• During heavy exercise, the demand for energy is high but the supply of oxygen to produce energy is limited. Therefore, anaerobic respiration takes places in the muscles cells to fulfil the demand for energy. This anaerobic breakdown of glucose leads to the formation of lactic acid in muscles. The accumulation of lactic acid in muscles leads to muscle cramps.
• HenceDuring heavy exercise, we get cramps in the legs due to the accumulation of Lactic acid.
• So, the correct answer is 'Lactic acid'.

• Glucose is the major source of energy in the body. excess glucose is stored in the form of glycogen in muscles and liver.
  
• Most of the glycogen is stored in liver i.e 10 % of the mass of liver and the muscles contain a low amount of glycogen i.e 1% of the muscle mass.
  
• When the glucose is completely utilized the animal feels exhausted, to overcome this the glycogen is degraded to lactic acid by enzymes in muscles and liver. 
• So, the correct answer is 'Exhausted'.

• The Burning of glucose during respiration requires oxygen. When a Muscle is subjected to continuous exercise there will be low availability of oxygen. This leads to the anaerobic breakdown of glucose which results in the formation of lactic acid. 
• This accumulation of lactic acid in the muscle causes muscle fatigue.
• Hence Muscles get fatigued due to the accumulation of lactic acid.
• So, the correct answer is 'Lactic acid'.

• Regor mortis is one of the recognizable signs of death, caused by the depletion of ATP followed by the chemical changes in muscles after death, causing the body to stiffen and locked in place, making it difficult to move and manipulate.
  
• When the ATP is depleted muscles can not contract and the body becomes stiff.
• It takes about 2 days for rigor mortis to fade, and once it does, decay sets in.
  
• Hence rigor mortis is due to depletion of ATP.
  
• So, the correct answer is 'Depletion of ATP'.
  

• The all-or-none law is the principle that the strength by which a nerve or muscle fibre responds to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential, the nerve or muscle fibre will give a complete response; otherwise, there is no response.
  
• Hence All or none law is not applicable for whole skeletal muscle.
  
• So, the correct answer is 'Whole skeletal muscle'.

• Generally, the glucose undergoes metabolism in the presence of oxygen to form ATP(adenosine triphosphate) which is essential for the muscle contraction but during vigorous exercise, the body cannot meet the oxygen demand and the glucose undergoes metabolism in the absence of oxygen which results in the formation and accumulation of lactic acid.
  
• Feeling of fatigue after vigorous exercise for some time is due to the accumulation of lactic acid in muscles.
• Hence lactic acid deposition leads to muscle fatigue.
• So, the correct answer is 'Muscle fatigue'.

• The all-or-none law is a principle that states that the strength of a response of a nerve cell(neuron) or muscle fibre is not dependent upon the strength of the stimulus. If a stimulus is above a certain threshold, a nerve or muscle fibre will fire.
• Hence All or none law is associated with muscle fibre and neuron.
• So, the correct answer is 'Both A and B'.
  

• Generally, the glucose undergoes metabolism in the presence of oxygen to form ATP (Adenosine triphosphate) which is essential for the muscle contraction but during strenuous exercise, the body cannot meet the oxygen demand and the glucose undergoes metabolism in the absence of oxygen which results in the formation and accumulation of lactic acid.
  
• This accumulated lactic acid is moved to the liver and converted into glycogen thorough Cori cycle.
• Cori cycle takes place in the liver.
• Hence In the anaerobic reaction of muscle, lactic acid is produced which causes painful tiredness. The lactic acid accumulated is catabolized by the mechanism of the Cori cycle in the liver.
• So, the correct answer is 'In liver by Cori cycle'

• Generally, the glucose undergoes metabolism in the presence of oxygen to form ATP(adenosine triphosphate) which is essential for the muscle contraction but during vigorous exercise, the body cannot meet the oxygen demand and the glucose undergoes metabolism in the absence of oxygen which results in the formation and accumulation of lactic acid.
  
• This Lactic acid generated during muscle contraction is elaborated to form glycogen in Liver through Cori cycle.
  
• So, the correct answer is 'Liver'.

Skeletal muscles show muscle fibres having well developed sarcoplasmic reticulum these muscles are easily fatigued during exercise. Skeletal muscles show voluntary movements.  These can be found in close association with limb bones.

The internal movements are performed by the involuntary muscles. The involuntary muscles function on their own and cannot be controlled willingly. The cardiac muscles are the involuntary muscles which are responsible for the pumping of the heart. The heart pumps and supplies the oxygenated blood to the different tissues in the body. The smooth muscles are responsible for carrying out peristalsis in the tube like structures of the alimentary canal.

The correct option is D.

• Generally, the glucose undergoes metabolism in the presence of oxygen to form ATP(adenosine triphosphate) which is essential for the muscle contraction but during vigorous exercise, the body cannot meet the oxygen demand and the glucose undergoes metabolism in the absence of oxygen which results in the formation and accumulation of lactic acid.
  
• Feeling of fatigue after running fast for some time is due to the accumulation of lactic acid in muscles.
  
• So, the correct answer is 'Accumulation of lactic acid in muscles'.

