Muscle stiffness in human ankle dorsiflexors: intrinsic and reflex components

Abstract

1. The purpose of this study was to evaluate the mechanical response to stretch in normal human ankle dorsiflexors at different levels of voluntary contraction. In an active muscle, the total mechanical response is the sum of the intrinsic response from the contractile apparatus, the response from passive tissues, and the reflex mediated response. Each of these components was investigated. 2. The total incremental stiffness was defined as the ratio between the torque increment and the amplitude of the stretch. In 14 subjects the total stiffness increased from approximately 0.6 N.m/deg to approximately 2.5 N.m/deg at 50% of MVC and remained constant (+/- 10%) from 30 to 80% of MVC. 3. The contribution to incremental stiffness from intrinsic muscle properties was measured during electrical stimulation of the deep peroneal nerve at 7-50 Hz. Intrinsic stiffness increased linearly with torque from approximately 0.5 N.m/deg to approximately 2.5 N.m/deg at 80% of MVC. 4. The reflex component (total minus intrinsic stiffness) had a maximum of 0.5-1.5 N.m/deg at 30-50% of MVC and was approximately zero at no and maximal contraction. For intermediate levels of contraction the reflex increased the stiffness with 40-100% of the intrinsic stiffness in this flexor muscle. 5. The reflex contribution to total stiffness began approximately 50 ms after onset of stretch and peaked 150-300 ms after onset of stretch. 6. Total, intrinsic, and reflex mediated stiffness were all nearly independent of the amplitude of stretch in the range from 2 to 7 degrees. The higher stiffness observed for 1 degree stretches could be due to "short range stiffness" of the cross bridges. 7. Stretching of a contracting muscle generates large force increments even for moderate amplitudes of stretch. Approximately half of this force increment is due to the stretch reflex, which makes the muscle stiffer than predicted from the intrinsic stiffness. These findings in human flexor muscles are surprisingly similar to previous findings in extensor muscles of the decerebrate cat.

Knee stabilization in patients with medial compartment knee osteoarthritis

“…Others have demonstrated that in both the upper (27,28) and lower (29,30) extremities, higher background muscle activity yields increased reflexive joint stiffness, potentially providing greater joint stability.…”

Mechanical properties of the plantarflexor musculotendinous unit during passive dorsiflexion in children with cerebral palsy compared with typically developing children

“…High interrater and intrarater reliability has previously been demonstrated for the measurement of passive torque and ankle displacement using this technique (intraclass correlation coefficient >0.86) 13 and the procedure has been shown to be highly responsive to change in stiffness characteristics. 14 Each participant lay supine with the foot placed in the ankle apparatus and positioned, by visual approximation, such that the point midway between the lateral and medial malleolus in the sagittal plane was aligned with the axis of rotation of the device. The participant's foot was secured with Velcro straps, the knee was placed in an extended position, and light pressure was applied by the researcher's (AA) hand above the knee over the thigh to ensure that knee position was maintained.…”

Systematic nonlinear relations between displacement amplitude and joint mechanics at the human wrist

“…The pattern of decrease of both stiffness and viscosity with increasing displacement amplitude has been found to be nonlinear, with an initial steep decrease followed by a slower rate of change (Kearney and Hunter, 1982;Sinkjaer et al, 1988). The ankle was studied in both instances and the pattern was found to be consistent in five subjects by Kearney and Hunter (Kearney and Hunter, 1982) but less consistent in three subjects studied by Sinkjaer (Sinkjaer et al, 1988).…”

“…This is followed by a sudden yield, after which the muscle stiffness remains constant at a lower level (Rack and Westbury, 1974). Studies of intact joint mechanics (with stretch reflex input) have reported that stiffness decreases as displacement amplitude increases, both in humans (Kearney and Hunter, 1982;Milner and Cloutier, 1998;Sinkjaer et al, 1988) and decerebrate cats (Nichols, 1985). Kearney and Hunter (1982) found a decrease in the stiffness of the ankle with increasing displacement up to 14° peak to peak amplitude, while Sinkjaer et al (1988) found that ankle stiffness decreased only between 1° and 2° of displacement and did not change from 2° to 7°.…”

“…Studies of intact joint mechanics (with stretch reflex input) have reported that stiffness decreases as displacement amplitude increases, both in humans (Kearney and Hunter, 1982;Milner and Cloutier, 1998;Sinkjaer et al, 1988) and decerebrate cats (Nichols, 1985). Kearney and Hunter (1982) found a decrease in the stiffness of the ankle with increasing displacement up to 14° peak to peak amplitude, while Sinkjaer et al (1988) found that ankle stiffness decreased only between 1° and 2° of displacement and did not change from 2° to 7°. Some of the above studies reported that joint viscosity also decreases with displacement amplitude, although Kirsch et al (1994) reported no change in viscosity in decerebrate cats.…”