Muscle atrophy and decline in muscle strength appear very rapidly with prolonged disuse or mechanical unloading after acute hospitalization or experimental
bed rest. The current study analyzed data from short-, medium-, and long-term
bed rest (5-120 days) in a pooled sample of 318 healthy adults and modeled the mathematical relationship between muscle strength decline and
atrophy. The results show a logarithmic disuse-induced loss of strength and
muscle atrophy of the weight-bearing knee extensor muscles. The greatest rate of muscle strength decline and
atrophy occurred in the earliest stages of
bed rest, plateauing later, and likely contributed to the rapid neuromuscular loss of function in the early period. In addition, during the first 2 wk of
bed rest, muscle strength decline is much faster than
muscle atrophy: on day 5, the ratio of
muscle atrophy to strength decline as a function of
bed rest duration is 4.2, falls to 2.4 on day 14, and stabilizes to a value of 1.9 after ∼35 days of
bed rest. Positive regression revealed that ∼79% of the muscle strength loss may be explained by
muscle atrophy, while the remaining is most likely due to alterations in single fiber mechanical properties, excitation-contraction coupling, fiber architecture, tendon stiffness,
muscle denervation, neuromuscular junction damage, and supraspinal changes. Future studies should focus on neural factors as well as muscular factors independent of
atrophy (single fiber excitability and mechanical properties, architectural factors) and on the role of extracellular matrix changes.
Bed rest results in nonuniform loss of isometric muscle strength and
atrophy over time, where the magnitude of change was greater for muscle strength than for
atrophy. Future research should focus on the loss of muscle function and the underlying mechanisms, which will aid in the development of countermeasures to mitigate or prevent the decline in neuromuscular efficiency.NEW & NOTEWORTHY Our study contributes to the characterization of muscle loss and weakness processes reflected by a logarithmic decline in muscle strength induced by chronic
bed rest. Acute short-term hospitalization (≤5 days) associated with periods of disuse/immobilization/prolonged time in the supine position in the hospital bed is sufficient to significantly decrease muscle mass and size and induce functional changes related to weakness in maximal muscle strength. By bringing together integrated evaluation of muscle structure and function, this work identifies that 79% of the loss in muscle strength can be explained by
muscle atrophy, leaving 21% of the functional loss unexplained. The outcomes of this study should be considered in the development of daily countermeasures for preserving neuromuscular integrity as well as preconditioning interventions to be implemented before clinical
bed rest or chronic gravitational unloading (e.g., spaceflights).