What you'll learn
- How skeletal muscle is organised from whole muscle down to sarcomeres.
- Why slow-twitch and fast-twitch muscle fibres are suited to different activities.
- How homeostasis uses negative feedback to keep internal conditions stable.
- How to interpret simple recovery data after exercise.
The big picture: why running challenges the body
When you run, skeletal muscles need much more ATP, oxygen and glucose than they do at rest. They also produce more carbon dioxide, heat and metabolic products such as lactate. If these changes were not controlled, enzymes and cells would stop working properly.
ATP
ATP, or adenosine triphosphate, is the immediate energy source used by cells. In muscle fibres, ATP is needed for cross-bridge cycling, active transport of ions, and many other energy-requiring processes.
So this topic links two ideas: specialised muscle fibres help produce movement, and feedback systems help keep the internal environment suitable for those fibres to keep working.
Skeletal muscle is organised in layers
Skeletal muscle is voluntary, striated muscle usually attached to bones by tendons. A tendon is a tough connective tissue that transmits the pulling force of muscle contraction to the skeleton.
A whole skeletal muscle is made of bundles called fascicles. Each fascicle contains many muscle fibres.
Muscle fibre
A muscle fibre is a long, multinucleate skeletal muscle cell. It contains many myofibrils, which are protein fibres specialised for contraction.
A myofibril is made of repeating units called sarcomeres. A sarcomere is the functional unit of contraction: it contains thin actin filaments and thick myosin filaments arranged between Z lines.
Sarcomeres and the sliding filament idea
During contraction, actin and myosin filaments slide past each other. The filaments themselves do not get shorter. Instead, the Z lines move closer together, so the sarcomere shortens.
A cross-bridge forms when a myosin head attaches to actin. ATP is hydrolysed to provide energy for the myosin head to move, pulling actin inward. Calcium ions are also essential because they allow the myosin-binding sites on actin to become exposed.
Contraction changes overlap
In a contracting sarcomere, actin and myosin overlap more. The sarcomere shortens, but the actin and myosin filaments remain the same length.
Predicting the effect of low ATP
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ATP is needed for myosin heads to detach from actin after a power stroke, so a shortage of ATP means cross-bridge cycling becomes less efficient.
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ATP hydrolysis also re-energises the myosin heads, so fewer myosin heads are ready to pull actin again.
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The sarcomeres cannot shorten normally, so the muscle produces less force and fatigues more quickly.
Slow-twitch and fast-twitch fibres
Different muscle fibres are adapted for different patterns of activity. A twitch is a single contraction and relaxation response of a muscle fibre after stimulation.
Slow-twitch fibres
Slow-twitch fibres, also called Type I fibres, contract relatively slowly but resist fatigue. They are adapted for endurance activities such as long-distance running and maintaining posture.
They usually have:
- many mitochondria for aerobic respiration
- a dense capillary supply for oxygen and glucose delivery
- lots of myoglobin, an oxygen-binding protein found in muscle
- relatively low force but high fatigue resistance
Fast-twitch fibres
Fast-twitch fibres, often called Type II fibres, contract rapidly and powerfully but fatigue more quickly. They are useful for sprinting, jumping and short bursts of intense activity.
They usually have:
- faster myosin ATPase activity, so ATP is hydrolysed rapidly
- larger glycogen stores for quick glucose supply
- fewer mitochondria than slow-twitch fibres
- lower myoglobin content, so they appear paler
- greater reliance on anaerobic respiration during intense activity
Anaerobic respiration
Anaerobic respiration is ATP production without oxygen. In mammals, it converts glucose to lactate and releases much less ATP per glucose molecule than aerobic respiration.
| Feature | Slow-twitch fibres | Fast-twitch fibres |
|---|---|---|
| Main role | Endurance and posture | Rapid, powerful movement |
| Respiration | Mostly aerobic | Often anaerobic during intense activity |
| Mitochondria | Many | Fewer |
| Myoglobin | High | Lower |
| Fatigue | Fatigue-resistant | Fatigue quickly |
| Typical activity | Marathon running | Sprinting |
Fast does not mean better
Fast-twitch fibres are not “better” than slow-twitch fibres. They are better for power, but slow-twitch fibres are better for sustained activity.
Choosing the likely fibre type
A biopsy from an athlete’s leg muscle shows many mitochondria, high myoglobin content and a dense capillary network.
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Many mitochondria indicate a high capacity for aerobic respiration, so the fibres can produce ATP steadily using oxygen.
