Physical strength
Physical strength is the ability of a person or animal to exert force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

 on physical objects using muscles
Skeletal muscle
Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system- i.e. it is voluntarily controlled. It is one of three major muscle types, the others being cardiac and smooth muscle...

. Increasing physical strength is the goal of strength training
Strength training
Strength training is the use of resistance to muscular contraction to build the strength, anaerobic endurance, and size of skeletal muscles. There are many different methods of strength training, the most common being the use of gravity or elastic/hydraulic forces to oppose muscle contraction...



An individual's physical strength is determined by two factors; the cross-sectional area of muscle fibers recruited to generate force and the intensity of the recruitment. Individuals with a high proportion of type I slow twitch muscle fibers will be relatively weaker than a similar individual with a high proportion of type II fast twitch fibers, but would have a greater inherent capacity for physical endurance. The genetic inheritance of muscle fiber type sets the outermost boundaries of physical strength possible (barring the use of enhancing agents such as testosterone), though the unique position within this envelope is determined by training. Individual muscle fiber ratios can be determined through a muscle biopsy. Other considerations are the ability to recruit muscle fibers for a particular activity, joint angles, and the length of each limb. For a given cross-section, shorter limbs are able to lift more weight. The ability to gain muscle also varies person to person, based mainly upon genes dictating the amounts of hormones secreted, but also on sex, age, health of the person, and adequate nutrients in the diet. A one rep maximum
One rep maximum
One rep maximum in weight training is the maximum amount of weight one can lift in a single repetition for a given exercise. One repetition maximum can be used for determining an individual's maximum strength and is the method for determining the winner in events such as powerlifting and...

 is the test to determine maximum muscular strength.

Strength capability

There are various ways to measure physical strength of a person or population. Strength capability analysis is usually done in the field of ergonomics
Ergonomics is the study of designing equipment and devices that fit the human body, its movements, and its cognitive abilities.The International Ergonomics Association defines ergonomics as follows:...

 where a particular task (e.g. lifting a load, pushing a cart, etc.) and/or a posture is evaluated and compared to the capabilities of the section of the population that the task is intended towards. The external reactive moments
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

 and forces on the joints are usually used in such cases. The strength capability of the joint is denoted by the amount of moment that the muscle force can create at the joint to counter the external moment.

Skeletal muscles produce reactive forces and moments at the joints. To avoid injury or fatigue, when person is performing a task, such as pushing or lifting a load, the external moments created at the joints due to the load at the hand and the weight of the body segments must be ideally less than the muscular moment strengths at the joint.

One of the first sagittal-plane
Sagittal plane
Sagittal plane is a vertical plane which passes from front to rear dividing the body into right and left sections.-Variations:Examples include:...

 models to predict strength was developed by Chaffin in 1969. Based on this model, the external moments at each joint must not exceed the muscle strength moments at that joint.

Mj/L < Sj

Where, Sj is the muscle strength moment at joint, j, and Mj/L is the external moment at the joint, j, due to load, L and the body segments preceding the joint in the top-down analysis.

Top-down analysis is the method of calculating the reactive moments and forces at each joint starting at the hand, all the way till the ankle and foot. In a 6-segment model, the joints considered are elbow, shoulder, L5/S1 disc of the spine
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

, hip, knee and ankle. It is common to ignore the wrist joint in manual calculations. Software intended for such calculation use the wrist joint also, dividing the lower arm into hand and forearm segments.

Prediction of static strength

Static strength prediction is the method of predicting the strength capabilities of a person or a population (based on anthropometry) for a particular task and/or posture (an isometric contraction
Isometric exercise
Isometric exercise or isometrics are a type of strength training in which the joint angle and muscle length do not change during contraction...

). Manual calculations are usually performed using the top-down analysis on a six or seven-link model, based on available information about the case and then compared to standard guidelines, such as the one provided by the National Institute for Occupational Safety and Health
National Institute for Occupational Safety and Health
The National Institute for Occupational Safety and Health is the United States’ federal agency responsible for conducting research and making recommendations for the prevention of work-related injury and illness. NIOSH is part of the Centers for Disease Control and Prevention within the U.S...

, to predict capability.
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