Biomechanics In Sport
Original Editor - Naomi O'Reilly
- 1 Introduction
- 2 Application
- 3 Principles
- 4 Lower Limb Biomechanics
- 5 Upper Limb Biomechanics
- 6 Sport Specific Biomechanics
- 7 References
Biomechanics in sport incorporates detailed analysis of sport movements in order to minimise the risk of injury and improve sports performance. Sport and exercise biomechanics encompasses the area of science concerned with the analysis of the mechanics of human movement. It refers to the description, detailed analysis and assessment of human movement during sport activities . Mechanics is a branch of physics that is concerned with the description of motion/movement and how forces create motion/movement. In other words sport biomechanics is the science of explaining how and why the human body moves in the way that it does. In sport and exercise that definition is often extended to also consider the interaction between the performer and their equipment and environment. Biomechanics is traditionally divided into the areas of kinematics which is a branch of mechanics that deals with the geometry of the motion of objects, including displacement, velocity, and acceleration, without taking into account the forces that produce the motion while kinetics is the study of the relationships between the force system acting on a body and the changes it produces in body motion . In terms of this there are skeletal, muscular and neurological considerations we also need to consider when describing biomechanics .
According to Knudson  human movement performance can be enhanced in many ways as effective movement encompasses anatomical factors, neuromuscular skills, physiological capacities and psychological/cognitive abilities. Biomechanics is essentially the science of movement technique and as such tends to be most utilised in sports where technique is a dominant factor rather than physical structure or physiological capacities . The following are of the areas where biomechanics is applied to either support performance of athletes or solve issues in sport or exercise:
- The identification of optimal technique for enhancing sports performance
- The analysis of body loading to determine the safest method for performing a particular sport or exercise task
- The assessment of muscular recruitment and loading
- The analysis of sport and exercise equipment e.g., shoes, surfaces and rackets.
Biomechanics is utilised to attempt to either enhance performance or reduce the injury risk in the sport and exercise tasks examined.
It is important to know several biomechanical terms and principles when examining the role of biomechanics in sport and exercise.
Forces and Torques
A force is simply a push or pull and it changes the motion of a body segment or the racket. Motion is created and modified by the actions of forces (mostly muscle forces, but also by external forces from the environment). When force rotates a body segment or the racket, this effect is called a torque or moment of force . Example - Muscles create a torque to rotate the body segments in all tennis strokes. In the service action internal rotation of the upper arm, so important to the power of the serve, is the result of an internal rotation torque at the shoulder joint caused by muscle actions (latissimus dorsi and parts of the pectoralis major and deltoid). To rotate a segment with more power a player would generally apply more muscle force.
Newton’s Laws of Motion
Newton’s Three Laws of Motion explain how forces create motion in sport. These laws are usually referred to as the Laws of Inertia, Acceleration, and Reaction. 
- Law of Inertia - Newton’s First Law of inertia states that objects tend to resist changes in their state of motion. An object in motion will tend to stay in motion and an object at rest will tend to stay at rest unless acted upon by a force. Example - The body of a player quickly sprinting down the field will tend to want to retain that motion unless muscular forces can overcome this inertia.
- Law of Acceleration - Newton’s Second Law precisely explains how much motion a force creates. The acceleration (tendency of an object to change speed or direction) an object experiences is proportional to the size of the force and inversely proportional to the object’s mass (F = ma). Example - If a player improves leg strength through training while maintaining the same body mass, they will have an increased ability to accelerate the body using the legs, resulting in better agility and speed. This also relates to the ability to rotate segments, as mentioned above.
- Law of Reaction - The Third Law states that for every action (force) there is an equal and opposite reaction force. This means that forces do not act alone, but occur in equal and opposite pairs between interacting bodies. Example - The force created by the legs “pushing” against the ground results in ground reaction forces in which the ground “pushes back” and allows the player to move across the court (As the Earth is much more massive than the player, the player accelerates and moves rapidly, while the Earth does not really accelerate or move at all). This action-reaction also occurs at impact with the ball as the force applied to the ball is matched with an equal and opposite force applied to the racket/body.
Newton’ Second Law is also related to the variable momentum, which is the product of an object’s velocity and mass. Momentum is essentially the quantity of motion an object possesses. Momentum can be transferred from one object to another. There are different types of momentum which each have a different impact on the sport.
Is momentum in a straight line. Example, Linear momentum is created as the athlete sprints in a straight line down the 100m straight on the track.
