The role of supraspinal excitability in the development of healthy-limb deficits following anterior cruciate ligament injury

In the current study, we examined how supraspinal and spinal excitability were altered bilaterally after unilateral anterior cruciate ligament reconstruction (ACLr). 7 participants with ACLr and 7 healthy controls underwent transcranial magnetic stimulation (TMS) and electrical stimulation. To evaluate supraspinal excitability, resting motor thresholds (RMT) and motor evoked potential (MEP) stimulus response curves (SRC) were used. To measure spinal excitability, H-reflex SRC gain was assessed. Mixed factorial ANOVAs were used to compare measures between limbs and between groups. Cohen’s d was used to assess effect sizes between groups. Data indicated no significant differences between subject groups or between limbs. However, large effect sizes were found between limbs for H-reflex gain and RMTs suggesting that ACLr can have an effect on some of the variables examined. This study identified decreases in strength in the injured limbs and that subjects with an ACL injury exhibited decreases in spinal and supraspinal excitability of the quadriceps compared to Healthy controls.

The dual effect of normobaric hypoxia on heart rate variability and substrate partitioning following interval cycling

Recent studies have shown the importance of the beat-by-beat changes in heart rate influenced by the autonomic nervous system (ANS), or heart rate variability (HRV). The purpose of this study was to examine the lasting effects of hypoxic exercise on HRV, and its influences on substrate usage. Results from this study could lead an increased understanding on this topic. Eight active healthy males (age: 31±11 years; height: 180±7 cm; weight: 83±8 kg; VO₂max (maximal oxygen consumption): 4.4±0.6 L•min⁻¹) underwent normoxic and hypoxic (FᵢO₂= 0.15) conditions during high-intensity interval (HIIT) cycling (70%-high interval, 35%-rest interval). Cycling intensity was determined by a peak power output cycling test. Each experimental session consisted of a basal metabolic rate determination, up to 45-minutes of HIIT cycling, and three 30-minute post-exercise metabolic rate measurements (spanning 3 hours and 15 minutes after exercise). During exercise, RPE was higher (p<0.01) and LAC (lactate) increased (p=0.001) at each point of time in hypoxia, with no change in normoxia. After hypoxic exercise, the SNS/PNS ratio (overall ANS activity) was significantly higher (p<0.01) and significantly decreased through time in both conditions (p<0.01). In addition, a significant interaction between time and conditions (p<0.02) showed a decrease in LAC concentration through time post-hypoxic exercise. The findings showed that a single bout of hypoxic exercise alters ANS activity post-exercise along with shifting substrate partitioning from glycolytic to lipolytic energy production. The significant decrease in LAC concentration post-hypoxic exercise supports the notion that hypoxic HIIT induces a greater muscle glycogen depletion leading to increased fat oxidation to sustain glycogenesis and gluconeogenesis to maintain blood glucose level during recovery.