Exercise induces a range of adaptations in muscle tissue, depending on the type and duration of activity. Such physical training can be broadly categorized into two types: endurance exercises and resistance exercises.

Endurance exercises

Endurance exercises involve running, swimming, or cycling, which require repetitive movements with low force output. When a person engages in endurance exercise, a few noticeable changes occur in their skeletal muscles. For instance, the number of capillaries surrounding the muscle fibers increases. Additionally, the number of mitochondria within the muscle fibers also increases. Another change is that the fibers synthesize more myoglobin. While these changes occur in all fiber types, they are particularly significant in slow oxidative fibers that rely heavily on aerobic pathways. The result of these changes is that the muscles become more efficient in their metabolism, leading to greater endurance, strength, and resistance to fatigue. In fact, regular endurance exercise can even cause fast glycolytic fibers to transform into fast oxidative fibers. However, the transformation is not total or straightforward; the extent of these changes varies significantly between individuals and depends on factors like training intensity, duration, and genetic predisposition.

Resistance exercises

On the other hand, resistance exercises, such as weightlifting, sprinting, or jumping, require more strength than stamina. Such activities primarily engage fast muscle fibers that rely on ATP and glycogen for energy. Consistent resistance training enlarges muscle cells, eventually increasing strength. During high-intensity resistance exercise, muscles are pitted against high-resistance or immovable forces, and strength, rather than stamina, is the focus. Just a few minutes of high-intensity resistance exercise every other day can lead to a 50% increase in muscle mass within a year, even for someone not particularly strong. The additional muscle bulk primarily results from the increased size of individual muscle fibers, particularly the fast glycolytic variety, rather than an increase in the overall number of muscle fibers.

In summary, endurance and resistance exercises are essential for building physical fitness. While endurance exercises focus on improving stamina, resistance exercises aim to increase strength. Incorporating both activities into a workout routine can help individuals build muscle mass and improve their physical performance.