No abstract

1. We investigated whether human subjects can activate selectively flexor pollicis longus (FPL) and digital portions of flexor digitorum profundus (FDP). These muscles were selected because they are the only flexors of the distal phalanges. 2. Electromyographic activity (EMG) was recorded with intramuscular electrodes from one digital component of the deep flexors (‘test’) while subjects lifted weights by flexing the distal interphalangeal joint of the other digits in turn (‘lifting’ digits). Only recording sites at which single motor units were recruited selectively at low forces were used. The weights lifted represented 2.5‐50% of the maximal voluntary contraction (MVC). We measured the lowest weight lifted which produced phasic and tonic coactivation in the ‘test’ muscle. 3. The extent of coactivation varied with the ‘distance’ between the test and lifting digits although no significant difference occurred in the pattern of coactivation thresholds among the digital flexors. The extent of coactivation increased when angular displacement or velocity at the distal interphalangeal joint of the lifting digit increased but was not critically dependent on restraint of the hand. 4. Because mechanical ‘connections’ could interfere with the ability to move a distal phalanx independently, the arms of nine cadavers were studied. The separation of tendons between the thumb (FPL) and the index portion of FDP, and between the index and middle portions of FDP, usually extended more proximally in the forearm than separation between the tendons to the middle and ring fingers and between the ring and little fingers. Direct intertendinous links were also noted. 5. It is not possible to direct a sufficiently focal motor command to flex selectively the distal joint of the fingers and thumb when forces exceeding 2.5% MVC are generated. For the middle, ring and little fingers in particular, movement of adjacent digits may also involve ‘in‐series’ mechanical links between adjacent components of FDP.

Context: The Hertel model of chronic ankle instability (CAI) is commonly used in research but may not be sufficiently comprehensive. Mechanical instability and functional instability are considered part of a continuum, and recurrent sprain occurs when both conditions are present. A modification of the Hertel model is proposed whereby these 3 components can exist independently or in combination.Objective: To examine the fit of data from people with CAI to 2 CAI models and to explore whether the different subgroups display impairments when compared with a control group.Design: Cross-sectional study.Patients or Other Participants: Community-dwelling adults and adolescent dancers were recruited: 137 ankles with ankle sprain for objective 1 and 81 with CAI and 43 controls for objective 2.Intervention(s): Two balance tasks and time to recover from an inversion perturbation were assessed to determine if the subgroups demonstrated impairments when compared with a control group (objective 2).Main Outcome Measure(s): For objective 1 (fit to the 2 models), outcomes were Cumberland Ankle Instability Tool score, anterior drawer test results, and number of sprains. For objective 2, outcomes were 2 balance tasks (number of foot lifts in 30 seconds, ability to balance on the ball of the foot) and time to recover from an inversion perturbation. The Cohen d was calculated to compare each subgroup with the control group.Results: A total of 56.5% of ankles (n 5 61) fit the Hertel model, whereas all ankles (n 5 108) fit the proposed model. In the proposed model, 42.6% of ankles were classified as perceived instability, 30.5% as recurrent sprain and perceived instability, and 26.9% as among the remaining groups. All CAI subgroups performed more poorly on the balance and inversionperturbation tasks than the control group. Subgroups with perceived instability had greater impairment in single-leg stance, whereas participants with recurrent sprain performed more poorly than the other subgroups when balancing on the ball of the foot. Only individuals with hypomobility appeared unimpaired when recovering from an inversion perturbation.Conclusions: The new model of CAI is supported by the available data. Perceived instability alone and in combination characterized the majority of participants. Several impairments distinguished the sprain groups from the control group.Key Words: ankle injuries, joint instability, postural balance, recurrent ankle sprains Key PointsN The proposed new model of chronic ankle instability is supported by data from previous studies and the current study.More subgroups are identified than in previous models, with perceived instability as a common link.N On balance tests, all groups with chronic ankle instability performed more poorly than control groups. N The model will allow the development of specific injury-rehabilitation and injury-prevention programs for subgroups of chronic ankle instability.