Question

1. a. Compare and contrast the forces that are generated and/or energy dynamics of a human running and a fish swimming. List at least one similarity and one difference. Ideas to compare include: direction of forces generated, how muscle contraction/relaxation is related to forces, where/when potential energy is stored.

b. Compare and contrast the actions of muscles as motors and brakes. List at least one similarity between the two and one difference between the two.

c. How can you decide if a work loop is depicting a muscle acting as a motor, brake, strut or spring? Describe the method you would use to make this decision.

Answer 1

A. Force is the act of pushing or pulling an object. Force has many forms, It can be shown as pressure, friction, gravity, spring and air resistance. When you are running the main types of forces that you deal with are friction and air resistance. Two other forces that can be seen in running are spring force and gravitational force. Forces like friction and air resistance can have a positive and a negative effect on your running.

The forces acting on a swimming fish are weight, buoyancy and hydrodynamic lift in the vertical direction, along with thrust and resistance in the horizontal direction. Thrust is the force acting in animal's direction. Lift is the force acting opposite in right angles to the thrust. Drag is the force acting in the opposite to the direction of movement. Drag is minimized by the streamlined shape of the fish. To swim efficiently in water, fishes have to overcome the drag, maintain their vertical position in the water column, and maintain an upright position. They should be able to change their direction to move efficiently. Pressure drag is the force needed to push water out of the way to swim forward. Frictional drag can cause turbulence, making it harder for water to flow smoothly across the fishes.Streamlined body shape reduces the pressure drag. The slime coat provides a smooth surface that allows laminar flow and minimizes the frictional drag. Fishes overcome the drag forces by pushing their body against water. If they have to move fast, their push should be hard. The fins give fish control over its movements by directing thrust, supplying lift and even acting as brakes. Thus, when thrust is greater than drag fishes will start swimming.

The energy stored as chemical potential energy in specific bonds within molecules in your muscle cells, specifically ATP molecules. Potential energy is stored energy. Energy can be stored in chemicals, by compressing a spring, or by doing work against gravity (ex: by placing an object on a higher shelf.) Muscles act like springs, so the chemical potential energy of muscles is converted, by the muscle applying a force, into kinetic energy of motion.

B. During normal daily activities, muscles are required to lengthen as frequently as they shorten to produce movement. Lengthening muscle actions are associated with high forces and low energy consumption, but can often result in muscle injury. Using specific myosin inhibitors and different temperatures examined the molecular mechanisms of stretch-induced force enhancement. The results, which suggest that lengthening force arises from the strain of both cross-bridge and non-cross-bridge components of the sarcomere, help to refine understanding of the molecular mechanisms of muscle contraction .

C.The work loop technique is used in muscle physiology to evaluate the mechanical work and power output of skeletal or cardiac muscle contractions via in vitro muscle testing of whole muscles, fiber bundles or single muscle fibers. a burst of electrical pulses is applied to the muscle at the beginning of each shortening-lengthening cycle to stimulate the muscle to produce force. Since force and length return to their initial values at the end of each cycle, a plot of force vs. length yields a 'work loop'In running, for example, muscles in each leg experience time-varying forces and time-varying shortening velocities as the leg decelerates and accelerates from heelstrike to toeoff.

As a “motor” the muscle does work on the environment resulting in positive work in the work loop in the counter-clockwise direction. When positive work happens the length of the muscle will increase followed by an increase in force before reaching a peak. When the peak is reached the muscle will shorten along with a decrease in force. An example of positive work being done in the environment would be scallops swimming.

As a “brake” the muscle is able to absorb energy from the environment.This then results in negative work in the work loop in a clockwise direction. The result is a shortening of the muscles as well as a decrease in force output. After the muscle is done absorbing the energy from the environment, the length of the muscle then returns to normal with increased force. In animals legs that act purely as “brakes” to stop the animal’s movement.

As a “spring” the muscles are able to alter between states of motion, thus producing negative work  this negative work results from the movement and changing of the muscles position in human legs in order to produce more energy. The “springs” in these muscles absorb the energy from the environment and redirect it, then outputting that absorbed energy to make repeated movements more energy efficient.

As a “strut” the muscle can output a force and then hold the muscle length. In fish movement the body moves back and forth to produce work but as the fish moves the muscles move the energy down the length of the fish. As the energy passes the muscle the muscle then holds as a “strut”. The length of the muscle as a “strut” remains constant.