The current study investigated male subjects involved in 520 d confinement. To the best of our knowledge no prior work has quantified physical activity and neuromuscular performance in this model. We found that physical activity, when measured in terms of average acceleration, reduced. This indicates that subjects changed body position less and/or changed their speed of walking less. Although the effects did not reach statistical significance, distance ambulated decreased marginally and average gait speed stayed the same or increased marginally. Explosive power during countermovement jump and force production during single-leg hopping, a test influenced predominately by the ankle joint plantarflexors 
, also reduced progressively during confinement and these effects were still present after controlling for changes in body mass. Grip force showed little change.
The findings from the actibelt® measure show that a reduction of physical activity occurred during confinement. Whilst the average gait speed remained the same and the distance ambulated decreased marginally, there was a strong significant reduction in the acceleration of the body’s center of mass. This implies that there was a reduction in the forces generated within the body – be it from changes in body position or other forms of acceleration.
In the assessment of neuromuscular function in the lower- and upper-limbs, we observed a pattern of greater decrements in neuromuscular performance in the lower-limb (countermovement jump power, multiple single-leg hopping) with little change in the upper-limb (grip force). It is possible that these changes occurred due to the new “lifestyle” of reduced physical activity in confinement. In models of extreme disuse, such as bed-rest and spaceflight, it is well known that greater loss in function, muscle mass and bone density occur in the lower-limbs with limited changes in the upper-limbs 
. With age, decline in peak countermovement jump power per unit body weight occurs in both elite athletes 
and the wider population 
, though the rates of decline seen in these earlier works were slower than seen in the current study. Associated with such decline, a number of mechanisms including inhibition of muscle regeneration via satellite cells 
and oxidative stress 
have been identified.
There are, however, some aspects of the finding of the current study do not support the interpretation that the changes in neuromuscular function are due to reduced physical activity. For example, the main reductions in countermovement jump power and single-leg hop force relative to body mass occur at around 40–80 days of confinement, with some indication of progressive losses beyond this time-point. This aspect of the findings does not support the idea of a 1
1 relationship between changes in activity levels and adaptation in lower-limb neuromuscular function.
The effects we observed during isolation were likely not due to changes in subject motivation. A concern could be that subjects may have been, with time, simply less motivated in general to move or perform maximal explosive tasks. However, there was a marginal, non-significant, increase in our measure of subject motivation. After isolation, the up-tick in neuromuscular performance compared to during isolation could, however, be associated with the parallel up-tick in subject motivational state. Also, the discordant changes between upper and lower-limbs on neuromuscular testing, rather than similar changes in both, further suggests that subject motivation or factors such as repeated testing is likely not the mediating factor.
The current study had some important limitations. Firstly, we did not have a control group that performed testing on the same schedule but did not undergo isolation. Comparable data are also not available in the literature. Also, the number of subjects was limited for logistical reasons. Despite this we were able to find significant effects by conducting a number of measures during the course of confinement, though ability to generalise to a wider population would be limited. Subjects also performed an exercise program during confinement. The type of exercise and order of their performance fixed according to the study protocol, but subjects did not always comply with the requested volume of exercise and detailed information on exercise actually executed was not made available to researchers participating in the study. Nonetheless, neuromuscular or motivational state variables did not differ significantly between the phases when these exercise programs were due to be performed. Average gait speed and distance walked were significantly increased when subjects were scheduled to perform running exercises (data not shown). Importantly, we found significant decrements in neuromuscular performance despite attempts to implement a countermeasure exercise program.
In conclusion the current study investigated physical inactivity and neuromuscular performance in prolonged 520 d confinement. Overall, the data suggest that prolonged confinement is a form of reduced physical activity with concurrent adaptation in lower-limb, rather than upper-limb, neuromuscular performance.