Muscle Dynamics: Roles, Kinetic Chains, and Strength Factors
Understanding Muscle Roles in Human Movement
In any movement, muscles perform different functions. For example, in a vertical jump, the extensor muscles of the lower limb (such as the gastrocnemius, quadriceps, and glutes) contract, pulling on bones and levers to cause movement. These muscles, which are the primary movers of a gesture, are called AGONISTS.
However, for the gesture to be successful, it is necessary for the opposing muscles (flexors of the leg: tibialis anterior, hamstrings, and iliopsoas) to relax, allowing for maximum effectiveness in the jump. These muscles, which oppose the main action, are called ANTAGONISTS.
Finally, trunk muscles, particularly those in the abdomen and waist, stabilize the bone segments of the trunk. This allows for greater vertical force and helps avoid losses of balance during the jump. These muscles are not the primary movers of the gesture, but they aid in its proper execution through a stabilizing function. They are called FIXATORS.
The Kinetic Chain: Coordinated Muscle Action
With this understanding, it becomes easier to grasp the concept of the kinetic chain. This refers to a series of joints and muscles that are involved in a coordinated manner in the execution of a movement. The greater the coordination between the different muscles involved, the more effective the kinetic chain and technical gesture will be. The chain always progresses from the point of support until the last segment to act in motion. A clear example is the kinetic chain observed in a medicine ball throw.
Factors Influencing Muscle Strength
Muscles cannot always develop the same force. Many factors influence muscle contraction. We can group these factors into four main categories:
Physiological and Anatomical Strength Factors
- Muscle Cross-Section: A thicker or larger muscle volume generally correlates with greater strength.
- Predominant Fiber Type: A higher percentage of fast-twitch muscle fibers leads to greater strength.
- Muscle Length: Longer muscle fibers typically generate more strength.
- Muscle Innervation: More innervated muscle fibers result in a greater capacity for muscle contraction.
Physical or Mechanical Strength Factors
- Joint Angle: The angle of the joint in a particular movement significantly impacts strength. For instance, at 90°, 100% strength is often reached, while at 180°, approximately 40% of maximum strength may be lost.
- Joint Rotation: The degree of joint rotation determines the intensity of the force produced.
- Muscle Pre-Stretching: If not excessive, pre-stretching of a muscle favors a greater contraction, thus producing a stronger effective force.
Psychological Aspects of Muscle Strength
- Motivation, Attention, and Concentration: These factors, along with willpower and a spirit of sacrifice, significantly influence strength.
- Emotional States: Emotional states can also alter nerve discharges, varying muscle strength.
Other Key Factors Affecting Muscle Force
- Sex: Generally, women have less strength than men due to lower concentrations of male hormones, which increase muscle mass.
- Age: Up to 12 years, strength is almost equal in girls and boys. In boys, strength increases very significantly between 14 and 17 years and reaches its maximum around age 30. In girls, the increase is less noticeable, reaching its maximum around 20-25 years, after which strength begins to decrease.
- Nutrition: A balanced intake of protein, carbohydrates, fats, minerals, and vitamins is essential for strength training and good muscle function.
- Muscle Temperature: A good warm-up increases the capacity for effective force. A warm muscle increases blood flow and oxygen supply, thereby improving its performance.
- Ambient Temperature: Average ambient temperatures favor the ability to contract, while excessively low and high temperatures reduce muscle contraction ability.
- Fatigue: Fatigue reduces the intensity and extent of muscle contraction and may cause injury.
- Training Level: Regular training increases muscle contraction capacity, while excessive fatigue or lack of sleep decreases it.