Dominic Stone
The human body has a remarkable ability to defend its temperature against a wide range of thermal conditions. Heat production is critical to maintaining core temperature, and skeletal muscles are the primary source of heat production in cold-exposed humans. This is achieved through voluntary contractions from exercising muscles and involuntary contractions from shivering. Even when relaxed, muscles can continue to produce heat through muscle-based thermogenesis. This process is essential for maintaining a stable body temperature in cooler environments.
| Characteristics | Values |
|---|---|
| Do muscles create heat? | Yes |
| How do muscles create heat? | Active muscle converts chemical energy into heat and work. |
| How do muscles create heat during exercise? | Heat production is the result of enhanced heat liberation during ATP production when aerobic metabolism becomes dominant. |
| How do muscles create heat during rest? | The calcium ion leak from the sarcoplasmic reticulum forces the calcium ion pump to work harder, producing more heat. |
| What is the role of muscle-generated heat? | Muscle-generated heat is important for thermoregulation, especially in cold environments. |
| What are the types of thermogenesis in skeletal muscles? | Shivering thermogenesis and non-shivering thermogenesis |
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What You'll Learn
- Heat is produced by contracting and non-contracting muscles
- Mammals can generate heat even when their muscles are at rest
- Shivering is a repetitive mode of involuntary contractions resulting in excessive heat production
- Heat production by contracting human skeletal muscle doubles over 3 minutes of intense dynamic exercise
- Calcium ion pumps in skeletal muscles help regulate how much heat is produced when the muscles are at rest
Heat is produced by contracting and non-contracting muscles
The act of muscle contraction itself also produces heat, with nearly 85% of the heat produced in the body being a result of muscle contraction. This heat production is exploited by shivering, which activates large muscles and increases glycolysis as the primary source of heat production. Fish and reptiles also use rhythmic muscle contractions to generate local heat.
Non-contracting muscles can also produce heat, as cold exposure recruits both muscle and BAT-based heat production. When BAT function is minimized, muscle thermogenesis increases to compensate, as seen in mice studies where the loss of BAT function resulted in increased SLN expression in skeletal muscle.
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Mammals can generate heat even when their muscles are at rest
Mammals produce heat through metabolic processes, and this heat production is particularly associated with muscle contractions during exercise. However, the heat generated by mammals is not solely due to muscle contractions, as metabolic processes also play a significant role in maintaining body temperature. This is evident in the fact that mammals, including humans, can regulate their body temperature within a narrow range, typically around 37°C, even when their muscles are at rest.
The heat produced by metabolic processes in mammals serves a critical function in warming the body. This internally generated heat, also known as metabolic heat, distinguishes mammals from ectotherms, who rely primarily on external sources of heat to regulate their body temperature. The ability to produce metabolic heat allows mammals to maintain a high and efficient metabolic rate, resulting in an elevated pace of life compared to ectotherms.
During intense dynamic exercise, the heat production by contracting skeletal muscles can double within 3 minutes, as observed in human subjects. This increase in heat production is attributed to the enhanced liberation of heat during ATP production, particularly when aerobic metabolism becomes dominant. However, it is important to note that the quantification of energy fluxes in contracting muscles is a complex task, especially in humans.
Additionally, the production of heat in mammals is influenced by their environment and the ambient temperature. For example, a tropical mammal like a sloth exhibits a sharp increase in metabolic rate when the temperature drops below 31°C. This thermogenic response is a distinguishing feature of mammals, enabling them to compensate for heat loss in colder conditions and maintain a constant body temperature.
In summary, mammals, including humans, have the remarkable ability to generate heat through metabolic processes, even when their muscles are at rest. This heat production is essential for maintaining a high metabolic rate and a constant body temperature, which contributes to the elevated pace of life characteristic of mammals.
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Shivering is a repetitive mode of involuntary contractions resulting in excessive heat production
Shivering is an involuntary movement of the body, which means that it is uncontrolled. It is a repetitive mode of involuntary contractions resulting in excessive heat production. When people are cold, the muscles in their body contract and relax rapidly to generate heat. This causes part or all of the body to shiver or shake.
