Can passive stretch inhibit motoneuron facilitation in the human plantar flexors?

Abstract

The purpose of the present study was to examine the possible inhibitory effect of passive plantar flexor muscle stretching on the motoneuron facilitatory system. Achilles tendon vibration (70 Hz) and triceps surae electrical stimulation (20 Hz) were imposed simultaneously in 11 subjects to elicit contraction through reflexive pathways in two experiments. In experiment 1, a vibration-stimulation protocol was implemented with the ankle joint plantar flexed (+10°), neutral (0°), and dorsiflexed (-10°). In experiment 2, the vibration-stimulation protocol was performed twice before (control), then immediately, 5, 10, and 15 min after a 5-min intermittent muscle stretch protocol. Plantar flexor torque and medial and lateral gastrocnemius and soleus (EMGSol) EMG amplitudes measured during and after (i.e., self-sustained motor unit firing) the vibration protocol were used as an indicator of this facilitatory pathway. In experiment 1, vibration torque, self-sustained torque and EMGSol were higher with the ankle at -10° compared with 0° and +10°, suggesting that this method is valid to assess motoneuronal facilitation. In experiment 2, torque during vibration was reduced by ∼ 60% immediately after stretch and remained depressed by ∼ 35% at 5 min after stretch (P< 0.05). Self-sustained torque was also reduced by ∼ 65% immediately after stretch (P < 0.05) but recovered by 5 min. Similarly, medial gastrocnemius EMG during vibration was reduced by ∼ 40% immediately after stretch (P < 0.05), and EMGSol during the self-sustained torque period was reduced by 44% immediately after stretch (P < 0.05). In conclusion, passive stretch negatively affected the motoneuronal amplification for at least 5 min, suggesting that motoneuron disfacilitation is a possible mechanism influencing the stretch-induced torque loss.

Acute effects of muscle stretching on physical performance, range of motion, and injury incidence in healthy active individuals: a systematic review

“…Decreases in electrically stimulated force after acute plantar flexor stretches (Trajano et al 2013(Trajano et al , 2014) may be considered evidence for the hypothesis; however, these reductions were disproportionally smaller than, and not correlated with, the loss of voluntary force and were not correlated with the recovery of force after stretch (Trajano et al 2014). As yet, practically meaningful muscle damage has not been reported after SS in humans.…”

“…The weighted estimates of the remaining 178 measures revealed a moderate 3.7% mean performance reduction (Table 1). Thus, although there are some occasions in which large or very large reductions are reported (e.g., Trajano et al 2014), SS generally induces moderate mean (<5%) performance impairments when testing is performed within minutes of stretching. Given the substantial between-study differences in poststretch changes (range, +5% to -20.5%), closer examination of the possible variables that influence the likelihood and magnitude of performance change after SS is required.…”

“…Thus, ischemia-reperfusion cycles induced by intermittent stretching appear to be particularly problematic. The mechanism by which these cycles impair force production is not known specifically, but it appears not to be caused by a decrease in intracellular free calcium concentration (Trajano et al 2014). …”

“…However, others report no changes in EMG amplitude (e.g., Herda et al 2008;Ryan et al 2008), so a consensus cannot be reached regarding changes in central drive as assessed using EMG. Nonetheless, reductions in the EMG/M ratio (reducing peripheral influences on EMG amplitude), voluntary activation levels (measured using the interpolated twitch technique), and V-wave amplitude (a variant of the H reflex providing evidence of voluntary drive to the motoneurons) have been observed after stretch (Trajano et al 2013(Trajano et al , 2014. Furthermore, these variables increased in the early period (<15 min) after stretch, and their changes were correlated with the changes in maximal force production (e.g., Trajano et al 2013Trajano et al , 2014.…”

“…Nonetheless, reductions in the EMG/M ratio (reducing peripheral influences on EMG amplitude), voluntary activation levels (measured using the interpolated twitch technique), and V-wave amplitude (a variant of the H reflex providing evidence of voluntary drive to the motoneurons) have been observed after stretch (Trajano et al 2013(Trajano et al , 2014. Furthermore, these variables increased in the early period (<15 min) after stretch, and their changes were correlated with the changes in maximal force production (e.g., Trajano et al 2013Trajano et al , 2014. Indeed, the simultaneous recoveries of voluntary force and central drive (EMG) have been observed in other studies.…”