What is the maximal ability of a muscle to generate force?

Explanation:

Índice Show

Where does the energy come from and where does it go?
Putting the relationships together

Flexibility

Flexibility is the range of movement through which a joint or sequence of joints can

move. Inactive individuals lose flexibility, whereas frequent movement helps retain the

range of movement. Through stretching activities, the length of muscles, tendons, and

ligaments is increased. The ligaments and tendons retain their elasticity through constant

use. Flexibility is important to fitness; a lack of flexibility can create health problems for

individuals. People who are flexible usually have good posture and may have less low-

back pain. Many physical activities demand a range of motion to generate maximum

force, such as serving a tennis ball or kicking a soccer ball.

Muscular Strength

Muscular strength is the ability of muscles to exert or resist force. Most activities do not

build strength in areas where it is needed—the arm-shoulder girdle and the abdominal—

trunk region. When you push, pull or lift objects, for example, your muscles are exerting

a force. Muscular strength is the ability of a muscle to produce force at high intensities

for short intervals. The greater muscular strength you have the easier and safer it is to

accomplish most daily activities.

Muscular Endurance

Muscular endurance is the ability of a muscle, or a group of muscles to exert force over

an extended period of time. Endurance postpones the onset of fatigue so that activity can

be performed for lengthy periods. Sport activities require muscular endurance, because

throwing, kicking, and striking skills have to be performed many times without fatigue.

When you rake leaves, shovel snow, or do sit-ups, you are performing acts of muscular

endurance. The better your muscular endurance, the longer you can continue activities

without fatigue.

Cardiovascular Fitness

Aerobic fitness offers many health benefits and is often seen as the most important

element of fitness. Cardiovascular endurance is the ability of the heart, the blood vessels,

and the respiratory system to deliver oxygen efficiently over an extended period of time.

To develop cardiovascular endurance, activity must be aerobic in nature. Activities that

are continuous and rhythmic in nature require that a continuous supply of oxygen be

delivered to the muscle cells. Activities that stimulate development in this area are paced

walking, jogging, biking, rope jumping, aerobics, and swimming. The better your

cardiovascular fitness, the easier it is to complete your daily tasks and still have energy to

enjoy other activities.

Body Composition

Body composition is an integral part of health-related fitness. Body composition is the

percent of fat, bone, and other tissues to lean body mass. After the thickness of selected

skin folds has been measured, the percentage of lean body mass can be calculated by

using formulas that have been developed using other, more accurate methods of

measuring body composition.

There are a number of factors that change the force developed by heart muscle cells. In a manner similar to that seen in skeletal muscle, there is a relationship between the muscle length and the isometric force developed. As the muscle length is increased, the active force developed reaches a maximum and then decreases. This maximum point is the length at which the heart normally functions. As with skeletal muscle, changes in length alter the active force by varying the degree of overlap of the thick myosin and thin actin filaments. The force developed by heart muscle also depends on the frequency at which the muscle is stimulated. As the stimulus frequency is increased, the force is increased until the maximum is reached, at which point it begins to decrease. An increase in the level of circulating epinephrine and norepinephrine from the sympathetic nervous system also increases the force of contraction. All these factors can combine to allow the heart to develop more force when required. At any given length the velocity of contraction is a function of the load lifted, with the velocity decreasing as the load is increased.

Demands on the heart vary from moment to moment and from day to day. In moving from rest to exercise, the cardiac output may be increased tenfold. Other increases in demand are seen when the heart must pump blood against a high pressure such as that seen in hypertensive heart disease. Each of these stresses requires special adjustments. Short-term increases in demand on the heart (e.g., exercise) are met by increases in the force and frequency of contraction. These changes are mediated by increases in sympathetic nervous system activity, an increase in the frequency of contraction, and changes in muscle length. The response to long-term stress (hypertension and thyrotoxicosis) results in an increase in the mass of the heart (hypertrophy), providing more heart muscle to pump the blood, which helps meet the increase in demand. In addition, subtle intracellular changes affect the performance of the muscle cells.

