Fluid Mechanics and Movement Analysis

B.2.2 Fluid Mechanics

B.2.2.1 Projectile Motion in Air

• The path of a projectile through the air is determined by various factors and forces.

B.2.2.2 Environmental Factors

• Environmental conditions like temperature, humidity, air pressure, wind, and salinity of water and altitude affect the external forces acting on an object.

B.2.2.3 Forces in Fluids

• Forces such as buoyancy, lift, and drag impact an object moving through a fluid (air or water).
• A projectile traveling through a fluid can be affected by:
  • Bernoulli's principle
  • The angle of attack
  • The Magnus effect

Introduction to Fluid Mechanics

• Humans and objects moving through fluids (gas or liquid) experience external resistive forces.
• Understanding the direction and magnitude of these forces, and how environmental conditions affect them, is crucial.
• These forces apply during flight (e.g., diving) or translation in contact with the ground (e.g., cycling).
• Forces are measured in Newtons (N).

Projectile Motion Explained

• Projectile motion refers to the movement of an object projected into the air or dropped, influenced only by gravity and air resistance.
• When air resistance is negligible, only gravity acts on the object. The horizontal component of velocity remains constant, while the vertical component changes.

Projectiles in Sports

• Many sports involve projectiles, either implements or the human body itself.
• Examples:
  • A football, golf ball, or volleyball in flight.
  • The human body during a long jump or the flight phase of running.

Factors Affecting Projectile Trajectory

• The trajectory and range of a projectile depend on:
  • Initial conditions of projection
  • Gravity
  • Air resistance

Initial Conditions of Projection

• Initial conditions include:
  • Initial velocity (v)
  • Angle of projection (\theta)
  • Height of release (h)

Initial Velocity

• The flight path is significantly affected by the initial velocity.
• The projectile's range is related to the square of the initial velocity; thus, a small increase in initial velocity greatly increases the range.

Angle of Projection

• The angle of projection is crucial for:
  • Maximum height of flight (e.g., volleyball serve over the net).
  • Accuracy (e.g., basketball free throw).
• The angle of projection significantly affects the range.
  • When the projection height equals the landing height, a 45° angle maximizes range.
  • If the landing height is higher than the projection height, the optimal release angle is greater than 45°.

Height of Release

• The height of projection also affects the projectile range; a higher projection height results in a longer range.
• Projection height is important for:
  • Beating an opponent (e.g., basketball jump shot or volleyball spike).
  • Maximizing possible values for initial velocity or angle.

Optimum Angle of Projection

• To achieve maximum range, the optimum projection angle depends on the initial velocity and, more importantly, the projection height.
  • If the projection height is above the landing area, the optimum projection angle is below 45° (e.g., shot-put).
  • If the projection height is below the landing area, the optimum projection angle is above 45° (e.g., basketball free throw).
  • If the projection height is the same as the landing height, the optimum angle is 45° (e.g., a goal kick in soccer).
• These angles are also influenced by air resistance and complex factors like muscle contraction speed.
• Long jumpers take off at 18°-27° rather than 45° due to these factors.

Typical Projection Angles in Sports

• Table 1 provides typical projection angles for various sporting activities.
• Examples:
  • Long jump take-off: 18-27°
  • High jump take-off
  • Shot-put: 35-42°
  • Basketball free throw: 50-60°
  • Tennis first serve: -3 to -15° (negative angles as the ball is served down into the court)
  • Golf drive: 10-20° (low angle because backspin creates lift)

Centre of Mass

• The centre of mass is the mathematical point around which the mass of a body or object is evenly distributed.
• It depends on the distribution of material in the body or object, affected by density and shape.
• The centre of mass doesn't need to lie within the material of the body or object.
  • Example: the centre of mass of a boomerang is in the space between the arms.
• This is also true for the human body, especially in sports like high jump or pole vault when clearing the bar.

Importance of Knowing the Centre of Mass

• It determines the stability of static positions.
  • If the vertical projection of a line downwards from the centre of mass lies within the base of support, the position is stable.
• It is the axis for all free airborne rotations of the body or object (e.g., somersaulting in diving).
• It acts as the reference point for whole-body or object translation (e.g., long jump).

Centre of Mass in Long Jump

• The trajectory of the centre of mass during take-off, flight, and landing is critical for understanding the distance jumped.
• Hay et al. (1986) measured the distances of phases L₁, L₂, L₃ of long jumps by four athletes.

Measurement of Centre of Mass

• The centre of mass can be measured in several ways (e.g., calculations from segmental positions and masses, reaction board, suspension of an object or model).
• Measurements are accurate to approximately 1-2 millimeters for the human body, accounting for errors from breathing, blood circulation, and inaccuracies in segment densities and positions.

Projectile Pathway of the Body's Centre of Mass

• Once the human body has been projected into the air, the athlete cannot change the pathway of the flight.
• The pathway of the centre of mass is a projectile path influenced by gravity, with air resistance considered negligible.
• During the flight, the athlete can move different body segments; however, the pathway of the centre of mass will not be affected.

