Here we explain the neural and mechanical mechanisms responsible for synchronizing sway and improving postural control during physical contact with another standing person. Postural control processes were modelled using an inverted pendulum under continuous feedback control. Interpersonal interactions were simulated either by coupling the sensory feedback loops or by physically coupling the pendulums with a damped spring. These simulations precisely recreated the timing and magnitude of sway interactions observed empirically. Effects of firmly grasping another person's shoulder were explained entirely by the mechanical linkage. This contrasted with light touch and/or visual contact, which were explained by a sensory weighting phenomenon; each person's estimate of upright was based on a weighted combination of veridical sensory feedback combined with a small contribution from their partner. Under these circumstances, the model predicted reductions in sway even without the need to distinguish between self and partner motion. Our findings explain the seemingly paradoxical observation that touching a swaying person can improve postural control.

Key points: Physiological hand tremor has a clear peak between 6 and 12 Hz, which has been attributed to both neural and resonant causes. A reduction in tremor frequency produced by adding an inertial mass to the limb has usually been taken as a method to identify the resonant component. However, adding mass to a limb also inevitably increases the muscular force required to maintain the limb's position against gravity, so ambiguous results have been reported. Here we measure hand tremor at different levels of gravitational field strength using a human centrifuge, thereby increasing the required muscular force to preserve limb position without changing the limb's inertia. By comparing the effect of added mass (inertia + force) versus solely added force upon hand acceleration, we conclude that tremor frequency can be almost completely explained by a resonant mechanical system. Human physiological hand tremor has a resonant component. Proof of this is that its frequency can be modified by adding mass. However, adding mass also increases the load which must be supported. The necessary force requires muscular contraction which will change motor output and is likely to increase limb stiffness. The increased stiffness will partly offset the effect of the increased mass and this can lead to the erroneous conclusion that factors other than resonance are involved in determining tremor frequency. Using a human centrifuge to increase head‐to‐foot gravitational field strength, we were able to control for the increased effort by increasing force without changing mass. This revealed that the peak frequency of human hand tremor is 99% predictable on the basis of a resonant mechanism. We ask what, if anything, the peak frequency of physiological tremor can reveal about the operation of the nervous system.

This study evaluated the ingestion of sodium bicarbonate (NaHCO₃) on postexercise acid-base balance recovery kinetics and subsequent high-intensity cycling time to exhaustion. In a counterbalanced, crossover design, nine healthy and active males (age: 23 ± 2 years, height: 179 ± 5 cm, body mass: 74 ± 9 kg, peak mean minute power (W_peak) 256 ± 45 W, peak oxygen uptake (V̇O_2peak) 46 ± 8 ml.kg^-1.min^-1) performed a graded incremental exercise test, two familiarization and two experimental trials. Experimental trials consisted of cycling to volitional exhaustion (T_LIM1) at 100% W_PEAK on two occasions (T_LIM1 and T_LIM2) interspersed by a 90 min passive recovery period. Using a double-blind approach, 30 min into a 90 min recovery period participants ingested either 0.3 g.kg^-1 body mass sodium bicarbonate (NaHCO₃) or a placebo (PLA) containing 0.1 g.kg^-1 body mass sodium chloride (NaCl) mixed with 4 ml.kg^-1 tap water and 1 ml.kg^-1 orange squash. The mean differences between T_LIM2 and T_LIM1 was larger for PLA compared with NaHCO₃ (-53 ± 53 vs. -20 ± 48 s; p = .008, d = 0.7, CI =-0.3, 1.6), indicating superior subsequent exercise time to exhaustion following NaHCO₃. Blood lactate [Bla^-] was similar between treatments post T_LIM1, but greater for NaHCO₃ post T_LIM2 and 5 min post T_LIM2. Ingestion of NaHCO₃ induced marked increases (p < .01) in both blood pH (+0.07 ± 0.02, d = 2.6, CI = 1.2, 3.7) and bicarbonate ion concentration [HCO₃^-] (+6.8 ± 1.6 mmo.l^-1, d = 3.4, CI = 1.8, 4.7) compared with the PLA treatment, before T_LIM2. It is likely both the acceleration of recovery, and the marked increases of acid-base after T_LIM1 contributed to greater T_LIM2 performance compared with the PLA condition.