| dc.description.abstract | Single-limb or whole-body immobilisation or inactivity can occur as a consequence of injury, illness, frailty, and surgery. Such periods of immobilisation result in decreased muscle strength which is in part due to muscle atrophy. However, the loss in muscle strength during immobilisation is typically greater and occurs faster than the loss of muscle size. As such, muscle atrophy cannot fully explain the immobilisation-induced loss in muscle strength. Muscle strength is strongly influenced by neural processes, so it is likely that changes in neuromuscular function (NMF) also contribute to immobilisation-induced loss of strength. Therefore, the aim of the thesis was to enhance the understanding of the early phases of immobilisation and the contribution that neuromuscular function may play in the observed declines in strength.
A systematic review of the literature on the effects of periods of segmental limb immobilisation, identified that declines in strength (~2% per day immobilised) were 5-fold greater than the reductions in muscle size (~0.4% per day). Muscle contractility was impaired with declines of ~1% per day in the rate of force development and ~1% per day in relaxation rate. Central drive was also impaired at 0.5% per day loss. The key finding of the systematic review was that the magnitude of muscle strength loss is greater than muscle atrophy in the first few days of immobilisation, and loss of contractility is an important contributing factor to functional loss especially in early stages of immobilisation. However, only 10% of the included studies investigated the effects of immobilisation for less than 7 days despite the results indicating that this is the period in which the largest rate of change in all outcome measures, other than muscle size, occurs. Based on the findings of the systematic review we then investigated the effects of periods of 12h and of 48 h single leg immobilisation via fixed angle knee brace on neuromuscular function of the knee extensors via contractions evoked by peripheral nerve electrical stimulation (PNS) and transcranial magnetic stimulation (TMS).
The 12h immobilisation protocol induced a significant 5% decrease in muscle strength but no statistically significant effects on contractility, excitability, or voluntary activation (all p > 0.05) compared to the non-immobilised limb. In a second experimental study, we investigated the effects of 48 h of single leg immobilisation, utilising the same outcome measures as well as muscle size measures with MRI. The experimental paradigm was altered to include a control group and data were interrogated with ANCOVA. Muscle strength declined by ~10%, despite no change in muscle size (-0.5%). Significant reductions in muscle contractility were observed with lengthening of time to peak twitch in resting muscle (PNS - 18%; TMS -16%) and during potentiated twitches (PNS -19%; TMS -19%). Alongside this elevations in central drive (7%) were observed using the twitch interpolation technique. In summary, 48 h of single limb immobilisation can cause significant reductions in muscle strength as a result of reductions in muscle contractility and supraspinal neural drive that cannot be ameliorated by increased spinal contributions to the voluntary activation. Finally, we interrogated the impact of different control paradigms for immobilisation research i.e., within person non-immobilised limb control versus a separate non-immobilised control group. This study explores the potential for cross education between immobilised and non-immobilised limbs and/or compensatory overload and hence fatigue of the non-immobilised leg, to confound the control comparison. Strength significantly declined 6.5% following 48h of immobilisation on the contralateral limb alongside increases in sarcolemma excitability (M-wave amplitude: Vastus lateralis 5%; M-wave area: Vastus lateralis 8%; Vastus medialis 13%), but no changes in contractility or central drive were observed. The data observed in the non-immobilised limb suggests an inability to produce maximal force but did not provide clear evidence as to why this had occurred.
The findings presented in the thesis have demonstrated the impact of short-term immobilisation on muscle strength and size, and neuromuscular function. During the early phase of immobilisation reduced contractility plays a key role in driving the observed declines in strength. The non-immobilised limb exhibits loss of strength and increased peripheral excitability, and thus studies examining the effects of single limb immobilisation should employ a control group as comparator. | en_GB |
