The Effect of Transcutaneous Electrical Nerve Stimulation (TENS) Applied to the Foot and Ankle on Strength, Proprioception and Balance: A Preliminary Study

Background: Transcutaneous electrical nerve stimulation (TENS) promotes upper motor neuron excitability which has the potential to improve function. As a precursor to clinical trials, we investigated the potential efficacy of TENS on strength, proprioception and balance in healthy older adults.

Method: Design: A paired-sample randomized crossover trial. No stimulation was the control. Intervention: A one-off session of TENS (Modulated frequency: 70-130Hz, 5 second cycle) via a conductive sock. Participants: 25 healthy older volunteers with no pre-existing balance or mobility limitations or contra-indications to TENS. Outcomes: Dorsiflexor and plantarflexor strength and proprioception using an isokinetic dynamometer and balance (postural sway and forward reach test). Analysis: Paired t-tests

Results: None of the parameters showed any significant changes with TENS (p>0.05).

Conclusions: The stimulation of cutaneous sensory nerve endings of the foot with the application of TENS showed no immediate effect on the ankle proprioception, lower leg muscle strength, and postural stability. The concern that TENS would have a distracting impact on sensation and balance was not supported according to these results.

Transcutaneous electrical nerve stimulation; Strength; Proprioception; Balance

Transcutaneous electrical nerve stimulation (TENS) is a well-known intervention which differentially influence a variety of outcomes related to pain [1-3]. Physical function used to be part of this outcome domain [4]. It has been shown that TENS increased the balance function on the “Timed up and GO” test as well as reducing the pain level in people with osteoarthritis [5], but it was unclear if this improvement is related to the pain reduction effects or direct promotion of functional outcomes using TENS. This has led to the intriguing notion that TENS could be used to promote motor performance or recovery in people with disabling neuromotor conditions. Cortical excitability changes can alter the individual experience to interact with the environment. The new situation can result in either an improvement or detrimental effects on the functional performance [6]. There is some evidence that TENS to the lower limb can enhance gait speed, functional recovery, spasticity and reflex activity in people with stroke [7,8], joint position sense in people with knee osteoarthritis [9] and postural sway in healthy volunteers [10-13].

Foot and ankle complex is the only segment which is in direct contact with supporting surface and has an important role in collecting somatosensoy feedbacks and regulating balance control [14]. Sensory impairements in the foot and ankle complex contribute to balance and activity probles in people with stroke [15]. Acessory sensory stimulation applied to the paretic hand improves the upper limb function in people with stroke [16]. If effective, the application of TENS at the foot and ankle has the potential to be a highly beneficial intervention as deficits of strength, joint position sense and balance control contribute to the increased risk of falls and limited mobility associated with older adults and many neurological conditions [17,18]. There is another debate that stimulation can be a distraction for people while they need their attention for keeping the balance and walking safe. Accordingly, it remains a major concern if applied TENS to the foot has any adverse effects on the balance parameters and how the people with disabling neurologic conditions would cope with this detrimental effect. To take this ethical issue into account, a priliminary study was needed to carry out with age-matched healthy controls. Augmenting plantar sensory feedbacks provides a potential mechanism to improve balance stability, even in the healthy subjects [19]. This exploratory trial was to check the components of the intervention and define the feasibility of a safe testing protocol according to the UK Medical Research Council Framework (MRC) for the evaluation of complex interventions [20]. Thus, as a precursor to trials in patient populations, we investigated the effect of TENS in healthy older adults who are assumed to have a lesser risk of falls within the testings compared with people with neurologic problems. To our knowledge, none of other studies applied the TENS at the foot and ankle in healthy subjects. It was hypothesized that applying TENS to the foot and ankle complex would improve the balance and postural control. The aim of this study was to investigate any possible effects of TENS on balance control and its possible underlying mechanisms in the lower leg like proprioception [21,22] and muscle strength [23].

A paired sample, randomized crossover trial of the immediate effects of TENS was used in which the participants acted as their own control and the randomization came from the order in which the TENS or control was given. All participants completed both control and stimulation conditions and all testing procedures. Ethics approval was obtained from the University of Salford’s Research Ethics Committee (Manchester, UK).

Participants

The intervention

Testing protocol

Proprioception and strength

Balance

Data processing and analysis

Twenty-five healthy volunteers (12 women and 13 men; age 56.8 ± 14.5 years; weight: 78.6 ± 14.5 Kg; height: 171.2 ± 6.7 cm) were recruited. All tolerated the TENS which had no significant effect (either positive or negative on any of the parameters measured) (Table 1).

As none of the study outcome measures showed a significant change with and without stimulation, a power analysis was undertaken (using GPower Software 3.1) to explore the impact of the small sample size on the result. Mean velocity of CoP was selected as the representative parameter for this analysis as it had been shown to be the more reliable and sensitive measure of postural sway [29]. The power of the current study was acceptable [power (1-β error prob)=0.85]. Alpha level was adjusted at 0.15 rather than traditional level of 0.05, as recommended for small group size [30].

The results of this study show no significant impact of TENS on ankle strength, joint position sense or balance control in healthy older adults. This is one of the very few reports of negative findings for supplementary sensory stimulation given via TENS or any other paradigm to healthy or disabled participants. It is impossible to ascertain whether this is merely due to reporting bias where other researchers have not published non-significant findings or due to methodological differences. Previous reports of TENS as a potential treatment paradigm in healthy adults used a similar paired-group randomized controlled design so it is unlikely that a different bias from the trial design is a contributor. However the previous reports on healthy volunteers used postural sway (a measure of balance impairment) as the primary outcome and gave stimulation to the knee or posterior calf [8,10,11] to influence the “ankle strategy” for standing postural which contrasts our stimulation of the foot and ankle using strength, proprioception and balance activity outcomes. Further methodological differences were the involvement of young adults (university students) who received a similar dose of TENS [8,10] or older adults who received sub-sensory stimulation [11]. Study heterogeneity is such that it is not possible to identify methodological differences to explain the different response to previous trials. One potentially important difference with previous studies was the method of delivering the TENS. Using a conductive sock might reduce the deliverability of the stimulation compared to the gel electrodes. More recent reports however showed a significant improvement in functional outcomes using stimulation delivered through the same sock in people with neurological condition [31,32]. Thus, it is not clear whether the difference in response is due to heterogeneity in the participants selected, the stimulation paradigm applied or the outcome measures used. Similarly, studies of TENS in patient groups are too heterogeneous in terms of the stimulation protocols, the selected participants and the outcome measures to postulate hypotheses about the possible discrepancies of our findings. The TENS effect was measured during peak effect. This approach was thought to be more likely to show any possible effect. Previous research demonstrated that TENS has the greatest effects when it is on or immediately after switching it off [1,33].

Limitations

In contrast to previous reports of postural control, we found that TENS did not affect the strength, proprioception and balance in healthy older volunteers. Further research is needed to systematically develop this potential intervention.