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High myoglobin and dense capillaries both improve oxygen supply, which supports long-duration activity.
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The evidence therefore points to a high proportion of slow-twitch fibres, which would suit endurance events such as long-distance running.
Homeostasis: keeping conditions within limits
The internal environment means the fluid conditions surrounding body cells, including temperature, pH, water potential, oxygen concentration, carbon dioxide concentration and blood glucose concentration.
Homeostasis
Homeostasis is the maintenance of a stable internal environment within narrow limits, despite changes inside or outside the body.
A homeostatic system usually has:
- a receptor, which detects a change
- a coordinator, which processes information and decides the response
- an effector, which carries out the response
- a set point, which is the target level the system works around
For example, human core body temperature is usually controlled close to 37 °C, although it naturally fluctuates slightly.
Negative feedback
Negative feedback
Negative feedback is a control mechanism where a change in a variable triggers responses that reverse or reduce the original change.
The key word is reduce. Negative feedback does not mean “bad feedback”; it means the response acts in the opposite direction to the original deviation.
During exercise, one major challenge is heat production from respiration and muscle contraction. Thermoregulation is mainly coordinated by the hypothalamus, a region of the brain involved in maintaining internal conditions.
Explaining temperature control during exercise
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If core body temperature rises above the set point, thermoreceptors detect the increase and send information to the thermoregulatory centre in the hypothalamus.
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The hypothalamus coordinates responses by sending impulses to effectors such as sweat glands and smooth muscle in skin arterioles.
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Sweat evaporates from the skin surface, removing heat energy, while vasodilation increases blood flow near the skin surface so more heat is lost by radiation.
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As body temperature falls back towards the set point, the original stimulus is reduced, so less corrective response is needed.
Negative feedback is not a flat line
Homeostatic variables usually fluctuate around a set point. A graph does not need to be perfectly horizontal to show homeostasis.
Why homeostasis matters for muscle fibres
Muscle contraction depends on enzymes, membranes and ion gradients. If temperature rises too far, enzymes may denature. If carbon dioxide builds up, blood pH can fall, which affects enzyme activity and protein shape.
During exercise, several systems work together:
- ventilation increases to bring in more oxygen and remove more carbon dioxide
- heart rate and stroke volume increase to deliver oxygen and glucose faster
- sweating and vasodilation increase heat loss
- liver glycogen may be broken down to help maintain blood glucose concentration
These are coordinated responses, not isolated reactions. Your body is constantly balancing oxygen delivery, heat loss, blood pressure and fuel supply.
Interpreting recovery data
After exercise, variables such as heart rate, breathing rate and body temperature usually return gradually towards resting values. This recovery can be described using a rate calculation.
For a temperature change:
r=ΔTΔtr = \frac{\Delta T}{\Delta t}r=ΔtΔTwhere rrr is the mean rate of temperature change, ΔT\Delta TΔT is the change in temperature, and Δt\Delta tΔt is the time interval.
Calculating a recovery rate
A runner’s core temperature falls from 38.6 °C to 37.4 °C over 12 min after exercise.
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Calculate the temperature decrease:
ΔT=38.6−37.4=1.2 ∘C\Delta T = 38.6 - 37.4 = 1.2\ ^\circ\text{C}ΔT=38.6−37.4=1.2 ∘C -
Substitute into the rate equation:
r=1.2 ∘C12 min=0.10 ∘C min−1r = \frac{1.2\ ^\circ\text{C}}{12\ \text{min}} = 0.10\ ^\circ\text{C min}^{-1}r=12 min1.2 ∘C=0.10 ∘C min−1 -
Interpret the sign in context: the temperature is falling, so the mean recovery rate is a decrease of 0.10 °C per minute.
Practical data
If you count pulse for 15 s and multiply by 4, any counting error is also multiplied by 4. Repeats, consistent timing and electronic sensors can improve reliability.
In the exam
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For muscle fibre questions, link structure to function: mitochondria, capillaries and myoglobin support aerobic endurance; glycogen and fast ATP use support rapid power.
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For negative feedback, name the receptor, coordinator, effector and response, then explain how the response reduces the original change.
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On graphs, use data with units and describe whether the variable is moving towards or away from the set point.
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For rate calculations, show the formula, substitute values with units, then state the result with appropriate units.
Check yourself
- Why do many mitochondria and high myoglobin content make slow-twitch fibres fatigue-resistant?
- In thermoregulation after exercise, what are the receptor, coordinator and effectors?
- How would you calculate the mean rate of temperature recovery from a graph?
