Human single muscle fibre function with 84 day bed-rest and resistance exercise

Area of Science:

• Exercise Physiology
• Muscle Biology
• Human Physiology

Background:

• Muscle atrophy and functional decline occur during prolonged inactivity, such as bed rest.
• Understanding the differential effects of inactivity and exercise on specific muscle fibre types is crucial for developing countermeasures.

Purpose of the Study:

• To investigate the impact of resistance exercise during bed rest on the contractile function of slow-twitch (MHC I) and fast-twitch (MHC IIa) muscle fibres.
• To determine if resistance exercise can mitigate the negative effects of bed rest on muscle fibre size and function.

Main Methods:

• Muscle biopsies were taken from vastus lateralis of men before and after 84 days of bed rest.
• Participants were divided into control (bed rest only) and resistance-exercised groups.
• Single muscle fibres were analyzed for contractile properties (force, velocity, power) and myosin heavy chain (MHC) isoform composition.

Main Results:

• Bed rest alone caused significant decreases in size, force, velocity, and power of both MHC I and IIa fibres.
• Resistance exercise largely maintained the size and contractile function of MHC IIa fibres but not MHC I fibres.
• MHC I fibres showed reduced normalized force and power with resistance exercise, while MHC IIa fibres maintained function with increased velocity.
• MHC isoform composition shifted towards faster fibres in both groups, more pronounced in the control group.

Conclusions:

• Resistance exercise during bed rest effectively preserves whole muscle function, primarily by maintaining fast-twitch (MHC IIa) fibre power.
• Slow-twitch (MHC I) fibres are more vulnerable to bed rest-induced decline and less responsive to the resistance exercise protocol.
• The observed shift in MHC isoform composition, favoring faster fibres, contributes to the maintenance of overall muscle strength and function during inactivity.