletes, who normally performed 16 training total sessions per week, were able to cut their losses in half by performing just about 20% of their normal weekly training volume!
Regarding the resistance training portion, your next question may be, “What if I can’t perform heavy resistance training even just once per week now that the gyms are closed and I don’t have heavy weights at home?” Not to worry. It is possible to maintain, and even build, strength by using as little as 30% of your 1 repetition maximum weight.1 Numerous studies have shown that low-load, high volume resistance training is equally as effective in improving strength and building muscle mass (aka hypertrophy) as high-load, low volume exercise.1 The key is that you perform each set of each exercise TO FAILURE (which basically translates to the point at which you would no longer be able to maintain good form or move through the entire range of motion due to excessive fatigue!). So, depending on your access to various weights at home, you may not reach the point of failure until you have performed 20-25 repetitions (or more!) vs. the 6-12 repetitions that you would normally perform with a heavier load at the gym.
"Rest and be thankful" - William Wadsworth
For those of you who are currently unable to perform your particular sport or cross train at all right now either due to the nature of your injury or due to the closures caused by COVID-19, as well as for those of you who simply don’t have the desire or motivation to train right now, it’s okay to take a complete break. Rest is important in all sports, as the body’s adaptions to hard training take place during the recovery periods between training. While those positive adaptations don’t require, and do start to decline with, extended time off, the body simply cannot continue to be pushed to its limits year round. This is why all sports are broken up into seasons and even professional athletes take a complete break from their sport, and often from all physical activity, at the end of their season. For instance, current and former marathon world record holders, Eliud Kipchoge and Paul Tergat, respectively, take about a month of complete rest after a marathon. Not only does this break allow time for them to physically recover, but it also provides adequate time to mentally and emotionally recover so that they are fully rejuvenated when they return to training. In general, taking a break from your sport, regardless of the reason, should give you a newfound love and appreciation for your sport and your body's abilities.
That being said, here is my advice to you during these uncertain times:
Stay strong and optimistic, and focus on what you CAN and WANT to do during this time! We’re all in this together!