The pelvic floor muscles (PFM) are crucial for maintaining urinary continence. Exercise programs intended to strengthen the core and PFM are considered a first line of treatment against stress and mixed urinary incontinence. Among people with spinal cord injury (SCI), more than 80% of the population experience adult neurogenic lower urinary tract dysfunction (ANULTD). PFM training programs may not have been attempted in people with SCI because it is often incorrectly assumed that they are unable to engage muscles of the core. Evidence from previous work has shown that sparing in abdominal function can be detected using manual palpation, surface electromyography (EMG) (1), and transcranial magnetic stimulation (TMS) (2). It remains unknown to what extent the PFM may be similarly spared in this population. 

The purpose of this study was to 1) characterize and compare activation patterns of pelvic floor, abdominal, and gluteal muscles elicited by different exercise maneuvers in able-bodied (AB) individuals and people with SCI and 2) evaluate corticospinal excitability to the PFM via transcranial magnetic stimulation.

Participants with motor-complete SCI and AB controls were recruited to this cross-sectional study. A nurse conducted the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) exam with each SCI participant to confirm their injury classification.

Activation of the PFM: 
Surface EMG electrodes were affixed bilaterally over the rectus abdominis, external oblique, erector spinae, and gluteus maximus. Bilateral PFM recordings were taken from levator ani by placing disposable surface electrodes perianally. Participants attempted a series of six validated maneuvers isometrically to determine if voluntary PFM activity could be elicited: trunk flexion, side bend (right and left), back extension, abdominal hollowing, Kegel, and gluteal contraction.  

The root mean square (RMS) of the EMG data was calculated over a 2 s window for each muscle during rest and the attempted maneuver in each trial. If the RMS value during a given contraction exceeded rest (mean + 2SD), then activation was considered ‘present’. Participants attempted each maneuver twice, and were given an ‘activation score’ of 0-2, corresponding to the number of trials with activation (converted to a percentage). EMG amplitude for each maneuver was also compared using a one-way repeated measures ANOVA with post-hoc Bonferroni-corrected pairwise comparisons.

Corticospinal Excitability to the PFM: 
Single-pulse transcranial magnetic stimulation (TMS) was applied over the medial aspect of the precentral gyrus using a MagStim 200 stimulator with a double cone coil (3). A custom navigation system ensured trial-to-trial coil positioning was within 2 mm. Stimuli were applied in blocks of 5 pulses beginning at sub-maximal intensities and increasing in intensity by 5-10% of the maximum stimulator output (%MSO). Participants were asked to attempt a submaximal contraction of their PFM for each stimulation by attempting a sit-up at 10% of their maximal effort which was verified using an EMG biofeedback system.

The onset (latency) of the motor-evoked potentials (MEPs) was defined as the time at which the MEP exceeded 2SD above the mean of baseline EMG activity for at least 2 ms. MEP amplitude was defined by the peak-to-peak amplitude of the raw EMG activity after this MEP onset and plotted against stimulation intensity to produce a recruitment curve. Each participant was also given a ‘MEP presence score’ based on the five trials completed at the highest %MSO they reached: a score of 0 if no MEP was present, a score of 1 if a MEP was present in some trials, or a score of 2 if the MEP was present in all trials.

Nine spinal cord injured participants (6 men, 3 women (1 nulliparous); mean age 43) and eight AB participants (4 men, 4 women (4 nulliparous); mean age 24) participated in this study. All SCI participants were motor-complete (7 AIS A, 2 AIS B) with injuries ranging from C6-T10 and were at least 1-year post-injury (range 1-33yr, mean 15yr). All participants used catheters to void (8 clean intermittent catheterization, 1 indwelling catheter) and six participants reported regular urine leakage.

The activation score ranged from 64-100% (mean 90%) for AB participants with the gluteal contraction and Kegel producing the most consistent responses. All but one SCI subject were able to successfully recruit their PFM during the attempted maneuvers, but activation score ranged from 3-64% (mean 27%). Trunk flexion was most effective for recruiting the PFM among the SCI participants. In the AB group, pairwise comparisons revealed a significant difference in EMG amplitude between rest and all maneuvers (p < 0.0001). In the SCI group, pairwise comparison revealed a significant difference in EMG amplitude between rest and right side bending (p = 0.0069), left side bending (p = 0.003), and trunk extension (p = 0.006) (Figure 1). 

MEPs in the PFM could be elicited in all AB participants which increased in amplitude with increasing %MSO. The average latency for the PFM was 17.5 ms (SD: 2.4 ms). Of the SCI participants, PFM MEPs were elicited bilaterally in six individuals and unilaterally in one individual, with an average latency of 21.8 ms (SD: 4.7 ms). For most subjects, the size of the MEP corresponded to the intensity of stimulation, but this relationship was not as well defined in comparison to the AB group (Figure 2). Four participants scored 100% on the MEP presence score, two scored 75%, one scored 25%, and the remaining two scored 0%.

Despite their clinical diagnosis as motor-complete, all but one SCI participant were able to activate their PFM voluntarily. Participants were generally unable to activate the PFM directly (no response to Kegel), but were able to activate this muscle group through synergistic maneuvers (e.g. trunk flexion). This synergist behaviour of the PFM during tasks that target other core muscles is consistent with other studies showing that the PFM is engaged during these tasks to guard against incontinence during instances of higher intra-abdominal pressure. PFM MEPs were elicited in seven of the SCI participants suggesting some sparing of descending pathways to this muscle group.

Our results are the first to present evidence for both voluntary activation of and cortical sparing to the PFM in the SCI population, opening up possibilities for PFM training programs, even in those classified as motor-complete. Future work should explore how the application of PFM training programs may improve ANULTD in this population.

Bjerkefors A, et al. (2009). Trunk muscle activation in a person with clinically complete thoracic spinal cord injury. Journal of Rehabilitation Medicine, 41, 390-392.
Bjerkefors A, et al. (2015). Assessment of abdominal muscle function in individuals with motor-complete spinal cord injury above T6 in response to transcranial magnetic stimulation. Journal of Rehabilitation Medicine, 47(2), 138-146.
Asavasopon S, et al. (2014). Cortical activation associated with muscle synergies of the human male pelvic floor. Journal of Neuroscience, 34(41), 13811-13818.