Health Related Fitness of Children with Disabilities: Muscular Endurance
Many children with disabilities also have very low levels of strength and have difficulty with any activity that requires adequate strength.25 Muscle strength and endurance is very important to the child with a disability because ADLs and IADLs, such as performing transfers or wheeling up a ramp, require adequate amounts of strength and endurance.26
Several studies have documented low levels of muscle strength and endurance in children with disabilities. Winnick and Short reported poor grip strength in 141 children with lower limb impairments.27 Olney and coworkers found that the ankle plantar flexors on the affected side of children with spastic hemiplegia were severely deficient in generating power during walking, which resulted in greater amounts of work being performed by muscle groups of the hip.28
Kramer and MacPhail noted that there was a direct relationship between knee extensor strength and efficient walking and gross motor ability in adolescents with CP.29 They believed that lack of strength in the knee extensors could be one reason why they were limited in standing, walking, running and jumping activities, and concluded that improvements in muscular strength may be associated with improvements in walking efficiency and functional abilities.
O'Connell, Barnhart, and Parks examined the relationship between muscular endurance and wheelchair propulsion in three children with cerebral palsy.30 Using a 6-RM test for elbow flexion, elbow extension; shoulder abduction, flexion, extension, internal and external rotation; and a combined shoulder flexion-elbow extension test with cuff weights, dumbbells and barbells, their data showed that muscular endurance and both anaerobic and aerobic wheelchair tasks were correlated. They recommended that a muscular endurance program be developed for persons who use a wheelchair for ambulation since there is a strong association with the ability to push a wheelchair and the amount of muscular endurance that a person possesses. They cautioned, however, that with such a small sample size, more research is needed on specific outcomes of muscular endurance training.
O'Connell and Barnhart published a similar study using the same data set to determine if resistance training could improve wheelchair propulsion in pediatric wheelchair users.31 The three children with spastic cerebral palsy performed resistance exercises three times a week for 9 weeks, using cuff weights, dumbbells and barbells. They performed three sets of 6-RM for each movement: elbow flexion and extension; shoulder abduction, flexion, extension, internal and external rotation; and a combined supine shoulder flexion-elbow extension. They concluded that progressive resistance exercise training appears to be able to improve muscular strength and wheelchair performance in children with cerebral palsy, and encouraged practitioners to develop a formal resistance training program to enhance and maintain wheelchair propulsion.
Damiano, Kelly and Vaughn studied the effects of quadriceps femoris muscle strengthening on crouch gait in 14 children with spastic diplegia.32 They wanted to determine if there was a causal link between resistance training and improved gross motor function. The subjects exercised three times a week for 6 weeks using ankle weights at loads of approximately 65% of maximum isotonic force production. Gait analyses were also performed before and after the study. The investigators found that the subjects were able to increase quadriceps femoris strength with resistance training, and that there was no increase in hamstring muscle force. They concluded: "Our study focused on quadriceps femoris muscle weakness as one component of the motor dysfunction seen in children with CP, and utilizing a traditional orthopedic approach rarely recommended with this population demonstrated clinical improvement in strength and ambulatory ability." However, they cautioned that several of the children developed knee hyperextension in mid-stance, which prompted the investigators to recommend the possibility of hamstring strengthening at the same time that the quadriceps are developed. Damiano, Vaughn and Abel continued their research and examined the muscle responses to heavy resistance training in children with cerebral palsy.33 They compared their findings to a control group of 25 children who did not have an impairment. The investigators found that all of the children who participated in the study had consistent and dramatic strength increases in the quadriceps muscle, with the majority attaining normal strength values. A concurrent increase in the strength of the hamstring muscles was not found. This eliminated the concern that heavy resistance exercise would elicit unwanted muscle activity in antagonistic muscles.
Damiano and Abel expanded their research to examine the effects within and across two distinct clinical subgroups: children with spastic hemiplegia (n=5) and children with moderate to severe spastic diplegia (n=6).34 The children ranged in age from 6 to 12 years. The investigators wanted to determine if a resistance training program would increase strength in certain targeted muscles. The untrained muscles were used as a basis for comparison. In the children with hemiplegia, strength changes on the more affected extremity were compared to corresponding muscles on the contralateral extremity. In the children with diplegia, the ipsilateral antagonist muscles were used as the untrained comparison. Eight muscle groups were tested using a hand-held dynamometer. A secondary purpose of the study was to determine if subjects could achieve functional gains in performance after the resistance training program. Computerized gait analysis was used for this purpose. Subjects were asked to exercise three times a week for 6 consecutive weeks. Velcro-attached free weights were used, and the load for each muscle was approximately 65 percent of the maximum isometric strength value. Each subject performed four sets with five repetitions in each set at each muscle group. Results of the study found that in the children with diplegia, there was a 69 percent increase in strength after training in the targeted muscle groups, and in the children with hemiplegia, there was a 20 percent increase. Functional changes were also found for the entire group. Subjects increased their maximal speed during fast walking. Damiano and colleagues reconfirmed the findings of previous research that resistance training in young individuals with cerebral palsy is safe and effective in improving strength and motor performance: "To our knowledge, there is no research to support the theory that resistance training increases the recruitment of other muscles that are already overactive or spastic, which would obviously be contraindicated if this were the case."
Damiano and Abel concluded that a short-term strength training program demonstrated positive functional outcomes. They also reiterated that the clinical concern that resistance training will cause an "inadvertent strengthening of the spastic antagonist muscle during training" is unfounded.34
Based on these training studies, children who have cerebral palsy and other neuromuscular disabilities should be involved in a resistance training program. In developing strength programs for ambulatory children with neuromuscular disabilities, it is important to assess individual muscle groups to determine any asymmetrical weakness that may impose a greater burden on walking, and to develop the functional muscle groups that are needed for daily activities.