Muscles do more positive than negative work in human locomotion

Muscle work during level walking and ascent and descent ramp and stairway walking was assessed in order to explore the proposition that muscles perform more positive than negative work during these locomotion tasks. Thirty four healthy human adults were tested while maintaining a constant average walking velocity in the five gait conditions. Ground reaction force and sagittal plane kinematic data were obtained during the stance phases of these gaits and used in inverse dynamic analyses to calculate joint torques and powers at the hip, knee and ankle. Muscle work was derived as the area under the joint power vs time curves and was partitioned into positive, negative and net components. Dependent t-tests were used to compare positive and negative work in level walking and net joint work between ascent and descent gaits on the ramp and stairs (P<0.010). Total negative and positive work in level walking was -34 J and 50 J, respectively, with the difference in magnitude being statistically significant (P<0.001). Level walking was therefore performed with 16 J of net positive muscle work per step. The magnitude of the net work in ramp ascent was 25% greater than the magnitude of net work in ramp descent (89 vs -71 J m-1, P<0.010). Similarly, the magnitude of the net work in stair ascent was 43% greater than the magnitude of net work in stair descent (107 vs -75 J step-1, P<0.000). We identified three potential causes for the reduced negative vs positive work in these locomotion tasks: (1) the larger magnitude of the accelerations induced by the larger ground reaction forces in descending compared to ascending gaits elicited greater energy dissipation in non-muscular tissues, (2) the ground reaction force vector was directed closer to the joint centers in ramp and stair descent compared to ascent, which reduced the load on the muscular tissues and their energy dissipating response, and (3) despite the need to produce negative muscle work in descending gaits, both ramp and stair descent also had positive muscle work to propel the lower extremity upward and forward into the swing phase movement trajectory. We used these data to formulate two novel hypotheses about human locomotion. First, level walking requires muscles to generate a net positive amount of work per gait cycle to overcome energy losses by other tissues. Second, skeletal muscles generate more mechanical energy in gait tasks that raise the center of mass compared to the mechanical energy they dissipate in gait tasks that lower the center of mass, despite equivalent changes in total mechanical energy. Originally Published Journal of Experimental Biology October, 2008 210(19)

Serum potassium, calcium and magnesium after resuscitation from ventricular fibrillation: A canine study

Serum electrolytes were measured before and sequentially for 3 hours after resuscitation from ventricular fibrillation in a canine model that was designed to approximate the human cardiac arrest and resuscitation process. Twenty anesthetized dogs were resuscitated from ventricular fibrillation; 7 required epinephrine during resuscitation and 13 did not. To control for the effects of anesthesia, 10 dogs were anesthetized and instrumented, but ventricular fibrillation was not induced.Serum potassium decreased from 3.7 ± 0.3 mmol/liter at baseline to 3.2 ± 0.4 mmol/liter 45 minutes after resuscitation in the experimental dogs resuscitated without epinephrine, as compared with 3.6 ± 0.3 to 3.4 ± 0.2 mmol/liter in control dogs (p = 0.07 versus control dogs by two-way analysis of variance) and returned toward baseline at the end of 3 hours. Serum calcium decreased from 9.6 ± 0.6 mg/dl at baseline to 8.9 ± 0.9 mg/dl at 5 minutes after resuscitation as compared with 9.4 ± 0.7 to 9.5 ± 0.7 mg/dl in control dogs (p < 0.05 versus control dogs) and returned to baseline by 3 hours. Serum magnesium decreased from 1.5 ± 0.1 to 1.3 ± 0.2 mEq/dl by 3 hours in resuscitated dogs as compared with 1.6 ± 0.2 to 1.5 ± 0.2 mEq/dl in control dogs (p = 0.06 versus control dogs). These changes in serum potassium, calcium and magnesium were independent of the administration of epinephrine during the resuscitation process. Changes in potassium were independent of arterial pH or bicarbonate therapy. Serum glucose increased after ventricular fibrillation but not in control dogs (p < 0.0005 versus control). No changes in other electrolytes were observed.Thus, serum potassium, calcium and magnesium decreased after resuscitation from ventricular fibrillation in this canine model. These data suggest that, although the hypokalemia seen after ventricular fibrillation in humans may in some cases precede the event, a decrease in potassium may develop after resuscitation

How do low horizontal forces produce disproportionately high torques in human locomotion?

