Many people have a difficult time distinguishing between muscular strength and muscular power, and hence understanding why increasing the former (e.g., via lifting weights) doesn't automatically lead to an increase in the latter. In posting this I hope to clear up some of the existing confusion about this issue.

 

The plot below shows the relationship between average (i.e., over the full 360 degrees) effective (i.e., tangential to the crank, therefore causing it to turn instead of stretch or compress) pedal force (AEPF) vs. circumferential pedal velocity (CPV, the velocity at which the pedal travels around in a circle). The colored dots are derived from power and cadence data recorded every 1 s (SRM crank) during a variety of races and training sessions, as described here. The solid black line represents this individual's maximal AEPF at any given CPV, as determining using Dr. Jim Martin's inertial load technique. AEPF is highest (at 918 N) when CPV is zero, and falls to zero when CPV is maximal (i.e., 3.63 m.s). In other words, when measured at the joint angles achieved during pedaling, this person's strength is 918 N.  The dark blue curved line therefore represents this person's maximal power of 833 W, which occurs when the product of AEPF and CPV is maximal. For this athlete, this is at a CPV of ~1.75 m/s, or around 120 rpm. As discussed elsewhere, it can be seen from inspection of this plot that even when time trialing at 300 W and 80 rpm (CPV=1.4 m/s), this individual uses only about one-fourth of their maximal AEPF, i.e., their strength. Indeed, even when sprinting at maximal power, AEPF is still only 50-60% of maximum - in other words, strength per se is not limiting, even during sprinting. This fact, in and of itself, implies that strength training would have limited effect on cycling performance.

 

But what of the effects of increasing strength on power output? Couldn't this still increase maximal power, even if absolute strength is not limiting? Indeed, this would be true if the increase in strength were entirely due to hypertrophy, and nothing else changed. This is illustrated by the dashed line, which represents the effects of increasing maximal AEPF (i.e., strength) by 25%. This percentage increase was chosen because it is typical of that achieved when already moderately-active individuals take up resistance training for several months, e.g., as a cyclist might do during their off-season. (Maximal CPV was assumed not to change, since this is really an intrinsic characteristic of the muscles, and if anything might be expected to decrease, not increase, with resistance training/hypertrophy.) As shown by the light blue line, this increase in maximal AEPF, and therefore in AEPF at every velocity greater than zero, would theoretically result in a corresponding increase in maximal power to 1041 W.

 

So if increasing strength can at least in theory increase power, then why do studies generally show that this does not happen, even when untrained individuals are studied and it is maximal/short-term, not submaximal/sustained, power that is measured? The reason is that a very large percentage of the increase in strength that occurs with resistance training is due to neurological adaptations, not due to muscle hypertrophy. Furthermore, these neurological adaptations are highly specific to the joint angles and velocities used in training. For example, with a proper resistance training program untrained subjects may increase their 1 repetition-maximum (i.e., the maximum amount of weight they can lift one time) during knee extension by almost 2-fold, but their strength (i.e., force at zero velocity) increases much less, i.e., by only 15-20%. The force generated at higher velocities increases even less, if at all, such that maximal power may actually be unchanged. In effect, the neurological adaptations to weight lifting - which do not readily or completely transfer to other exercise modes, such as cycling - tend to alter or distort the force-velocity relationship, elevating force at low velocities but not higher velocities. Conversely, studies of individuals performing endurance training (cycling) have shown that maximal force increases at the velocities typically encountered when pedaling, but not at lower velocities.

 

So what's the bottom line? It is this: maximal power is generated when large muscles contract simultaneously, forcefully, and quickly. Weight training is an excellent way of increasing strength, because not only does it lead to an increase in muscle size, but also to improvements in synchronicity (simultaneous activation of multiple motor units) and activation (therefore more force from the same muscle). In fact, these neural adaptations are the primary mechanism leading to increases in strength, at least over the short (and even intermediate) term - but since they do not readily transfer to other modes of exercise, it is the more limited hypertrophic response that has the greatest effect on maximal cycling power.

 

Specificity, specificity, specificity....