Movement analysis and its applications practice questions.

A volleyball player halts her hitting arm abruptly straight after contact and reports shoulder soreness. Which phase of the spike is being performed poorly?

• Preparation, because the muscles are not stretched sufficiently before the hit
• Force production, because the arm is not accelerated fully during the swing towards the ball
• Recovery, because she does not return to a balanced ready position
• Follow-through, because the deceleration is applied over a shorter time

Answer: Follow-through, because the deceleration is applied over a shorter time

The follow-through allows the same change in arm velocity to be spread over a longer time, so the force on the shoulder structures is smaller. Cutting it short concentrates that force into a brief interval, which is a recognised loading pattern in overhead athletes.

A hockey player extends her arms so that the stick head moves on a radius of 1.00 m instead of 0.80 m, while keeping the angular velocity at 15 rad s-1. What happens to the linear velocity of the stick head?

• It falls from 15 to 12 m s-1
• It rises from 12 to 15 m s-1
• It is unchanged at 15 m s-1, because the angular velocity is unchanged
• It rises from 12 to 18.8 m s-1

Answer: It rises from 12 to 15 m s-1

Linear velocity is given by v = r x omega, so 0.80 x 15 = 12 m s-1 becomes 1.00 x 15 = 15 m s-1. Lengthening the effective radius is how throwers, hitters and strikers convert the same angular velocity into a faster implement.

A vertical jump is analysed on a force platform alongside video. Which of the recorded measurements is a kinetic variable rather than a kinematic one?

• The knee joint angle at the lowest point of the countermovement
• The vertical velocity of the centre of mass at take-off
• The peak vertical ground reaction force during take-off
• The time taken to complete the countermovement

Answer: The peak vertical ground reaction force during take-off

Kinetic variables describe the forces that cause motion, whereas kinematic variables describe the motion itself in terms of displacement, velocity, acceleration and time. Only the force platform trace measures a force directly, which is why it is added to video when the cause of a movement is under study.

A coach films a long jumper from the side at 100 frames per second and measures the knee angle at take-off. Which limitation of this two-dimensional analysis is most relevant to that measurement?

• Movement out of the filming plane is not recorded accurately
• Joint angles cannot be obtained from a video recording at all
• Angular velocity cannot be calculated from a sequence of frames
• The method yields only qualitative information about the movement

Answer: Movement out of the filming plane is not recorded accurately

A single camera records only the plane it faces, so any rotation or displacement towards or away from the lens distorts the measured angle; three-dimensional analysis needs at least two synchronised cameras. The frame rate here is high enough to capture take-off, so temporal resolution is not the limiting factor.

A diver leaves the board and then pulls into a tight tuck, after which she rotates noticeably faster. Which quantity stays constant while she is in the air?

• Her moment of inertia
• Her angular velocity
• Her angular momentum
• The radius of her rotation

Answer: Her angular momentum

With no external turning force acting once she has left the board, angular momentum is conserved, so drawing the mass closer to the axis reduces the moment of inertia and the angular velocity must rise to compensate. Opening out again slows the rotation, which is how a diver controls the entry.

What determines the flight path of a long jumper's centre of mass after take-off?

• The velocity, angle and height at the moment of take-off
• The leg movements made by the athlete while airborne
• The position of the arms at the moment of landing
• The horizontal velocity measured over the final stride only

Answer: The velocity, angle and height at the moment of take-off

Once the athlete is airborne, gravity is the only significant external force, so the flight path of the centre of mass is fixed by the conditions at the board. Movements in the air rearrange the body around that path and can add several centimetres at landing, but they cannot move the path itself.