Levers in movement and sport practice questions.

In a standing dumbbell curl the elbow is the fulcrum, the biceps brachii inserts close to that joint and the dumbbell is held in the hand. Which lever class is operating?

• First class, because the fulcrum lies between the effort and the resistance
• Second class, because the resistance lies between the fulcrum and the effort
• Third class, because the effort lies between the fulcrum and the resistance
• Second class, because the effort arm is shorter than the resistance arm

Answer: Third class, because the effort lies between the fulcrum and the resistance

The order fulcrum, effort, resistance defines a third-class lever, the commonest arrangement in the body. Because the effort arm is shorter than the resistance arm, mechanical advantage is below 1 and the muscle force exceeds the load.

A dancer rises onto the balls of both feet. Taking the metatarsophalangeal joints as the fulcrum and body weight as acting at the ankle, which arrangement makes this a second-class lever?

• Resistance between the fulcrum and the effort
• Effort between the fulcrum and the resistance
• Fulcrum between the effort and the resistance
• Effort and resistance acting at the very same point

Answer: Resistance between the fulcrum and the effort

With the toes as fulcrum, body weight in the middle at the ankle and the plantar flexors pulling on the calcaneus behind it, the resistance sits between fulcrum and effort. That gives an effort arm longer than the resistance arm, so mechanical advantage exceeds 1.

A badminton player flicks the wrist to accelerate the racket head. Most levers in the human body, including this one, are third class. What does that arrangement provide, given that its mechanical advantage is below 1?

• A muscle force smaller than the load being moved
• A larger moment for the same amount of muscle force
• Greater stability of the joint throughout the whole range of motion
• A large range and speed of movement at the far end of the limb

Answer: A large range and speed of movement at the far end of the limb

A short effort arm forces the muscle to produce more force than the load, but a small amount of muscle shortening sweeps the far end of the limb through a long arc very quickly. Range and speed, not force, are the pay-off.

A javelin thrower releases with the throwing arm fully extended rather than with the elbow flexed. What is the mechanical consequence at the shoulder?

• The effort arm lengthens, so the mechanical advantage of the lever rises above 1
• The resistance arm shortens, so the muscles at the shoulder need to produce less force
• The resistance arm lengthens, so the hand travels faster at the same angular velocity
• The moment of the implement about the shoulder is unchanged by arm position

Answer: The resistance arm lengthens, so the hand travels faster at the same angular velocity

Lengthening the resistance arm raises the moment that the shoulder muscles must overcome, but for a given angular velocity the linear speed of the hand follows v = r x omega. Throwers accept the extra muscular cost in order to gain release velocity.

A softball hitter 'chokes up', gripping the bat several centimetres further from the knob. What is the main mechanical effect on the swing?

• The effort arm of the wrist and forearm muscles becomes longer
• The mechanical advantage of the lever system decreases
• The resistance arm shortens, so a smaller moment swings the bat
• The fulcrum of the lever moves from the hands to the elbow

Answer: The resistance arm shortens, so a smaller moment swings the bat

Choking up moves the hands closer to the bat's centre of mass, shortening the resistance arm, so a smaller muscular moment starts and adjusts the swing more quickly. The trade-off is a shorter arc at the striking end, so bat-end speed and hitting distance fall — the mechanical advantage rises rather than decreases.

A student writes that, because the elbow forms a third-class lever during a biceps curl, the elbow flexors need to produce a force smaller than the weight of the dumbbell. Which correction is required?

• The elbow flexors must produce a force larger than the weight of the dumbbell
• The elbow forms a second-class lever during a curl, so the whole statement is the wrong way round
• The two forces are equal, because the effort arm and the resistance arm are the same length
• The statement holds only while the forearm is vertical

Answer: The elbow flexors must produce a force larger than the weight of the dumbbell

A third-class lever has its effort between the fulcrum and the resistance, so the effort arm is the shorter one and the mechanical advantage is below 1. The arrangement buys range and speed at the end of the limb, and it is paid for with a larger muscle force.