The role of mechanical and neural restraints to joint range of motion during passive stretch

The combined effects of transcutaneous electrical nerve stimulation (TENS) and stretching on muscle hardness and pressure pain threshold

[Purpose] This study aimed to clarify the immediate effects of a combined transcutaneous electrical nerve stimulation and stretching protocol. [Subjects] Fifteen healthy young males volunteered to participate in this study. The inclusion criterion was a straight leg raising range of motion of less than 70 degrees. [Methods] Subjects performed two protocols: 1) stretching (S group) of the medial hamstrings, and 2) tanscutaneous electrical nerve stimulation (100 Hz) with stretching (TS group). The TS group included a 20-minute electrical stimulation period followed by 10 minutes of stretching. The S group performed 10 minutes of stretching. Muscle hardness, pressure pain threshold, and straight leg raising range of motion were analyzed to evaluate the effects. The data were collected before transcutaneous electrical nerve stimulation (T1), before stretching (T2), immediately after stretching (T3), and 10 minutes after stretching (T4). [Results] Combined transcutaneous electrical nerve stimulation and stretching had significantly beneficial effects on muscle hardness, pressure pain threshold, and straight leg raising range of motion at T2, T3, and T4 compared with T1. [Conclusion] These results support the belief that transcutaneous electrical nerve stimulation combined with stretching is effective in reducing pain and decreasing muscle hardness, thus increasing range of motion.

To stretch or not to stretch: the role of stretching in injury prevention and performance

Stretching is commonly practiced before sports participation; however, effects on subsequent performance and injury prevention are not well understood. There is an abundance of literature demonstrating that a single bout of stretching acutely impairs muscle strength, with a lesser effect on power. The extent to which these effects are apparent when stretching is combined with other aspects of a pre-participation warm-up, such as practice drills and low intensity dynamic exercises, is not known. With respect to the effect of pre-participation stretching on injury prevention a limited number of studies of varying quality have shown mixed results. A general consensus is that stretching in addition to warm-up does not affect the incidence of overuse injuries. There is evidence that pre-participation stretching reduces the incidence of muscle strains but there is clearly a need for further work. Future prospective randomized studies should use stretching interventions that are effective at decreasing passive resistance to stretch and assess effects on subsequent injury incidence in sports with a high prevalence of muscle strains.

The relative lengthening of the myotendinous structures in the medial gastrocnemius during passive stretching differs among individuals

The increase in passive torque during muscle stretching may constrain the range of motion of a joint. As passive torque can vary substantially among individuals, the present study examined whether the relative lengthening of the myotendinous structures of the medial gastrocnemius (MG) during passive stretching differs among individuals. Sixteen subjects performed passive stretching of the plantar flexor muscles from ankle angles ranging from 10° plantar flexion (−10°) to 30° dorsiflexion (+30°). Changes in passive torque, muscle architecture (fascicle length and pennation angle) of the MG and electromyographic activity of MG and soleus were recorded. The results showed that passive torque produced by the plantar flexors increased exponentially ( r2 = 0.99; P < 0.001) with ankle dorsiflexion, whereas MG fascicle length increased linearly from 57.6 ± 9.1 to 80.5 ± 10.3 mm ( P < 0.001), and pennation angle decreased linearly from 21.2 ± 4.2 to 14.4 ± 3.1° ( P < 0.001) when the ankle joint angle was moved from −10° to +30°. The relative contribution of muscle (fascicles and aponeuroses) and tendon elongation to the change in length of the muscle-tendon unit (MTU) at 30° dorsiflexion was 71.8 and 28.2%, respectively. However, the adjustment differed across individuals during MTU lengthening; in subjects (62.5%) with small, passive stiffness, the elongation of the free tendon was less and that of the fascicles larger than for subjects (37.5%) with greater stiffness. In conclusion, the results indicate that the strain of muscle and tendon varies among individuals, and difference in the relative compliance of these structures influences MTU lengthening differently during passive stretching.

The role of passive muscle stiffness in symptoms of exercise-induced muscle damage

We examined whether passive stiffness of an eccentrically exercising muscle group affects the subsequent symptoms of muscle damage. Passive hamstring muscle stiffness was measured during an instrumented straight-leg-raise stretch in 20 subjects (11 men and 9 women) who were subsequently classified as "stiff" (N = 7), "normal" (N = 6), or "compliant" (N = 7). Passive stiffness was 78% higher in the stiff subjects (36.2 +/- 3.3 N.m.rad(-1)) compared with the compliant subjects (20.3 +/- 1.8 N.m.rad(-1)). Subjects then performed six sets of 10 isokinetic (2.6 rad.s(-1)) submaximal (60% maximal voluntary contraction) eccentric actions of the hamstring muscle group. Symptoms of muscle damage were documented by changes in isometric hamstring muscle strength, pain, muscle tenderness, and creatine kinase activity on the following 3 days. Strength loss, pain, muscle tenderness, and creatine kinase activity were significantly greater in the stiff compared with the compliant subjects on the days after eccentric exercise. Greater symptoms of muscle damage in subjects with stiffer hamstring muscles are consistent with the sarcomere strain theory of muscle damage. The present study provides experimental evidence of an association between flexibility and muscle injury. Muscle stiffness and its clinical correlate, static flexibility, are risk factors for more severe symptoms of muscle damage after eccentric exercise.