Neuromuscular Physiology: Force-Velocity Relationship

Neuromuscular Physiology: Force-Velocity Relationship

• Action potentials start in a motor neuron, leading to neuromuscular junction activity.
• This involves acetylcholine release, T-tubule activation, sarcoplasmic reticulum activity, and Calcium ion (Ca++) interaction with troponin C.
• Electrolyte balance (Ca++, Na+, K+, Cl-) is crucial for these processes.
• Troponin (I, C, T) binds to actin and myosin.
• Action potentials uncover active sites for actin-myosin binding.
• ATP energy alters the myosin head's shape, causing it to swivel and generate force, which results in muscle shortening.
• ATP is converted to ADP + P + E (energy).
• The myosin head detaches with a new ATP, returning to its starting shape without energy use.
• Relaxation occurs when Ca++ is pumped back into the sarcoplasmic reticulum.

Types of Muscle Actions Based on Length

• Concentric actions: Muscles shorten while producing force (force > weight).
• Example: lifting a weight in a biceps curl.
• Eccentric actions: Muscles lengthen while producing force (force < weight).
• Example: lowering a weight during a biceps curl.
• Isometric actions: Force is generated, but muscle length remains constant (force = weight).

Muscle Actions Based on Joint Angle or Force Production

• Isokinetic actions involve constant angular velocity while generating force.
• Requires specialized equipment like 'Biodex' that provides accomodating resistance.
• Isotonic actions assume fixed weight equates to constant force production.
• Isotonic is a misnomer, better described as dynamic.
• 'Nautilus' equipment, with kidney-shaped cams, are designed to mimic isotonic conditions.

Force-Velocity Curve

• The curve shows relationship between shortening velocity (or angular velocity) and maximal force production.
• It depends on of muscle actions.

Complete Force-Velocity Relationship

• Muscles generate more force during eccentric actions.
• There is more force per fiber in eccentric actions, known as negatives.

Rationale for Negatives in Resistance Training

• It increases force per fiber and specificity.

Power-Velocity Relationship

• Power is defined as force multiplied by velocity.
• High force production is critical in activities with slow speed.
• Power lifting, arm wrestling and football lineman are examples.
• Otherwise, power is the determining factor (rate of force production), such as in running, jumping, and throwing.

Characteristics of the Power-Velocity Relationship

• Determined by the force-velocity relationship.
• When force is high, velocity is low, and vice versa.
• Rare to find peak power or force, it's the combination of force, speed, and power generated under competition that matters.

Training Considerations

• Strength training is best for moving greater than body weight.
• High-speed training is for moving less than than body weight.
• Typical strength training increases maximal force.
• Plyometrics and light weight training can help achieve peak power at higher velocities, optimal for moving less than body weight.
• For legs, peak power is 10-20% about body weight.