Abstract
What is the central question of this study? The effects of long-term muscle disuse on neuromuscular function are unclear because disuse studies are typically short term. In this study, we used a new model (unilateral transtibial amputees) to investigate the effects of long-term disuse on quadriceps neuromuscular function. What is the main finding and its importance? Kinetic analysis (knee-extension moments during gait) indicated habitual disuse of the amputated limb quadriceps, accompanied by lower quadriceps muscle strength (60-76%) and neural activation (32-44%), slower contractile properties and altered muscle architecture in the amputated limb, which could not be predicted from short-term disuse studies. The purpose of this study was to determine: (i) whether individuals with unilateral transtibial amputations (ITTAs), who habitually disuse the quadriceps muscles of their amputated limb, provide an effective model for assessing the effects of long-term muscle disuse; and (ii) the effects of such disuse on quadriceps muscle strength and neuromuscular function in this population. Nine ITTAs and nine control subjects performed isometric voluntary knee extensions of both limbs to assess maximal voluntary torque (MVT) and the rate of torque development (RTD). The interpolated twitch technique and EMG normalized to maximal M-wave were used to assess neural activation, involuntary (twitch and octet) contractions to assess intrinsic contractile properties, and ultrasound images of the vastus lateralis to assess muscle architecture. Clinical gait analysis was used to measure knee kinetic data during walking at an habitual speed. The ITTAs displayed 54-60% lower peak knee-extensor moments during walking in the amputated compared with intact/control limbs, but the intact and control limbs were comparable for loading during walking and muscle strength variables, suggesting that the intact limb provides a suitable internal control for comparison with the disused amputated limb. The MVT and RTD were ∼60 and ∼75% lower, respectively, in the amputated than intact/control limbs. The differences in MVT appeared to be associated with ∼40 and ∼43% lower muscle thickness and neural activation, respectively, and the differences in RTD appeared to be associated with the decline in MVT coupled with slowing of the intrinsic contractile properties. These results indicate considerable changes in strength and neuromuscular function with long-term disuse that could not be predicted from short-term disuse studies.
Highlights
Prolonged disuse of skeletal muscle poses a considerable threat to neuromuscular functional capacity and health (Narici & de Boer 2011)
Long-term disuse of the amputated limb in individuals with unilateral transtibial amputations (ITTAs) was evidenced from the ~60% lower peak knee extensor moments during walking compared to the intact and control limbs
Short-term intervention studies suggest that maximum voluntary torque (MVT) decreases exponentially over time following unloading, plateauing out after ~90 days; the results of this study suggest the strength declines with longer-term disuse are considerably more than could be predicted from short-term intervention studies
Summary
Prolonged disuse of skeletal muscle poses a considerable threat to neuromuscular functional capacity and health (Narici & de Boer 2011). Just nine days of disuse causes considerable reductions in muscle strength, typically measured as maximum voluntary torque (MVT; Rozier et al 1979) or rate of torque development during contractions performed from rest (RTD; Bamman et al 1998). Studies show reductions in quadriceps MVT of approximately 2% per day for the first ten days (Berg & Tesch 1996, Puthucheary et al 2017, Rozier et al 1979), slowing to ~1% per week for up to 30 days, with an eventual plateau resulting in average strength losses of around 23% after 120 days of disuse Longterm muscle disuse is a default position for many clinical populations (Brown et al 2004) and the sedentary, yet it is unclear how both MVT and RTD change with long-term, habitual disuse, as typical disuse study models last
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