Taking a day off from training as a result of sore calf muscles may seem like a hassle, but as Benjamin Franklin so famously said, "An ounce of prevention is worth a pound of cure." This principal rings true in the world of strength training because working out with sore muscles doesn't allow the muscles to fully recover, leaving them susceptible to injury and decreasing their ability to build overall strength. Since the goal of resistance training is to optimize your potential for muscle development, it's best to give muscles a break when they're sore. A day off might not always be necessary though, as unrelated muscle groups can still benefit from training.
Delayed Onset Muscle Soreness
Muscle soreness that sets in 48 to 72 hours after training is known as Delayed Onset Muscle Soreness, or DOMS. DOMS is the result of tiny tears in the muscle fibers and connective tissues and the body's biological response working to repair the muscle. As resistance training or weight lifting is repeated the muscle group begins to adapt, garnering strength and becoming less susceptible to DOMS.
Training While Sore
When a muscle is not given enough time between workouts to fully recuperate, it is noticeably weaker in the next training session. However, the long-term effects are far graver. Continually working out sore muscles can lead to muscle dysfunction as the tiny tears sustained during resistance training do not have a chance to heal properly, leading to macro-trauma rather than bigger, stronger muscles. This macro-trauma can cause chronic inflammation and scar tissue which can inhibit muscle growth, alter the movement of joints and induce pain and weakness.
Muscle Recovery Period
To continually improve strength and endurance, muscles must be given an appropriate recovery period. Although recovery is always based on the intensity of a workout, as a general rule, the American Council on Exercise and the National Strength and Conditioning Association recommend at least 48 hours rest between intense workouts that result in DOMS. Recovery time will improve as muscles begin to adapt to the training regime though, so it's safe to train again once all DOMS has faded.
Training Frequency Recommendations
While the number of training days per week depends on a person's experience level and the type of training performed, the American Council on Exercise recommends that beginners perform a moderate-intensity, total-body workout two to three alternating days per week with specific muscle groups being trained only two times per week. So if your calves are sore one day, opt for an abdominal and upper-body workout instead of a run or leg workout.