The body prefers to generate most of its energy using aerobic methods. During physical exercise such as lifting heavy weights, energy production is faster than our bodies can adequately deliver oxygen. In those cases, the working muscles generate energy anaerobically. This energy comes from glucose through a process called glycolysis, in which glucose is broken down or metabolized into a substance called pyruvate. When the body has plenty of oxygen, pyruvate is shuttled to aerobic pathway to be further broken down for more energy. But when oxygen is limited, the body temporarily converts pyruvate into a substance called lactate and accumulates in the muscle. 

• When you exercise, your muscles move more vigorously than when you are at rest. Their metabolic rate increases. 
• They need more energy, so they produce more of the chemical energy molecule ATP. 
• Your muscles begin converting glucose into lactic acid instead of energy when anaerobic exercise takes over, power output drops and fatigue sets in. Hence, when Muscles run out of oxygen muscles begin converting glucose into lactic acid.

Generally, the glucose undergoes metabolism in the presence of oxygen to form ATP(adenosine triphosphate) which is essential for the muscle contraction but during strenuous exercise, the body cannot meet the oxygen demand and the glucose undergoes metabolism in the absence of oxygen which results in the formation and accumulation of lactic acid.Hence during strenuous exercise,glucose is converted into lactic acid.

• During a vigorous physical exercise, lactic acid and carbon dioxide are formed in the muscles of the body.
• The production of lactic acid can be prevented by the flow of required oxygen to the muscles thus, our body uses more oxygen than replenished.
  
• Therefore, after a vigorous exercise people pant due to to decrease in the oxygen level in the body.
• So, the correct answer is 'Oxygen debt'.

All or none law is the principle that states that the strength by which a muscle fiber responds to a stimulus is independent of the strength of the stimulus. If that stimulus exceeds the threshold potential, the muscle fiber will give a complete response, otherwise, there is no response. 

• Lactic acid is formed and accumulated in the muscle under conditions of high energy demand, rapid fluctuations of the energy requirement and insufficient supply of O2.
•  During exercise, there is a progressive reduction in the ability to produce muscle forces processes within the nervous system, as well as within the muscles contribute to this fatigue. 
•  Finally, the small-diameter muscle afferents that increase firing with exercise and fatigue. Hence, Lactic acid causes fatigue in muscles due to the Sensitivity of neuromuscular junction.

• Myofibril activity is required for muscle contraction on the molecular level. 
• When ATP binds to myosin, it separates from the actin of the myofibril, which causes a contraction.
  
• The muscle contraction cycle is triggered by calcium ions binding to the protein complex troponin, exposing the active-binding sites on the actin.
  
• ATP then binds to myosin, moving the myosin to its high-energy state, releasing the myosin head from the actin active site.
• When ATP is exhausted resulting in fatigue, lactic acid is produced 

Muscle fatigue is the reduction in force of contraction of a muscle after prolonged stimulation. In the absence of oxygen, skeletal muscle of human beings can contract for a short time, but it gets fatigued soon. This is due to the fact that in the absence of oxygen, products of glycolysis mainly lactic acid is not disposed of and accumulates in the muscles. This leads to muscle fatigue and pain in the muscles. A muscle gets fatigued sooner after a strenuous exercise than after a mild exercise. Faster breathing for sometime after a strenuous exercise supplies extra oxygen disposes off excess lactic acid and muscle fatigue disappears.

RICE-Rest, Ice, Compression, Elevation. As soon after an injury like an ankle sprain, fracture pain can be relived with RICE. Rest and protect the area of injury.

Intracellular acidosis due mainly to lactic acid accumulation has been regarded as the most important cause of skeletal muscle fatigue. Inorganic phosphate, which increases during fatigue due to the breakdown of creatine phosphate appears to be a major cause of muscle fatigue.

Rigor mortis is the state of rigidity that develops in the muscles after death. It is used to determine or estimate the time of death. This refers to the stiffening of the body after death due to loss of Adenosine Triphosphate (ATP) from the body's muscles. ATP provides the energy to the muscles which help them to work and without this, the muscles become stiff and inflexible. 

Muscle twitch is the response of a muscle to a single electrical stimulation. It is divided into three phases: latent period - time between stimulation and beginning of shortening, contraction phase - time of shortening, relaxation phase- time when muscle is returning to resting length (longest phase).

Rigor mortis is stiffening of the body caused by contraction of muscles after death. In humans it occurs four hours after death. Due to absence of oxygen after death, the ATP is not formed which is required for separation of actin and myosin. When oxygen is no longer present, the body may continue to produce ATP via anaerobic glycolysis. When the body's glycogen is depleted, the ATP concentration diminishes, and the body enters rigor mortis because it is unable to break those bridges. So, the correct answer is 'Contraction of muscles after death'.

The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball. It is frequently said that they are the strongest and fastest contracting muscles due to the constant blinking action they have to do. Eye movements do require high speed movements, and eye muscles are exercised continuously even during rapid eye movement of sleep. 

At the maximal stimulus, all motor units are recruited. All muscle cells in the whole muscle are contracting. Any stimulus above the maximal stimulus is called a supramaximal stimulus (voltage) and the tension will not increase beyond that elicited by a maximal stimulus. Thus, the correct answer is option A.