Air Resistance

• Both the overall pathway and the range of the projectile are affected by air resistance.
• The ratio of weight to air resistance influences the object's flight path.
• Faster-moving objects experience more air resistance.
  • For slow-moving objects, air resistance is small compared to gravity, so the object follows a typical projectile flight path.
  • For fast-moving objects, increased air resistance causes faster deceleration, affecting the flight path.
• Example: In badminton, the shuttlecock moves at high velocities but decelerates quickly due to high air resistance relative to its weight. The shape of the projectile pathway is not symmetrical.
• Feather shuttlecocks experience greater air resistance compared to synthetic ones due to their lower mass.

Environmental Conditions

• Environmental conditions, such as temperature, humidity, wind, air pressure, and salinity of water and altitude, affect the forces acting on an object.
• Temperature affects the density of air and water; however, the effect on forces is minimal within typical athletic temperature ranges (0-30°C) due to small variations in density values.

B.2.3 Movement Analysis and its Applications

B.2.3.1 Phases of Movement Approach

• A "phases of movement" approach is used to break down and describe movements.

Introduction to Movement Analysis

• Movement analysis requires identifying different phases of a physical task.
• Performance is observed or video recorded to identify biomechanical flaws or inefficient movement patterns independently for each phase to prevent injuries or improve performance.

Phases of Movement Approach Explained

• Phases of movement are categorized into different stages to help athletes and coaches understand and analyze skill execution.
• Key phases include:
  • Preparatory phase
  • Force production phase
  • Critical instant
  • Follow-through (for discrete skills) or Recovery phase (for continuous skills)
• Discrete skills have a fixed beginning and end with a short duration (e.g., throwing a ball or jumping).
• Continuous skills have no clearly defined beginning and end with a longer duration (e.g., swimming or running).

Discrete Skills

Preparatory Phase

• The initial stage where the athlete prepares mentally and physically for skill execution.
• Athletes focus on body position, balance, and optimal positioning.
• Anticipation of the skill's demands is a key component.

Force Production Phase

• Involves the synchronized movement of the skeletal and muscular systems to produce the movement and force needed to perform the task.
• In discrete skills (e.g., golf swing or tennis serve), this phase applies force to an object.
• In continuous skills (e.g., running, cycling, or swimming), it refers to applying force against the ground, pedals, or water to move the body.
• Good technique, timing, and coordination help maximize force application and performance.

Critical Instant

• The culmination of the preparation and force-producing phases.
• The specific moment within the skill execution that significantly influences the skill's outcome.
• Examples:
  • Point of contact/impact in striking sports (e.g., tennis, baseball, cricket).
  • Moment of contact loss in jumping sports (e.g., diving, trampolining, volleyball).
  • Point of release (e.g., throwing a javelin).

Follow-Through

• Occurs immediately after the critical instant in discrete skills.
• Involves continuing the motion and maintaining correct body alignment and form.
• Helps prevent injury, enhance accuracy, and improve the overall feel and control of the skill.

Discrete Skill Movement Analysis Example

• Consider the phases of movement of the forehand drive in tennis as a discrete skill with a distinct beginning and end.
• Phases are:
  • Preparatory phase
  • Force production phase
  • Critical instant
  • Follow-through

Continuous Skills

• For continuous skills (e.g., running, cycling, or swimming), phases of movement are identified based on key events (e.g., "heel strike" in running or "hand entry" in swimming).
• Each phase is analyzed, and inefficient movement patterns are identified.

Recovery Phase

• Takes place after one cycle or repetition of the movement.
• In swimming, the recovery phase occurs after completing one stroke as the swimmer positions their arms for the next stroke.
• A smooth and efficient recovery is essential for maintaining momentum and rhythm in continuous skills.

Continuous Skill Movement Analysis Example

• Consider the phases of movement in front crawl swimming.
• An arm stroke is divided into four phases based on the positions of the hand and shoulder on the horizontal axis:
  • Entry phase: begins when the hand enters the water.
  • Pull phase: begins when the hand starts moving backwards (on the horizontal axis).
  • Push phase: begins when the hand is aligned vertically with the shoulder.
  • Recovery phase: begins when the hand exits the water.
• Analysis includes time variables (duration of each phase) or spatial parameters (joint angles or distance covered).

Benefits of Movement Analysis for Health and Safety

• Helps identify biomechanical flaws or inefficient movement patterns that may contribute to injury risk.
• Healthcare professionals can collaborate with sports coaches and athletes to design targeted injury prevention programs, corrective exercises, and rehabilitation protocols.
• Video analysis, biomechanics, and motion capture technologies are used to assess and quantify the different phases of movement precisely.

Benefits of Movement Analysis for Sporting Performance

• Allows coaches to identify technical flaws in athletes' movements.
• By breaking down skills into phases, coaches can provide targeted feedback and design training interventions to enhance technique, consistency, and efficiency.
• Other applications include:
  • Optimizing biomechanics to improve power, speed, and performance.
  • Developing tactical insights of games players to develop effective strategies and improve decision-making.
  • Injury management, assessing progress, and readiness for an athlete to resume sporting activities.
  • Helping to track and manage an athlete's workload and reduce the risk of overuse injuries.