Institute for Health and Social Care Research Introduction
Humans develop force through the contraction of muscle, which is transmitted via tendon to bone causing movement or action against resistance. In order to generate external force muscles must first overcome the slack in the tendon (Wilson et al. 1994). As tendon stiffness increases force transmission from muscle to bone will be more rapid. This has been demonstrated practically by Bojsen-Moller et al. (2005) who found tendon stiffness to be related to rate of force development and electromechanical delay.
The use of stretching exercises to improve range of motion is common practice among both recreational and competitive athletes (Kubo et al. 2002). However, stretching has be shown to acutely decrease performance in; one repetition maximum exercises (Kokkonen et al. 1998; Nelson and Kokkonen 2001), various jump exercises (Young and Elliott 2001), maximal isokinetic efforts (Marek et al. 2005) and sprint performance (Nelson et al. 2005). Evetovich et al. (2003) reported a decrease in muscle stiffness as indicated by an increase in mechanomyography following static stretching. Similarly Magnusson et al. (1996) showed a decrease in muscle stiffness after repeated static stretching, stiffness was defined as the change in torque (in Newton-meters) divided by angular change in position (in radians) and was expressed as the slope of the torque-position curve, (measurements were made during the dynamic phase of a passive stretch). In addition to muscle stiffness recent methodological advances have enabled tendon stiffness to be measured in vivo using ultrasonography. Using this method Kubo et al. (2001) found gastrocnemius tendon stiffness in males to decrease significantly following static stretching. This decrease in muscle/tendon stiffness has been proposed as a mechanism for diminished performance following stretch (Cornwell et al. 2002).
Kubo et al. (2003) showed that there are gender differences in the viscoelastic properties of tendon structures. They found that the stiffness and Young’s modulus of the gastrocnemius tendon was significantly lower in women (16.5 ± 3.4 N/mm, 277 ± 25 MPa) than in men (25.9 ± 7.0 N/mm, 356 ± 32 MPa). These observations may partly explain the difference in measures of performance between genders, especially those which are dependant on force generation in the early stages of muscle contraction such as electromechanical delay and rate of force development.
As tendon mechanical properties between genders have been shown to be different, it may be that there are differences in the effect an acute stretch has on the structure of the tendon. By its nature if a set force is applied to a tendon which is more compliant it will stretch further and so experience a greater strain than a stiffer tendon. It is therefore suggested that stretching will have a greater effect in terms of changes in stiffness in females whose tendons are initially more compliant that males.
Therefore, the present study aims to compare the effects of stretching on tendon stiffness between genders.
Method Subjects
Sixteen males, mean age 22 years ± 2 (SD), height 181.1cm ± 6.6 and body mass
83.4kg ± 11.4, and 10 females, mean age 20 years ± 1 (SD), height 166.0cm ± 7.1 and
body mass 65.3kg ± 10.4 participated in the study. Participants were recruited from
the university population and were all habitually active; all females were currently taking an oral contraceptive which contained oestrogen. Prior to selection all subjects were screened for evidence of previous lower limb injury and any other medical condition which could have prevented them from participating. The investigation was approved by the Salford University Institutional Ethics Committee and all subjects gave their written informed consent to participate in the experiment. The study conformed to the principles of the World Medical Associations Declaration of Helsinki.
Experimental Design
The mechanical properties of the medial gastrocnemius tendon, in both males and females, were examined prior to and immediately post completion of a passive dorsiflexion stretch.
Measurement of Torque
A dynamometer (Kin Kom) was used to measure the torque output during isometric plantar flexion. The participants were seated on the dynamometer with the knee fully
extended and the hip flexed to 900. The foot was fixed in a neutral anatomical
position, where the sole of the foot was at 900 to the tibia. The ankle joint axis was
visually aligned with the pivot point of the dynamometer leaver and the foot was securely fixed to the dynamometer foot plate with Velcro straps. Prior to the test, the participants performed three warm up isometric planter flexions in order to accustomise themselves to the procedure. The participants were instructed to gradually develop force from relaxed to maximum voluntary contraction (MVC) over a 10s time period. The task was repeated three times with a 1 minute rest between trials. The torque signals were analogue-to-digital converted at a sampling rate of 2 KHz (Testpoint, Keithley instruments, UK) and saved for further analysis.
Measurement of Tendon Elongation
Tendon elongation measurements were taken during the graded isometric plantar flexion test using a 7.5MHz, 40mm linear array, B-Mode ultrasound probe (AU5, ESAOTE BIOMEDICA, Italy) with a depth resolution of 49.3mm. The probe was placed in the sagittal plane over the myotendinous junction of the medial head of the gastrocnemius muscle and fixed in position. An echo-absorptive marker was placed between the probe and the skin to act as a fixed reference from which measures of elongation could be made. The ultrasound image was displayed in real time on the ultrasound monitor. The S-VHS output video signal from the ultrasound apparatus was fed to a computer and captured at 25Hz using Adobe Premier Pro. The force output from the dynamometer and the ultrasound video were synchronised using a trigger (Digitimer, UK) that provided a visual marker on the ultrasound image and force trace. Stills from the ultrasound video were taken at times corresponding 10% force increments from 0 to 100% MVC and the displacement of the gastrocnemius
myotendinous junction were digitised using computerised image analysis (Image J, Wayne Rasband National Institute of Health, USA).