Although horizontal ground forces are only ~15% of vertical forces, they account for 47% and 33% of the metabolic cost in walking and running. To explain these disproportionately high metabolic costs we hypothesized that low horizontal ground forces generate relatively high torques on body segments during locomotion and this is mediated by long moment arms. We compared external force moment arms and discreet torques applied to the body segments by horizontal and vertical forces during walking and running. Sixteen subjects (21.9 ± 1.9 years) walked at 1.5 m/s and 10 subjects (23.2 ± 2.0 years) ran at 3.83 m/s. Segmental torques in the sagittal plane were partitioned into components due to horizontal and vertical forces and quantified by their angular impulses. The mean (±S.E.) ratios of horizontal to vertical ground forces (GF ratio) and angular impulses (AI ratio) in walking were 0.131 (±0.003, 95% C.I. 0.124 to 0.137) and 0.530 (±0.018, C.I. 0.497 to 0.569). Results were similar in running. In both gaits the AI ratios were significantly greater than the GF ratios because the respective C.I.s did not overlap. The horizontal forces produced 53% and 41% as much angular impulse on the body segments as did the vertical forces in walking and running despite being only 13% as large. In the two movements the moment arms for the horizontal forces averaged across foot, leg, thigh, and trunk body segments were 3.8 fold larger than those for the vertical forces. The data supported the hypothesis and suggest that the relatively low horizontal vs vertical forces accounted for a disproportionately higher percentage of the angular impulses placed on the body segments and this effect was due to relatively long moment arms for horizontal forces. These results partially explain the relatively large metabolic cost of generating relatively low horizontal forces. Originally published Journal of Biomechanics, Vol. 41, No. 8, 200

How do low horizontal forces produce disproportionately high torques in human locomotion?

Although horizontal ground forces are only ~15% of vertical forces they account for 47% and 33% of the metabolic cost in walking and running. To explain these disproportionately high metabolic costs we hypothesized that low horizontal ground forces generate relatively high torques on body segments during locomotion and this is mediated by long moment arms. We compared external force moment arms and discreet torques applied to the body segments by horizontal and vertical forces during walking and running. Sixteen subjects (21.9 ± 1.9 years) walked at 1.5 m/s and 10 subjects (23.2 ± 2.0 years) ran at 3.83 m/s. Segmental torques in the sagittal plane were partitioned into omponents due to horizontal and vertical forces and quantified by their angular impulses. The mean (±S.E.) ratios of horizontal to vertical ground forces (GF ratio) and angular impulses (AI ratio) in walking were 0.131 (±0.003 95% C.I. 0.124 to 0.137) and 0.530 (±0.018 C.I. 0.497 to 0.569). Results were similar in running. In both gaits the AI ratios were significantly greater than the GF ratios because the respective C.I.s did not overlap. The horizontal forces produced 53% and 41% as much angular impulse on the body segments as did the vertical forces in walking and running despite being only 13% as large. In the two movements the moment arms for the horizontal forces averaged across foot leg thigh and trunk body segments were 3.8 fold larger than those for the vertical forces. The data supported the hypothesis and suggest that the relatively low horizontal vs vertical forces accounted for a disproportionately higher percentage of the angular impulses placed on the body segments and this effect was due to relatively long moment arms for horizontal forces. These results artially explain the relatively large metabolic cost of generating relatively low horizontal forces. Originally published Journal of Biomechanics Vol. 41 No. 8 200