Vertical reaction forces and kinematics of backward walking underwater Leticia Calado Carneiro, Stella Maris Michaelsen *, Helio Roesler, Alessandro Haupenthal, Marcel Hubert, Eddy Mallmann Center of Health and Sport Sciences, Santa Catarina State University, Floriano ´polis, SC, Brazil 1. Introduction Hydrotherapy has been investigated as a form of treatment for various conditions including osteoarthritis [1], post-operative recovery after total hip-replacement surgery [2], chronic back pain [3] and coronary artery disease [4]. Similarly, its use has been studied for the prevention of physical changes accompanying the ageing process [5]. Numerous exercises have been proposed in widely varying programs of hydrotherapy, among them backward walking (BW), which is included in some rehabilitation protocols [1–5]. Walking backward on an underwater treadmill has been shown to elicit more electromyogram (EMG) activity in the paraspinal muscles compared to BW on land. When compared to forward walking (FW) on an underwater treadmill, BW elicits more paraspinal, vastus medialis and tibialis anterior activity as well as higher physiological and perceived exertion responses [6–8]. The aquatic environment presents the advantage of reducing weight bearing due to buoyancy, but this varies with the immersion level [9–11] and velocity of movement [12]. Despite reduction in vertical ground reaction forces (VGRF) in FW underwater, Barela et al. [11] did not find significant differences in the range of motion of the ankle, knee or hip joints comparing the FW kinematics on land and in water. On land, several authors have explored BW kinematics [13–17] and EMG patterns during BW compared to FW [14–16]. Some studies have explored the temporal–spatial characteristics of BW [18], but few authors appear to have investigated the kinetics of backward walking [19] or running [20] on land. In the case of rehabilitation, it is necessary to determine the load that the patient can tolerate based on the injury suffered; this load can then serve as a foundation for the prescription of walking underwater [21]. However, little is known about the loads generated on the locomotor apparatus during BW in an aquatic environment. Forward walking (FW) has been studied for longer than BW and its characteristics are already well defined. The shape of the force  time curve for the VGRF recorded during FW on land resembles an ‘‘M’’, which demonstrates the presence of two clear force peaks with a deflection between them. The first force peak (FFP) of VGRF arises from the contact of the foot with the ground. The second force peak (SFP) corresponds to the propulsion phase of walking [22]. In the case of walking underwater the peaks are not as distinct and the curve takes the form of a trapezoid [9–11]. The physiological and biomechanical demands of BW and FW differ [15]. In BW the VGRF in the contact phase with the toes on the ground (FFP) is greater than the phase of foot lift (SFP) [19]. Gait & Posture 35 (2012) 225–230 A R T I C L E I N F O Article history: Received 16 July 2010 Received in revised form 2 September 2011 Accepted 6 September 2011 Keywords: Walking backward Ground reaction force Walking underwater Kinematics A B S T R A C T The aim of this study was to compare the first and second peaks of the vertical ground reaction force (VGRF) and kinematics at initial contact (IC) and final stance (FS) during walking in one of two directions (forward  backward) and two environments (on land  underwater). Twenty-two adults (24.6  2.6 years) walking forward (FW) and backward (BW) on a 7.5 m walkway with a central force plate. Underwater immersion was at the height of the Xiphoid process. Ten trials were performed for each condition giving a total of 40 trials where the VGRF and kinematic data were recorded. Two-way repeated measures analysis of covariance was used with a combination of environment and direction of walking: FW on land, FW underwater, BW on land and BW underwater (entered as between-subjects factor) and repeated measures of VGRF peaks (first and second) or angles (at IC and FS). Walking velocity was included as a covariate. Both VGRF peaks were reduced when participants walked underwater compared to on land (p < .001). For BW, in both environments, the second peak was lower than the first (p < .001; for both). During BW at IC the ankle is more dorsiflexed and the knee is more flexed, both on land and underwater. At FS, there was no difference between the ankle angle for FW and BW in both environments. At IC, in FW and BW the knee and hip are more flexed underwater. BW underwater involves a lower VGRF and more knee and hip flexion than BW on land. ß 2011 Elsevier B.V. All rights reserved. * Corresponding author at: Department of Physical Therapy, Motor Control Lab, Santa Catarina State University, 358, Pascoal Simone Street, Zip Code 88080-350 Coqueiros, Floriano ´ polis, SC, Brazil. Tel.: +55 48 3321 8600. E-mail address: michaelsenstella@hotmail.com (S.M. Michaelsen). Contents lists available at SciVerse ScienceDirect Gait & Posture jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/g aitp os t 0966-6362/$ – see front matter ß 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.gaitpost.2011.09.011