Shivering is a natural response to cold temperatures, and it occurs when the body tries to generate heat to maintain a normal body temperature. During muscle contraction, heat is generated through both myosin-mediated adenosine triphosphate (ATP) hydrolysis and Ca2+ transport driven by the SERCA pump. This process is known as muscle thermogenesis, and it is the primary mechanism for heat production in most vertebrates.
High-intensity shivering activates large muscles and increases glycolysis as the main source of heat production. Since no external work is done during shivering, most of the chemical energy is released as heat within the muscle tissue. This can lead to excessive heat production, and constant shivering can be detrimental as it exhausts the muscles. Therefore, non-shivering thermogenic mechanisms have evolved to better adapt to colder environments. For example, fish and reptiles can exploit rhythmic muscle contractions to generate local heat when needed.
Shivering can also be caused by factors other than cold temperatures, such as fever, low blood sugar, or strong emotions like anxiety or excitement. In these cases, shivering is usually temporary and should stop once the underlying cause is addressed or resolved. For example, if a person is shivering due to a fever, treating the infection or illness causing the fever should also stop the shivering.
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Heat production by contracting human skeletal muscle doubles over 3 minutes of intense dynamic exercise
The rate of heat storage in the knee extensor muscles was highest during the first 45 seconds of exercise, at 70-80 J s-1, and then gradually declined to 14 ± 10 J s-1 at 180 seconds. This is supported by the observation that an elevation in temperature in the active muscle can be observed during the first few contractions (1-3 seconds).
The increase in heat production is also tightly coupled with changes in heat liberation during ATP production in the metabolic reactions involved early in exercise. After 60 seconds of exercise, when aerobic metabolism provided 82-89% of the ATP resynthesis, the match between total energy turnover and total metabolic input was more closely aligned.
Heat dissipation from dynamically contracting muscles includes heat transfer to the core of the body and to surrounding tissues or the environment. Heat conductance through tissues in the human body is a slow process, so heat exchange with the surroundings of the exercising thigh can be minimised by a thermostat isolation system.
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Calcium ion pumps in skeletal muscles help regulate how much heat is produced when the muscles are at rest
Muscle contractions produce heat, and this is exploited by the body when it shivers in response to cold temperatures. The main task of skeletal muscle is to contract and relax for body movement and posture maintenance. The contractile properties of muscle fibres are dependent on the variable expression of proteins involved in calcium signalling and handling. Calcium (Ca2+) is the main regulatory and signalling molecule for all muscle fibres.
Calcium ion pumps in skeletal muscles are responsible for calcium reuptake into the sarcoplasmic reticulum. The SR (sarcoplasmic reticulum) is a calcium store within the muscle cell. When the muscle is at rest, calcium is pumped into the SR and bound to the protein calsequestrin. This process is powered by active-transport Ca++ pumps, which are, in turn, energised by ATP.
During muscle contraction, calcium is released from the SR into the cytosol, where it activates a series of contractile proteins. The release of calcium ions from the SR is triggered by a signal from the motor neuron. The rate of force production, resistance to fatigue, and energy metabolism of skeletal muscle are all influenced by calcium regulation.
Calcium regulation is also important in muscle development and ageing. Eun Hui Lee and coworkers at the Catholic University of Korea in Seoul reviewed the molecular mechanisms regulating calcium entry into skeletal muscle cells. They found that calcium regulation is critical in fine-tuning every function of skeletal muscle, from contraction and release to development, ageing, and disease.
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Frequently asked questions
Yes, muscles create heat.
Active muscle converts chemical energy into heat and work. The calcium ion pumps in skeletal muscles work to keep the level of calcium ions steady, and the resistance to rising calcium levels allows for a steady calcium ion leak, which forces the calcium ion pump to work harder, producing more heat.
Heat production is critical to defending the body's core temperature. Humans have a reduced capacity to preserve heat in cold environments, so heat production is essential to survival in such conditions.
Muscles create heat during exercise and when at rest.
During intense dynamic exercise, oxidation is the primary energy-liberating pathway. Heat is transferred to the core of the body and to surrounding tissues or the environment.
