Passive stretches are performed by a therapist, who manually moves and holds the joint at its end-range position. Even though several studies have investigated the effects of passive stretching on range of motion (ROM) and the muscle tendon unit, the evidence is contradictory. It remains unclear whether stretching can influence fascicle length or whether stretching results only in improved stretch tolerance.
Ankle foot orthoses (AFOs) cause a mechanical constraint to the ankle and foot for an extended period. While one study showed only little changes in the muscle belly and tendon length, a simulation study showed that the lengthening abilities of the medial gastrocnemius muscle tendon unit varied during gait between types of AFOs. Therefore, it is suggested that a proper orthotic design might be able to promote the increase in sarcomeres in series.
In serial casting, the ankle joint is stretched in an end-range position over a prolonged period by applying a plaster. One study showed that prolonged casting resulted in an increased tendon rather than muscle length.
All three stretching methods as stand-alone treatment to improve muscle-tendon length appear to be ineffective. Could combined methods be a better approach?
- Botulinum toxin (botox) injections and serial casting
Serial casting is commonly applied in combination with intramuscular botox injections. The idea behind this is that botox reduces muscle spasticity and thereby allows for more ROM, which can in turn be maintained by serial casting. Although previous studies showed functional improvements after the combination of botox and serial casting, other animal studies showed that botox increased muscle stiffness and that a prolonged application resulted in muscle atrophy. This emphasises the importance of muscle physiological studies in children with CP.
- Strengthening and stretching
Another idea to increase the muscle tendon length during a passive stretch is by increasing tendon stiffness. Previous studies have shown that mechanical loading of the tendon can result in increased tendon stiffness. A study investigated this effect in children with CP and showed increased muscle strength, tendon stiffness and fascicle lengthening after this combined treatment.
- Electrical stimulation and stretching
One study proposed that the effect on stretching might improve when the agonist muscle receives a tensile stimulus and thereby reciprocally inhibits the stretched muscle. This was investigated in healthy individuals and showed improved ROM compared to stretching alone. However, this has not been investigated in children with CP yet.
- Specific collagenase enzymes and stretching
Recently, a new approach to reduce muscle stiffness is receiving attention. The idea is to inject specific collagenase enzymes into the muscle to digest collagen in the extracellular matrix and thereby reduce muscle stiffness. This theory is currently investigated in animal studies and the knowledge has not been transferred yet into human studies.
- Eccentric fascicle loading
High mechanical stretching to the muscle can also be achieved during eccentric strength training exercise. A few studies have resulted in increased strength, increased muscle fascicle length and ROM. However, the improvements might result from altered neuromuscular control rather than changes in the muscle structure.