Is rotational momentum and is created by the rotations of the various body segments e.g. The open stance forehand uses significant angular momentum. The tremendous increase in the use of angular momentum in ground strokes and serves has had a significant impact on the game of tennis. One of the main reasons for the increase in power of the game today is the incorporation of angular momentum into ground stroke and serve techniques. In tennis, the angular momentum developed by the coordinated action of body segments transfers to the linear momentum of the racket at impact.
The Center of Gravity (COG) is an imaginary point around which body weight is evenly distributed. The center of gravity of the human body can change considerably because the segments of the body can move their masses with joint rotations. This concept is critical to understanding balance and stability and how gravity affects sport techniques. 
The direction of the force of gravity through the body is downward, towards the center of the earth and through the COG. This line of gravity is important to understand and visualise when determining a person's ability to successfully maintain balance. When the line of gravity falls outside the Base of Support (BOS), then a reaction is needed in order to stay balanced.
The center of gravity of a squash racket is a far simpler process and can usually be found by identifying the point where the racket balances on your finger or another narrow object. 
Balance is the ability of a player to control their equilibrium or stability. You need to have a good understanding of both static and dynamic balance:
The ability to control the body while the body is stationary. It is the ability to maintain the body in some fixed posture. Static balance is the ability to maintain postural stability and orientation with center of mass over the base of support and body at rest.
The ability to control the body during motion. Defining dynamic postural stability is more challenging, Dynamic balance is the ability to transfer the vertical projection of the center of gravity around the supporting base of support. Dynamic balance is the ability to maintain postural stability and orientation with center of mass over the base of support while the body parts are in motion.
Lower Limb Biomechanics
As humans ambulation is our main form of movement, that is we walk upright and are very reliant on our legs to move us about. How the foot strikes the ground and the knock on effect this has up the lower limbs to the knee, hips, pelvis and low back in particular has become a subject of much debate and controversy in recent years.
Lower limb biomechanics refers to a complex interplay between the joints, muscles and nervous system which results in a certain patterning of movement, often referred to as ‘alignment’. Much of the debate centers around what is considered ‘normal’ and what is considered ‘abnormal’ in biomechanical terms as well as the extent to which we should intervene should abnormal findings be found on assessment. This section examines the biomechanics of the lower extremity in particular the anatomy and biomechanics of the foot and ankle, the impact of Q Angle on the mechanics of the hip and knee and finally the implications of this on gait.
The foot and ankle form a complex system which consists of 26 bones, 33 joints and more than 100 muscles, tendons and ligaments. It functions as a rigid structure for weight bearing and it can also function as a flexible structure to conform to uneven terrain. The foot and ankle provide various important functions which includes: supporting body weight, providing balance, shock absorption, transferring ground reaction forces, compensating for proximal malalignment, and substituting hand function in individuals with upper extremity amputation/paralysis all which are key when involved with any exercise or sport involving the lower limbs. This page examines in detail the biomechanics of the foot and ankle and its role in locomotion . Go to Page
An understanding of the normal anatomical and biomechanical features of the patellofemoral joint is essential to any evaluation of knee function. The Q angle formed by the vector for the combined pull of the quadriceps femoris muscle and the patellar tendon, is important because of the lateral pull it exerts on the patella .
The direction and magnitude of force produced by the quadriceps muscle has great influence on patellofemoral joint biomechanics. The line of force exerted by the quadriceps is lateral to the joint line mainly due to large cross-sectional area and force potential of the vastus lateralis. Since there exists an association between patellofemoral pathology and excessive lateral tracking of the patella, assessing the overall lateral line of pull of the quadriceps relative to the patella is a meaningful clinical measure. Such a measure is referred to as the Quadriceps angle or Q angle. It was initially described by Brattstrom . Go to Page
Sandra J. Shultz  describes gait as: “...someone’s manner of ambulation or locomotion, involves the total body. Gait speed determines the contribution of each body segment. Normal walking speed primarily involves the lower extremities, with the arms and trunk providing stability and balance. The faster the speed, the more the body depends on the upper extremities and trunk for propulsion as well as balance and stability. The legs continue to do the most work as the joints produce greater ranges of motion through greater muscle responses. In the bipedal system the three major joints of the lower body and pelvis work with each other as muscles and momentum move the body forward. The degree to which the body’s center of gravity moves during forward translation defines efficiency. The body’s center moves both side to side and up and down during gait.” Bipedal walking is an important characteristic of humans.This page will present information about the different phases of the gait cycle and important functions of the foot while walking . Go to Page
Upper Limb Biomechanics
Correct biomechanics are as important in upper limb activities as they are in lower limb activities. The capabilities of the upper extremity are varied and impressive. With the same basic anatomical structure of the arm, forearm, hand, and fingers, major league Baseball Pitchers pitch fastballs at 40 m/s, swimmers cross the English Channel, gymnasts perform the iron cross, and olympic boxers in weight classes ranging from flyweight to super heavyweight showed a range of 447 to 1,066 pounds of peak punching force.
