There are clear associations with increasing age and declining neuromuscular control [19
]. Even though the monosynaptic short latency stretch reflex exists at the lowest level of the nervous system’s postural control hierarchy [22
], enhancement could lead to improvements in maintenance of balance through faster corrections to postural adjustments. The monosynaptic short latency stretch reflex response is dependent on a precise balance of excitatory and inhibitory information from neural receptors in the spinal cord to produce motor neuron excitability [23
]. The muscle spindle provides essential information on muscle length and relays this to the spinal cord for integration and appropriate response via the α- and γ-motor neurons. When the muscle is in an unstretched state, action potentials are generated at a constant rate, whereas when stretched, the muscle spindle is activated and the action potential firing rate increases [24
]. Unfortunately, we were unable to quantify the effect of Aqua Titan on the stretch reflex response in older adults, due to the inability to produce the ankle jerk response in a number of our participants, an issue that has been noted in previous literature [25
]. Nevertheless, Hughes, et al. [9
] observed an almost certain large reduction in short latency stretch reflex among their younger participants with Aqua Titan tape application, using the Achilles tendon tap method. The reduced reflex time suggests enhanced neuromuscular functioning via faster nerve conduction; although the mechanism behind this improvement remains untested. Increased action potential firing rate can lead to faster neural communication, but an action potential is an energetically expensive process; a sensory neuron expends between 30–75-times more energy during an action potential, compared to when in a state of rest [26
]. An important aspect that impacts the energy cost of neural processing is the amount of noise (redundant information) amongst the signal production [27
]. If the amount of noise involved in a sensory feedback system can be reduced, said system becomes more efficient [27
]. The previous evidence by Korte [7
] showing reduced action potential firing rates with the application of Aqua Titan material makes us hypothesize that there is an enhancement in neural signaling efficiency. Therefore, it may be this efficiency (rather than increased firing rate) that has led to past improvements in the short latency stretch reflex, and this may be a potential mechanism behind the enhanced balance observed in the current study. Completing the balance task without any visual input places more emphasis on the vestibular and somatosensory systems and infers that the observed improvement with Aqua Titan is due to enhanced neuromuscular feedback and/or coordination.
Interestingly, the improvement in balance was only noted at 38 h into recovery, but not at 14, or 62 h. There is the possibility that the neutral result at 62 h may be driven by neural acclimation, but similar fluctuations in Aqua Titan-induced benefits to recovery have also been observed in previous studies. Wadsworth, et al. [11
] reported a reduction in peak run velocity on Day 3, but an improvement on Day 5 of recovery. In the same study, there were inconsistencies between recovery days for the ROM measures; some improvements being conclusive on one day, but trivial on another, or vice versa. If there are potential oscillations in the Aqua Titan effect on recovery, these will need to be investigated further to provide insight into the optimal time and duration of application.
Although we did not observe any improvement in walking performance or economy (contrary to previous research [10
]), the extent of Aqua Titan coverage needs to be considered. Rowlands, et al. [10
] employed whole body coverage of Aqua Titan using tight fitting garments and socks; however, only the lower legs were exposed to Aqua Titan in the current study. Whilst an Aqua Titan effect at the muscle-tendon complex was observed, the small extent of coverage may not have been enough to induce improvements in walking performance or efficiency. The association between Aqua Titan coverage and improvements in performance/efficiency needs to be investigated further.