Physiologic Effects of Heat

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Heat can produce a multifactorial effects in body tissues and systems:


Increased circulation


Increased nerve conduction velocity

Increased pain threshold

Changes in muscle strength


Increased metabolic rate


Increased collagen extensibility


Five Mechanisms of Heat Exchange

Heat Exchange

  1. Conduction - two surfaces directly touching each other (e.g. hot and cold packs)
  2. Convection - circulation between two mediums of different temperatures (e.g. fluidotherapy, whirlpool)
  3. Conversion - nonthermal energy becomes thermal (e.g. ultrasound)
  4. Radiation - direct transfer of energy without a medium for transfer (e.g. ultraviolet radiation)
  5. Evaporation - transfer of energy that causes a liquid to gas state with resultant cooling (e.g. sweating, vapocoolant sprays)

Key point: superficial thermal agents (like a hot pack) do not increase temperature below a depth of 12 cm.Once the heat source is removed, temperature changes dissipate almost immediately.

The increased circulation rates to muscles results in bringing cooler blood to the treatment area, and homeostasis with muscle temperatures is maintained. Application of heat to reduce pain from a muscle strain is based on increasing the pain-inhibiting nerves and receptors found in the skin and tendons. Decreased muscle guarding decreases pain.

Exercise (isometric, or other) combined with heat produces the greatest increase to circulation and metabolic rates in body tissues

Goals for Using Heat as an Intervention

Impairments and Goals

Treatment considerations

Functional Outcome

Decrease pain


Position patient for comfort

Improve functional activity or improved functional ROM

Decrease spasm

Position patient so stretch to affected muscles is reduced

Improved mobility

Increase tissue extensibility

Position patient so stretch on affected soft tissue is on slight stretch

Improve functional ROM (e.g., reaching, bending)

Increase circulation

Promote tissue healing


Screen for benefits in facilitating inflammatory responses (histamine and prostaglandin release)

Improve function



Clinical Indicators for Heat Application

Chronic musculoskeletal conditions show ability to benefit from heat. Impairments resulting from chronic spinal pain, sprains and strains, and postural dysfunctions are examples where therapeutic heat can be an integral part of promoting functional recovery.

Conditions and impairments associated with loss of soft tissue extensibility (e.g. scar tissue, myofascial syndromes, joint disorders) can also benefit from thermal agents.