Geriatric Research, Education, and Clinical Center, Salt Lake City VAMC, UT, USA.
Department of Internal Medicine, University of Utah, Salt Lake City, UT, USA.
J Physiol. 2018 Jun;596(12):2301-2314. doi: 10.1113/JP275817. Epub 2018 May 8.
This investigation assessed the influence of group III/IV muscle afferents on small muscle mass exercise performance from a skeletal muscle bioenergetics perspective. Group III/IV muscle afferent feedback was attenuated with lumbar intrathecal fentanyl during intermittent isometric single-leg knee-extensor all-out exercise, while P-MRS was used to assess skeletal muscle bioenergetics. Attenuation of group III/IV muscle afferent feedback improved exercise performance during the first minute of exercise, due to an increase in total ATP production with no change in the ATP cost of contraction. However, exercise performance was not altered during the remainder of the protocol, despite a sustained increase in total ATP production, due to an exacerbated ATP cost of contraction. These findings reveal that group III/IV muscle afferents directly limit exercise performance during small muscle mass exercise, but, due to their critical role in maintaining skeletal muscle contractile efficiency, with time, the benefit of attenuating the muscle afferents is negated.
The direct influence of group III/IV muscle afferents on exercise performance remains equivocal. Therefore, all-out intermittent isometric single-leg knee-extensor exercise and phosphorous magnetic resonance spectroscopy ( P-MRS) were utilized to provide a high time resolution assessment of exercise performance and skeletal muscle bioenergetics in control conditions (CTRL) and with the attenuation of group III/IV muscle afferent feedback via lumbar intrathecal fentanyl (FENT). In both conditions, seven recreationally active men performed 60 maximal voluntary quadriceps contractions (MVC; 3 s contraction, 2 s relaxation), while knee-extensor force and P-MRS were assessed during each MVC. The cumulative integrated force was significantly greater (8 ± 6%) in FENT than CTRL for the first minute of the all-out protocol, but was not significantly different for the second to fifth minutes. Total ATP production was significantly greater (16 ± 21%) in FENT than CTRL throughout the all-out exercise protocol, due to a significantly greater anaerobic ATP production (11 ± 13%) in FENT than CTRL with no significant difference in oxidative ATP production. The ATP cost of contraction was not significantly different between FENT and CTRL for the first minute of the all-out protocol, but was significantly greater (29 ± 34%) in FENT than in CTRL for the second to fifth minutes. These findings reveal that group III/IV muscle afferents directly limit exercise performance during small muscle mass exercise, but, due to their critical role in maintaining skeletal muscle contractile efficiency, with time, the benefit from muscle afferent attenuation is negated.
本研究从骨骼肌生物能量学的角度评估了 III/IV 类肌梭传入神经对小肌肉量运动表现的影响。在间歇性等长单腿伸膝全力运动中，通过腰穿芬太尼抑制 III/IV 类肌梭传入神经反馈，同时使用磷磁共振光谱（P-MRS）评估骨骼肌生物能量学。抑制 III/IV 类肌梭传入神经反馈可在运动的前 1 分钟内提高运动表现，这是由于总 ATP 生成增加，而收缩的 ATP 成本没有变化。然而，尽管总 ATP 生成持续增加，但在协议的其余部分运动表现没有改变，这是由于收缩的 ATP 成本恶化所致。这些发现表明，III/IV 类肌梭传入神经直接限制小肌肉量运动时的运动表现，但由于它们在维持骨骼肌收缩效率方面的关键作用，随着时间的推移，抑制肌梭传入神经的益处被抵消。
III/IV 类肌梭传入神经对运动表现的直接影响仍存在争议。因此，采用全力间歇性等长单腿伸膝运动和磷磁共振光谱（P-MRS），在对照条件（CTRL）和通过腰穿芬太尼（FENT）抑制 III/IV 类肌梭传入神经反馈的情况下，提供运动表现和骨骼肌生物能量学的高时间分辨率评估。在两种情况下，7 名有规律运动的男性进行 60 次最大自主股四头肌收缩（MVC；3 秒收缩，2 秒放松），同时在每次 MVC 时评估伸膝力和 P-MRS。在全力方案的前 1 分钟，FENT 时的累积积分力明显大于（8±6%）CTRL，但在第 2-5 分钟时没有明显差异。在整个全力运动方案中，FENT 时的总 ATP 生成明显大于（16±21%）CTRL，这是由于 FENT 时的无氧 ATP 生成（11±13%）明显大于 CTRL，而氧化 ATP 生成没有明显差异。在全力方案的前 1 分钟，FENT 时的收缩 ATP 成本与 CTRL 没有明显差异，但在第 2-5 分钟时，FENT 时明显大于（29±34%）CTRL。这些发现表明，III/IV 类肌梭传入神经在小肌肉量运动时直接限制运动表现，但由于它们在维持骨骼肌收缩效率方面的关键作用，随着时间的推移，抑制肌梭传入神经的益处被抵消。
