Foot sole cutaneous stimulation mitigates neuromuscular fatigue during a sustained plantar flexor isometric task

Electrical cutaneous stimulation of the foot sole does not enhance rate of torque development during maximal effort isometric plantarflexion in females

Rate of torque development (RTD) measures how rapidly one can generate torque and is crucial for balance and athletic performance. Fast RTD depends on the rapid recruitment of high threshold motor units (MUs). Cutaneous electrical stimulation has been shown to alter MU excitability, favoring high threshold MUs via reduced recruitment thresholds. A strong coupling exists between foot sole cutaneous mechanoreceptors and motor neurons of lower-limb muscles, yet it remains unknown if cutaneous input can impact RTD via modulation of MU excitability. This study aimed to investigate whether electrical stimulation across the heel could alter plantarflexion RTD. 11 young and healthy females underwent eight sets of five explosive isometric plantarflexion contractions on a dynamometer while sitting with hip, knee and ankle angles of 80°, 110°, and 90°, respectively. All participants achieved > 95 % voluntary activation of their plantar flexors. Four sets of contractions were performed with heel cutaneous electrical stimulation (1.0 ms pulses delivered at 300 Hz, at 2 × perceptual threshold) and four sets with no stimulation. Instantaneous RTD values were analyzed in 25 ms epochs from onset to 250 ms. No significant differences were observed between stimulation conditions within each epoch, thus our results suggest that electrical cutaneous stimulation does not alter RTD in this population.

Recovery of pinch force sense after short-term fatigue

The aim of this study was to identify the exact origin of force sense and identify whether it arises centrally or peripherally. The present study was designed to analyze the effects of short-term fatigue on pinch force sense and the duration of these effects. During the fatigue protocol, twenty (10 men and 10 women; Mage = 22.0 years old) young Chinese participants were asked to squeeze maximally until the pinch grip force decreased to 50% of its maximal due to fatigue. Participants were instructed to produce the target force (10% of maximal voluntary isometric contraction) using the same hand before and after fatigue (immediately, 10, 30, 60, 180, 300 s). The results showed significantly higher absolute error immediately after fatigue (1.22 ± 1.06 N) than before fatigue (0.68 ± 0.34 N), and 60 s (0.76 ± 0.69 N), 180 s (0.67 ± 0.42 N), and 300 s (0.75 ± 0.37 N) after fatigue (all P < 0.05) but with no effect on the variable error (P > 0.05). It was also revealed that there was a significant overestimate of the constant error values before (0.32 ± 0.61 N) and immediately after fatigue (0.80 ± 1.38 N, all P < 0.05), while no significant overestimation or underestimation exceeded 300 s after fatigue (P > 0.05). Our study results revealed that short-term fatigue resulted in a significant decrease in force sense accuracy, but it did not affect force sense consistently; however, force sense accuracy recovered to a certain extent within 10 s and 30 s, whereas it recovered fully within 60 s, and force sense directivity improvement exceeded 300 s after fatigue. The present study shows that the sense of tension (peripherally) is also an important factor affecting force sense. Our study supports the view that the periphery is part of the origin of force sense.

Adolescents running in conventional running shoes have lower vertical instantaneous loading rates but greater asymmetry than running barefoot or in partial-minimal shoes

Footwear may moderate the transiently heightened asymmetry in lower limb loading associated with peak growth in adolescence during running. This repeated-measures study compared the magnitude and symmetry of peak vertical ground reaction force and instantaneous loading rates (VILRs) in adolescents during barefoot and shod running. Ten adolescents (age, 10.6 ± 1.7 years) ran at self-selected speed (1.7 ± 0.3 m/s) on an instrumented treadmill under three counter-balanced conditions; barefoot and shod with partial-minimal and conventional running shoes. All participants were within one year of their estimated peak height velocity based on sex-specific regression equations. Foot-strike patterns, peak vertical ground reaction force and VILRs were recorded during 20 seconds of steady-state running. Symmetry of ground reaction forces was assessed using the symmetry index. Repeated-measures ANOVAs were used to compare conditions (α=.05). Adolescents used a rearfoot foot-strike pattern during barefoot and shod running. Use of conventional shoes resulted in a lower VILR (P < .05, dz = 0.9), but higher VILR asymmetry (P < .05) than running barefoot (dz = 1.5) or in partial-minimal shoes (dz = 1.6). Conventional running shoes result in a lower VILR than running unshod or in partial-minimal shoes but may have the unintended consequence of increasing VILR asymmetry. The findings may have implications for performance, musculoskeletal development and injury in adolescents.

Sensory enhancement of warm-up amplifies subsequent grip strength and cycling performance

PURPOSE

In sport and exercise, warm-ups induce various physiological changes that facilitate subsequent performance. We have shown that delivering patterned stimulation to cutaneous afferents during sprint cycling mitigates fatigue-related decrements in performance, and that repeated sensory stimulation amplifies spinal reflex excitability. Therefore, the purpose of this study was to assess whether sensory enhancement of warm-up would affect subsequent high-intensity arm cycling performance.

METHODS

Participants completed three experimental sessions, in which they randomly performed either a control, stim, or sleeve warm-up condition prior to maximal duration arm cycling. During the control condition, warmup consisted of low-intensity arm cycling for 15 min. The stim condition was the same, except they received alternating pulses (400 ms, 50 Hz) of stimulation just above their perceptual threshold to the wrists during warm-up. The third condition required participants to wear custom fabricated compression sleeves around the elbow during warm-up. Grip strength and spinal reflex excitability were measured before and after each warm-up and fatigue protocol, which required participants to arm cycle at 85% of peak power output until they reached volitional fatigue. Peak power output was determined during an incremental test at minimum 72 h prior to the first session.

RESULTS

Both sensory enhanced warm-up conditions amplified subsequent high-intensity arm cycling performance by ~ 30%. Additionally, the stim and sleeve warm-up conditions yielded improvements in grip strength (increased by ~ 5%) immediately after the sensory enhanced warm-ups. Ergogenic benefits from the sensory enhanced warm-up conditions did not differ between one another.

CONCLUSION

These findings demonstrate that enhanced sensory input during warm-up can elicit improvements in both maximal and submaximal performance measures.