Measurement of Electromyographic Activity
Two silver/silver chloride bipolar electrodes (Medicotest UK, type N10A), with a 20mm inter-electrode distance (centre to centre) were placed midline on the muscle belly, halfway between the centre of the belly and the distal myotendinous junction of the tibialis anterior. A ground electrode (Medicotest, UK, type Q10A) was placed at an electrical neutral site (lateral malleolus of the ankle). Electrodes were aligned perpendicular to the direction of muscle fibres. The participants’ skin was carefully prepared through a process of shaving, abrasion (Nuprep, SLE Ltd) and cleaning with alcohol before the electrodes were attached in order to minimise resistance. The Electromyographic signals were high and low pass filtered between 10 and 500 Hz respectively (Neurolog filters NL 144 and NL 134, Digitimer, UK), preamplified (x1000), (Neurolog remote AC preamplifier NL 824, Digitimer, UK), amplified (x2) (Neurolog isolation amplifier, NL 820, Digitimer, UK) and A/D converted at a rate of 2000Hz (KPCI 3101, Keithley instruments, UK). Post acquisition, a moving root mean square filter (100 ms window) was used to filter the electromgogram (EMG) signal.
In addition to the plantar flexion efforts participants performed three dorsi flexion MVC’s. The dorsiflexions were performed with the subject in the same position as previously described for the plantar flexion efforts. The force produced due to dorsiflexor muscles coactivation during the plantar flexion efforts was approximated assuming a linear relationship between EMG amplitude of the dorsiflexor muscles and force.
Co-contraction = TibAnt EMG during Plantarflexion X Maximal Dorsiflexion
Torque TibAnt EMG during Dorsiflexion Torque
Moment Arm Determination
The moment arm length of the gastrocnemius tendon was obtained using the tendon travel method (An et al. 1984). The displacement of the myotendinous junction
caused by rotating the ankle from 50 of dorsiflexion to 50 of plantarflexion was
recorded using ultrasonography (using the same probe and positioning described above in the measurement of elongation). The tendon moment arm length at the ankle
angle of 00 was obtained from the ratio of change in tendon displacement in mm to
change in angle in radians.
Calculation of Tendon Force
The torque values obtained from the dynamometer were converted into forces experienced by the tendon by dividing by the length of the tendon moment arm as measured using the tendon travel method explained above. Correction for relative muscle physiological volume of the gastrocnemius was applied in the calculation of final forces as per Fukunaga et al. (1996). Antagonistic co - contraction torque was determined and corrected for during the measures using sEMG (see ‘measurement of electromyographic activity’ section).
Calculation of Tendon Stiffness
The elongation of the tendon at loads corresponding to 0-100% of the plantarflexion force produced was measured at 10% intervals. The force elongation relationship was plotted and a second order polynomial fit was applied. Tendon stiffness was
calculated at 10% intervals of MVC and was defined as the slope of the force displacement relationship.
Stretch
A five minute passive dorsiflexion stretch was administered to each participant. The participant was positioned in the dynamometer as in the plantar and dorsi flexion
efforts (knee straight, hip at 900). The participants were instructed to remain as
relaxed as possible during the stretch and to ensure this was the case the EMG activity of the medial gastrocnemious and tibialis anterior was monitored (see measurement of electromyographic activity section for details). The foot plate of the dynamometer (to which the participants foot was attached) was moved into dorsiflexion so that it recorded a passive torque of 35-40Nm above baseline, this torque level was monitored and maintained throughout the stretch.
Statistics
Descriptive data included means (SEM). The significance of the difference between before and after passive stretching was analyzed by a paired Student’s t-test. The level of significance was set to P<0.05.
Preliminary Results
Females were shown to have smaller tendon moment arms (46.78 ± 5.3mm) than males (53.42 ± 2.6mm).
Medial gastrocnemius tendon stiffness (at 100% MVC) prior to stretching was greater
in males (68.9 ± 7.8 Nmm-1) than females (53.5 ± 8.5 Nmm-1). With female’s stiffness
being equal to 77.6% of males.
Post stretching there was no significant change in male’s tendon stiffness (68.3 ± 7.1
Nmm-1) whereas females showed a 26.42% decrease in stiffness (p<0.05) to a value
of 39.2 ± 6.1 Nmm-1 (see figure 1). -40 -35 -30 -25 -20 -15 -10 -5 0 100% 90% 80% 70% 60% 50% 40% 30%