Stronger Muscles and Joints: The Physical Reshaping of a Runner’s Body
When someone decides to take up running, they often think about cardiovascular fitness and losing weight. What many don’t realize is that running fundamentally transforms the body’s muscular and skeletal systems. The physical reshaping that occurs goes far deeper than just burning calories during a cardio workout. Every stride a runner takes sends forces through their body that reshape muscles, strengthen joints, and create adaptations that extend far beyond the running track.
The Impact Forces That Shape Everything
Every time a runner’s foot strikes the ground, their body absorbs approximately three to five times their body weight in force. This massive impact is the beginning of a cascade of physical changes. Without adequate muscle strength to control these forces, particularly in the quads and hips, the knees bear the brunt of this impact. This is why the transformation of muscle tissue is so critical for runners. The body must develop the strength to manage these repetitive, high-impact forces, or injury becomes inevitable.
The muscles surrounding the joints act as shock absorbers and stabilizers. When a runner begins their training, these muscles are often unprepared for the demands placed upon them. Over time, through consistent running and complementary strength training, these muscles adapt and grow stronger. This adaptation is not just about bigger muscles; it’s about muscles that can control movement, absorb impact, and maintain proper alignment through thousands of repetitive strides.
How Running Changes Muscle Activation Patterns
One of the most fascinating aspects of how running reshapes the body involves changes in muscle activation patterns. When runners become fatigued, their muscles activate differently than they do when fresh. Research has shown that fatigue induced by running causes changes in the way muscles are activated, which in turn causes changes in the mechanics of movement and joints during running. This happens because fatigue alters the motor system itself.
During the heel strike phase of running, both knee flexor and extensor muscles show high levels of activity. These muscles work together to maintain knee motion and stability. However, when fatigue sets in, the activation patterns of these muscles change. For instance, studies have found that the rectus femoris activity during heel strike is significantly reduced in certain foot types after fatigue. This reduction in muscle activity can lead to changes in joint mechanics and reduced joint co-contraction, which is the simultaneous contraction of muscles on opposite sides of a joint that provides stability.
The body’s response to this challenge is to adapt. Over weeks and months of running training, the neuromuscular system learns to maintain better muscle activation patterns even as fatigue accumulates. This neural adaptation is just as important as the physical growth of muscle tissue itself. The brain and nervous system become more efficient at recruiting the right muscles at the right time, even when the body is tired.
The Transformation of Leg Muscles
The legs undergo perhaps the most dramatic transformation in a runner’s body. The quadriceps, hamstrings, glutes, and calves all develop in response to running. However, the development is not uniform. Different running workouts and training methods emphasize different muscles.
The quadriceps muscles on the front of the thigh become particularly strong in runners. These muscles are responsible for extending the knee and propelling the body forward. However, research has shown that some runners develop what is called quadriceps dominance, where the quadriceps become disproportionately strong compared to the hamstrings. This imbalance can actually increase the risk of injury. The hamstrings, which are located on the back of the thigh, need to be nearly as strong as the quadriceps to maintain proper knee function and prevent excessive stress on the knee joint.
The glutes, particularly the side glutes or hip abductors, are often neglected by runners who focus only on running without supplemental strength training. Yet strength in the side glutes stabilizes every stride and spares the knees from overload. These muscles are critical for maintaining proper hip alignment and preventing the knee from collapsing inward during running. When the glutes are weak, other muscles must compensate, leading to poor movement patterns and increased injury risk.
The calves and other muscles in the lower leg also strengthen significantly. These muscles help control the foot and ankle during the push-off phase of running. A runner’s calf muscles become noticeably more defined and powerful as training progresses. This is not just cosmetic; stronger calves mean better propulsion and more efficient running mechanics.
The Role of Strength Training in Reshaping the Body
While running itself builds some muscle strength, running alone cannot increase muscle strength to the degree needed for optimal performance and injury prevention. This is why strength training is so important for runners. Supplemental strength training targets specific muscle groups and creates the kind of overload stimulus that leads to significant muscle growth and strength gains.
Strength training exercises for runners typically focus on the legs, core, and upper body. Glute bridges, for example, activate the glutes, hamstrings, and core, supporting hip extension and power generation. This exercise strengthens the muscles needed for a strong push-off and helps maintain proper posture during long runs, improving running efficiency and reducing fatigue. Bulgarian split squats and Romanian deadlifts improve overall lower body strength and help address muscle imbalances.
Upper body strength training is equally important, though often overlooked. Strong arms, shoulders, and back muscles help runners maintain good posture and form, especially in the final miles of a marathon or long run. Bent-over rows strengthen the lats, rhomboids, delts, traps, and biceps, helping runners build a strong upper back and arms. This exercise improves arm swing efficiency and posture, contributing to better running mechanics and more effective energy transfer from the arms to the legs.
The combination of running and strength training creates a synergistic effect. Running builds cardiovascular fitness and muscular endurance, while strength training builds the raw strength needed to control movement and prevent injury. Together, they reshape the body in ways that neither could achieve alone.
Joint Health and Adaptation
Contrary to popular belief, running does not necessarily damage joints. A 2023 meta-analysis that examined studies including over 7,000 runners and 6,900 non-runners found that runners had a lower prevalence of knee pain. Those who run with a moderate training program and supplemental strength training have better cartilage health than the average person. This suggests that running, when done properly, actually strengthens and improves joint health.
However, the key phrase is “when done properly.” Excessive running without adequate strength training and recovery can lead to joint problems. If someone does anything in excess, that activity can potentially wear down the joint that is taking the brunt of the stress. With running, this includes the knees, hips, and ankles. Individuals with risk factors like higher body mass and lower muscle mass may be at increased risk of knee injury or arthritis if they do a lot of high intensity or high volume running