In the pressure-overload type of hypertrophy (hypertensive heart disease), the pumping system of the sarcoplasmic reticulum responsible for calcium removal is slowed while the contractile protein myosin shifts toward slower cross-bridge cycling. The outcome is a slower, more economical heart that can meet the demand for pumping against an increase in pressure. At the molecular level the slowing of calcium uptake is caused by a reduction in the number of calcium pumps in the sarcoplasmic reticulum. The change in the maximum velocity of shortening and economy of force development occur because each myosin cross-bridge head cycles more slowly and remains in the attached force-producing state for a longer period of time.

In the thyrotoxic type of hypertrophy, calcium is removed more quickly while there is a shift in myosin. At the molecular level there are more sarcoplasmic reticular calcium pumps, while the myosin cross-bridge head cycles more rapidly and remains attached in the force-producing state for a shorter period of time. The result is a heart that contracts much faster but less economically than normal and can meet the peripheral need for large volumes of blood at normal pressures.

Because vertebrate smooth muscle is located in the walls of many hollow organs, the normal functioning of the cardiovascular, respiratory, gastrointestinal, and reproductive systems depends on the constrictive capabilities of smooth muscle cells. Smooth muscle is distinguished from the striated muscles of the skeleton and heart by its structure and its functional capabilities.

As the name implies, smooth muscle presents a uniform appearance that lacks the obvious striping characteristic of striated muscle. Vascular smooth muscle shortens 50 times slower than fast skeletal muscle but generates comparable force using 300 times less chemical energy in the process. These differences in the mechanical properties of smooth versus striated muscle relate to differences in the basic mechanism responsible for muscle shortening and force production. As in striated muscle, smooth muscle contraction results from the cyclic interaction of the contractile protein myosin (i.e., the myosin cross bridge) with the contractile protein actin. The arrangement of these contractile proteins and the nature of their cyclic interaction account for the unique contractile capabilities of smooth muscle.

Whether you are a student who wants to be fitter, a netballer who wants a faster more powerful throw, a sprinter who wants to win that race or a weightlifter who wants to lift heavier weights, you are trying to make your muscles work better.

There are three major factors that affect how well your muscles perform – strength, power and endurance.

Strength

Muscle strength is also a result of the combination of three factors:

Physiological strength, which depends on factors such as muscle size, the cross-sectional area of the muscle and responses to training.
Neurological strength, which looks at how weak or how strong the signal is that tells the muscle to contract.
Mechanical strength, which refers to a muscle’s pulling force and the way those forces can be changed using bones and joints as levers.

When we talk about the strength or muscles, we are describing the maximum force a muscle can exert. Muscle strength is directly dependant upon the size of the cross-sectional area of muscle, so if after a period of training, you increase your muscle size by 50%, you will also increase the force the muscle can develop by 50%.

For every 1 square centimetre of cross sectional area, muscle fibres can exert a maximum force of approximately 30–40 newtons (the weight of a 3–4 kg mass).

Example: Emily can lift 21 kg (210 newtons force) using muscles that have a cross-sectional area of 6 cm2. Use this formula to work out how many newtons per square centimetre her muscles can pull with:

Emily’s friend Alisha has larger muscles that have a cross-sectional area of 8 cm2. Use this formula to work out what weight Alisha should be able to lift if her muscle tissue is similar to Emily’s:

Power

When muscles contract or stretch in moving a load they do work, and energy is transferred from one form to another. The power of muscles refers to how quickly the muscles can do this work and transfer the energy.

Example: A weightlifter lifts 100 kg up a distance of 1.5 m. 100 kg has a weight force of 1000 newtons. Use this formula to calculate the work done (energy transferred) by the weightlifter:

If the weightlifter lifts the 100 kg explosively and takes only 0.5 seconds to make the lift, use this formula to calculate the power their muscles produce:

Where does the energy come from and where does it go?

The energy for muscle contraction comes from glucose transported by the blood and deposited in muscle tissues. In the weightlifter example, the energy has been transformed to gravitational potential energy. Also, heat energy will be generated in the muscle tissues themselves. This means that the muscles will have transferred even more energy than the amount calculated above.

Putting the relationships together

There are three different equations that can be simplified to make an even more useful equation:

Because

the formula can be rewritten: power = force × velocity

Sports scientists use this formula to measure the power profiles of particular sets of muscles by measuring both the force of the muscles and the speed with which they are contracting or lengthening. They have found that the greatest power is produced when the load is much less than the maximum load on the muscles.