Internal Forces in the Human Body: Direct vs Indirect

Question 1

What are External Forces?

Answer 1

External forces can change the state of a system (gravity, ground reaction forces, air resistance)

Question 2

What are Internal Forces?

Answer 2

Internal forces are generated within the system. (musculoskeletal forces).

Question 3

Name some Internal Forces.

Answer 3

• Joint Contact forces
• Ligament forces and other soft tissue forces
• Muscle/Tendon forces

Question 4

What are the direct ways of measuring muscle/tendon forces and what are their Limitations/Problems?

Answer 4

Direct ways = Buckle transducers and Optic fibres
Limitations = Invasive procedure
Ethical issues
Tendon suitability - not all tendons are suitable
Calibration

Question 5

How can we indirectly measure msucle force?

Answer 5

EMG - high correlations exist between muscle force and muscle activity in clinical isometric settings
Inverse Dynamics = calculates the internal joint forces and then subsequently the net turning torque (moment) of a joint. Torque = f*d

Question 6

What kinetic and kinematic equipment can be used to calculate internal forces?

Answer 6

Kinetics - force plates, linear transducers, accelerometers

Kinematics - High speed cameras, Infared auto tracking, Cineradiography

Question 7

What are the limitations of calculating segmental inertial properties?

Answer 7

Each segment is uniform in density and rigid
The segmental mass can be considered as a single point at the segment’s centre of mass
The segmental moment of inertia remains constant throughout the movment.

Question 8

What are the limitations to inverse dynamics?

Answer 8

• Articulating surfaces in the joints are smooth and do not create a frictional force
• The forces act through the centre of the joint and the centre of the joint does not change in relation to the corresponding connecting segments
• Foot segment is assumed to be rigid however doesn’t include the metatarsal segment and joint.

Question 9

What is Muscle Power?

Answer 9

Muscle power = Muscle moment multiplied by angular velocity.

MP - MMa.w

Question 10

What are the mechanical and performance variables of speed & power tests?
(Vertical jumps, CMJ, Drop Jumps)

Answer 10

• Impulse
• Takeoff velocity
• Power
• Height of the Jump (CG)
• RFD
• Speed-strength (jump squat)
• RSI (drop jumps)

Question 11

What happens with ankle and hip contribution during sprinting?

Answer 11

Initially hip contribution is more in the early stages of the sprint however towards the end of the sprint as speed reaches its max the ankle contribution exceeds other joints.