Passive ankle-foot joint stiffness and physical performance

Abstract

The toe flexor muscles maintain body balance during standing and provide push-off force during walking, running, and jumping. Additionally, they are important contributing structures to maintain normal foot function. Thus, weakness of these muscles may cause poor balance, inefficient locomotion and foot deformities. The quantification of metatarsophalangeal joints (MPJ) stiffness is valuable since it is considered as important factor in toe flexor muscles function. Ankle joint stiffness has been investigated for performance and clinical assessment. Previous researches suggested that eccentric exercise of ankle joint muscles increased its passive stiffness, hence improving running and jumping performance. Conversely, excessive ankle joint stiffness deteriorates ankle function. Measurement of MPJ and ankle joint stiffness is still largely depended on manual skills as current devices do not have good control on alignment, angular joint speed and displacement during measurement. Therefore, this study introduces an innovative dynamometer and protocol procedures for MPJ and ankle joint torque measurements with precise and reliable control of foot alignment, angular rotation speed and displacement. Within-day and between-day test-retest experiments on MPJ and ankle joint torque measurements were conducted on ten and nine healthy male subjects respectively. Intraclass-correlation coefficients (ICC) of averaged peak torque of both joints in within-day and between-day test-retest experiments were ranging from 0.91 to 0.96, and the joint torque was similar to the measurements of other studies. The results indicated the innovative device is systematic and reliable for the measurements and can be used for multiple scientific and clinical purposes. To investigate the relationship of ankle-foot stiffness and performance outcome, 99 male subjects, aged between 18 to 30, were recruited for experiment. Passive MPJ and ankle joint torques were collected by the dynamometer. Physical outcome was evaluated by vertical stiffness (Kvert), which was determined by body deceleration and displacement of body mass during hopping on force platform. Pearson's correlation was analyzed. It was found that vertical stiffness was significantly correlated to passive ankle joint torque (Tankle) and MPJ torques in both sitting and standing postures (p<0.005, 0.43>R>0.29). To further examine the relationship of ankle-foot passive stiffness and physical performance, fifty-one out of 99 subjects were selected and divided into three groups according to their regular physical activities. There were 15 marathon runners, 19 basketball players and 17 miscellaneous group of athletes. It was hypothesized that the passive ankle joint torque of marathon runners is stronger because higher elastic energy can be stored and reused in stretching and shortening cycles during running for better running economy (RE). Secondly, it was hypothesized that passive MPJ torque of basketball players is stiffer because basketball players should run around the stage in different directions, sudden stop running, jump shooting and maintain the body balance. All these actions require strong toe flexors. One-way ANOVA was used to test between group differences. Post-hoc test showed that Tankle of basketball players was significantly stiffer than that of miscellaneous athletes by about 24% (p=0.03). MPJsit and MPJstand of basketball players were stiffer than that of marathon runners and miscellaneous athletes by about 24% and 32% respectively (p=0.01). The results supported the second hypothesis that the toe plantar flexors of basketball players were stronger but not supported the first hypothesis that the ankle joints of marathon runners were stiffer for better RE. This study concluded that strengthening both ankle and toe plantar-flexors could improve basketball players' performance. In the last experiment, the Passive MPJ torque in sitting and standing position, total leg stiffness, vertical stiffness and RE during sub-maximal running were examined. It was found that RE, leg and vertical stiffness during sub-maximal running was correlated with passive MPJ torque. The improvement of RE can be explained by the reduction of contact time with increased toe strength, and hence improve leg and vertical stiffness. It was suggested that RE for sub-maximal running would be improved by increasing toe flexors strength. For further research, the dynamometer can be used to evaluate surgical outcome of toe deformity correction in term of range of motion, stiffness and maximum force output of the toes. Besides, it can be used as a conventional dynamometer to determine ankle joint stiffness for stroke patient to determine the range of motion and the force output of ankle and MPJ.