The structure of the upper extremity is composed of the shoulder girdle and the upper limb. The shoulder girdle consists of the scapula and clavicle, and the upper limb is composed of the arm, forearm, wrist, hand, and fingers. However, a kinematic chain extends from the cervical and upper thoracic spine to the fingertips. Only when certain multiple segments are completely fixed can these parts possibly function independently in mechanical roles.
This section reviews the anatomical structures enabling these different types of movement and examines the biomechanics or ways in which the muscles cooperate to achieve the diversity of movement of which the upper extremity is capable.
Scapulohumeral rhythm (also referred to as glenohumeral rhythm) is the kinematic interaction between the scapula and the humerus, first published by Codman in the 1930's. This interaction is important for the optimal function of the shoulder. When there is a change of the normal position of the scapula(describe) relative to the humerus, this can cause a dysfunction of the scapulohumeral rhythm. The change of the normal position is also called scapular dyskinesia. Various studies of the mechanism of the shoulder joint that have attempted to describe the global motion capacity of the shoulder refer to that description, Can you evaluate the shoulder to see if the function is correct and explain the complex interactions between components involved in placing the hand in space? Go to Page
Sport Specific Biomechanics
Running is similar to walking in terms of locomotive activity. However, there are key differences. Having the ability to walk does not mean that the individual has the ability to run . There are some differences between the gait and run cycle - the gait cycle is one third longer in time, the ground reaction force is smaller in the gait cycle (so the load is lower), and the velocity is much higher. In running, there is also just one stance phase while in stepping there are two. Shock absorption is also much larger in comparison to walking. This explains why runners have more overload injuries .
Cycling was initially invented by Baron Carl von Drais in 1817, but not as we know it. This was a machine which initailly had two wheels that were connected by a wooden plank with a rudder device for steering. It involved people running along the ground whilst sitting down; giving them the name of a 'running machine' (in all senses) or a velocipied. This was solely used by the male population at the time of invention. The velocipied then made a huge design development in the 1860's at the Michaux factory in Paris. They added leaver arms to the front wheel which were propelled by pedals at the feet. This was the first conventional bicycle, and since then and up until the current day the bicycle has made great design and technological advances .
A survey in 2014 estimated that over 43% of the United Kingdom population have or have access to a bike and 8% of the population aged 5 and above cycled 3 or more times a week. With such a large amount of people cycling, whether it be professional, recreational or for commuting this increase the chance of developing an injury, so it is time we understood the biomechanics of cycling . Go to Page
Baseball pitching is one of the most intensely studying athletic motions . Although focus has been more on the shoulder movement, entire body movement is required to perform baseball pitching. Throwing is also considered one of the fastest human motions performed, and maximum humeral internal rotation velocity reaches about 7000 to 7500o/second . Go to Page
Tennis biomechanics is a very complex task. Consider hitting a tennis ball. First the athlete needs to see the ball coming off their opponent's racket. Then, in order, they have to judge the speed, spin, trajectory and, most importantly, the direction of the tennis ball. The player then needs to adjust their body position quickly to move around the ball. As the player returns prepares to hit the ball the body is in motion, the ball is moving both in a linear and rotation direction if there is pin on the ball, and the racket is also in motion. The player must coordinate all these movements in approximately a half a second so they strike the ball as close to the center of the racket in order to produce the desired spin,speed and direction for return of the ball. A mistake in any of these movements can create an error. 
The International Tennis Federation (ITF) provides detailed resources on tennis biomechanics including a number of presentations below.
These articles provide some more detailed information on Serve and Ground Stroke Biomechanics and also look at the implications for strength training and rehabilitation.
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- Cycling UK. Cycling UK Cycling Statistics. http://www.cyclinguk.org/resources/cycling-uk-cycling-statistics#How many people cycle and how often? (accessed 24 May 2015)
- Seroyer ST, Nho SJ, Bach BR, Bush-Joseph CA, Nicholson GP, Romeo AA. The Kinetic Chain in Overhand Pitching: Its Potential Role for Performance Enhancement and Injury Prevention. Sports Health: A Multidisciplinary Approach. 2010 Mar 1;2(2):135-46.
- Tennis Mind Training. Basics of Tennis Biomechanics. http://tennis-mind-training.com/tennis-biomechanics.html#sthash.ptoeFJzA.dpuf (accessed: 1 June 2016)